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Here at Warm Glass UK we are proud to offer all our customers UNLIMITED TECHNICAL SUPPORT on products bought from us. Most of the questions we get asked about glass are answered on this page, and if you can't find what you are looking for, then simply call us on 01934 863344, we really are here to help. Prefer to e-mail us? Use the enquiry form at the bottom of this page and we will get back to you within one working day.

For kiln schedulesonline education videostechnical videos and Bullseye tipsheets and projects, covering all aspects of glass work from glass cutting to kilncasting, see our Knowledge Base.

There are also help sheets under a great many of our individual products - just find the product you are interested in on this site, and check for a pdf link.

Frequently asked questions:

glass studio tips

Glass Basics:

Fundamentals of working with glass

Bubbles in fused glass

Get the most out of your glass studio tools and equipment.

What is the difference between each of the shelf primers you stock?

Shelf primer (or kiln wash) is used to prevent your glass from sticking to the kiln shelf or mould during firing. We sell an number of different types and each have their particular uses:

Bullseye Shelf Primer: Our most popular kiln wash, this is a good general purpose primer. It has a helpful pink tint so you can see exactly where you have applied it (the tint burns out on firing). It also dries quickly between layers and is easy to apply evenly with a soft haik brush. Another benefit is that if you fire under 706C, then you will not need to re-apply the primer for another firing. A new coating should be applied after a full fuse (766C or higher). 

Hi-Fire Shelf Primer: An excellent shelf primer, this primer is specifically designed to perform well at high temperatures for techniques such as raking. It is also good for bead release and seems to work well at lower firing temperatures as well, making it a good all-round primer. It also has a pink tint to help show where it has been applied, and goes on nicely in even layers. It cleans from the shelf and glass easily after use.

Primo Primer: We recommend this primer for use with casting moulds, such as the Colour de Verre dragonfly mould or nano bead mould.  Although slightly more difficult to mix and apply (Tip: leave it for an hour after adding the water before mixing again and applying), it is truly excellent at releasing work from these complex moulds, leaving great detail and a smooth finish. It also needs minimal clean up afterwards.  

Boron Nitride Spray: It is expensive, but a little goes a long way and it is ideal for stainless steel moulds such as the floral former because, unlike other primers, you do not have to heat the mould for the primer to adhere. It also gives a fantastically smooth finish to your glass, minimising cold work. Not recommended for firing temperatures above product.

Bullseye Thinfire Paper: This product removes the need to prepare and apply a primer. A more expensive option than primers because the paper only lasts for one firing, but preparation times are almost zero. Just put the paper on the kiln shelf and place your work on top.  Bullseye Thinfire leaves a very smooth finish too. We use this extensively in our studio.

For more information, see the product pages or watch our YouTube video: Help Me Choose: Shelf Primers.

I would like to glue glass to glass. What glue do you recommend that dries clear?

There are several options for glueing glass to glass, the simplest is silicone with aquarium silicone being the clearest. Single part silicone is good for glueing large areas together. Alternatively, professional laminating glues are available from Bohle and you can either use 2 part silicone glue or ultra violet curing glue, both are very strong and very expensive.

What product would you recommend to create moulds in which to cast glass? I would like to use it several times.

castalot for glass casting mouldsThe best product for multiple glass casting is Castalot Casting Plaster.

I have made some place mats using two layers of Bullseye fused glass. How tough and heat resistant will they be?

Bullseye glass is as strong as most other glasses but is not toughened or heat resistant like Pyrex. Assuming that the glass has been annealed properly Bullseye glass can be used for placemats, plates, bowls and coasters without issue.

What can I expect from Bullseye rods?

Bullseye rods are given two different grades, based on their characteristics - T-grade rods, which are formulated for flameworking and are not recommended for kilnforming, and F-grade rods, which can be used for both flameworking and kilnforming. On the whole, F-grade rods behave very similarly to sheet glass, with a few differences which are mainly due to the fact that rods are thicker than the 3mm of sheet glass. View this tipsheet for more information on Bullseye rods.

What is the specific gravity of Bullseye glass?

The specific gravity of Bullseye glass is 2.5.

How big are individual frit granules?

Bullseye frit comes in 5 different sizes:

Powder which has grain sizes 0.2mm and finer;
Fine which has grain sizes from 0.2mm to 1.2mm;
Medium which has grain sizes from 1.2mm to 2.7mm;
Coarse which has grain sizes from 2.7 - 5.2mm;
Extra Large (available in clear only) which has grain sizes 5.2mm - 30mm.

Is Bullseye glass food safe?

Some Bullseye glass colours contain lead and cadmium, and Bullseye recommend capping these colours with clear glass if they are to be used with food. View the list of these glasses here.

What does it mean when glass is described as a 'striker'?

Some glass will appear very pale or even colourless in its unfired state, but when fired it will mature, or 'strike' to its target colour. Striking colours can be affected by temperature, atmosphere and the heat history of the piece, so we would recommend testing the glass before firing to make sure that the optimum target colour is reached.

What is 'Curious' glass?

Bullseye have very specific requirements that their hand-made glass must satisfy in order to be deemed of a good enough quality to be sold as standard stock. Because of the hand-making process, inevitably some glass will not reach these requirements. This glass is classed as 'Curious' and is sold at a discounted price. Curious glass is fusible and all sheets are unique, which can make for very interesting pieces.

Why do I need to clean glass before firing it, and what should I use to clean it?

Glass needs to be cleaned before firing, as this removes anything that may contaminate the surface of your fired piece, such as cutting fluid, oils, dust, sticker residue or fingerprints to name just a few. These contaminants may be visible on the finished piece even if they are not visible before firing, and could cause devitrification, which is the growth of crystals on the surface of the glass. Some people ask whether fluids such as washing up liquid or hand soap, which contain detergents, are appropriate to clean glass with. These cleaning fluids are not suitable for glass cleaning, as they can cause devitrification. Our Professional Glass Cleaner is perfect for this job.

How do I know whether to use 2mm or 3mm glass?

When it is heated in a kiln, glass naturally wants to assume a thickness of 6mm unless it is constrained by dams or a mould. This means that either 2 layers of 3mm glass, or 3 layers of 2mm glass can be stacked and the glass will not distort in the kiln. For more information, see 6mm Rule.

Will all Bullseye glass behave the same in my kiln?

Not all Bullseye glass behaves identically when heated. As different glasses have different viscosities there may be some slight variation depending on what glass you are using.

What can I use fibre paper and fibre rope for?

using fibre for kilncastingFibre paper and fibre rope are very versatile, and different thicknesses lend themselves to different tasks:

1mm fibre paper is useful for lining a box casting or for lining the base of your kiln. Some people use this fibre as a separator paper in place of kiln wash, however does not give as smooth a finish as Thinfire paper. Opal glasses tend to stick to fibre papers more than clear and transparent coloured glasses, for this reason we do not recommend fibre paper is used as a shelf separator unless it is used on combination with another separator such as Thinfire paper.

 3mm Fibre Paper is great as a separator in box casting, lining the box or protecting dams. Just cut some strips and arrange them inside the box. You can also use 3mm fibre paper to add texture to glass in a simple process known as kiln carving. It can also be used to insulate against thermal shock and improve annealing, or you can cut and construct with it for bas relief design.

6mm Fibre Paper is also very versatile. As well as kiln carving, you can cut and construct with it for bas relief design or use it to form low profile dams when making pieces using frit. Use with fibre hardener to create rigid moulds that can withstand more than one firing. 

We don’t recommend firing onto any of these fibres above 804C (for kiln carving) unless you are planning to do some coldworking.

Fibre rope can be used to create lateral holes in pendants. try using two strands of the rope to achieve a hole large enough for a necklace chain. Fibre rope can also be used in kiln carving to create shapes in glass.

To get started with kiln carving, try our Kilncarving Kit.

The fibre we sell is Biosoluble Fibre Paper. This is not the same as Ceramic Fibre (which we have never sold). The fibre paper we sell is made from an Alkaline-earth Silicate Wool, unlike Ceramic Fibre, AES Wool is bio-soluble and if inhaled does not accumulate in the lungs. This is an excellent alternative to ceramic fibre but it can be itchy so we recommend using a FFP3 Dust Mask when using this product, and cleaning up using a vacuum with a HEPA filter.

Can Bullseye Glass be fused and cast with other CoE90 glass?

During manufacture, Bullseye glass is only tested against Bullseye glass so it cannot be guaranteed to be compatible with other CoE90 glass. The only way of ensuring compatibility is to conduct your own tests.

Which cutter is best, pistol grip or pencil grip?

toyo glass cutterIt is simply a matter of personal taste. Some prefer the pencil grip while others like the pistol grip. We use both types regularly here at Warm Glass. For more information, take a look at our YouTube video: Help Me Choose... Glass Cutters.

What is fusing?

Fusing is the technique of joining two or more pieces of glass in a kiln by heating them to very high temperatures. In order to successfully fuse glass together, the glass must be compatible.

What is slumping?

Slumping is a technique that uses gravity to shape glass into the form of a pre-made mould. The glass and mould are placed in the kiln, and when the glass is heated it slowly drops into the mould under its own weight. This is a popular method for creating bowls and platters, but there are many other possibilities of pieces to be made using this technique.

What is tack-fusing?

Tack-fusing is a process in which two or more pieces of glass are fired in a kiln at quite a low temperature, so that they just stick together but retain most of their original features. You can use this technique to retain or create texture in your pieces.

I have been using fibre paper as a mould. I cut shapes then use my 'waste' glass (the very small pieces) in it.  This works well but obviously I can only get one firing per piece of paper.  I have noticed the fibre hardener on your site.  If I was to soak the fibre paper in this product, would I get more than one firing with each 'mould'?

Yes the hardener will make the fibre hard enough to use for repeated firings. However, you will need to use Primo primer or Bullseye shelf primer on the form to allow the glass to cleanly release. You can also use iridised glass against the mould for a clean release.

Bubbles in kiln glass

Bubbles in Glass:

Using or avoiding bubbles

Bubbles in fused glass

Bubbles are a natural occurrence in fused glass and some artists have learned to control bubbles to form geometric patterns and other effects. If bubble appear where you don’t want them it is usually caused by firing fast and trapping air between glass layers.

How do I minimise the look of bubbles in my piece?

small bubbles in glassIf you tend to get lots of little bubble trapped between layers of glass, try sifting Bullseye Clear powder between the glass layers before firing. This actually traps more bubbles, but they are much smaller and do not attract the eye. See the Bullseye Quick Tip: Powder Power for more information.

How do I avoid bubbles when using frit or stringers between fused layers?

frit causing bubbles in glassWhen using frit or stringers it’s always a good idea to run your frit or stringer design right to the edge of the glass, this will allow air to track out of the design instead of getting trapped and causing bubbles.

What is causing HUGE bubbles between the shelf and the fused glass?

huge bubbles in glassSome bubbles are not between layers but come all the way through the glass and sometimes appear as a large hole in the glass. Large bubbles or holes in a piece are usually down to one of three things:

1. The main culprit is fusing thin glass too fast causing an 'apple pie' effect. This is where the glass seals around the edge, trapping air in the middle which then forms a bubble which turns into a hole when it pops. Use the super bubble squeeze firing schedule or use 2 layers of 3mm for the base. Bubbles in designs which use a single 3mm base with frit on top are very common and difficult to resolve without using fibre on the shelf to allow the air to track out.

2. A shelf which has been primed or has not been fired for a long time often holds moisture even though it looks dry, this moisture can turn to steam during the firing which can blow bubbles. If you get these large bubbles in a 6mm thick piece, the solution is to dry your kiln-shelf by firing it to 260C for 20 minutes with the kiln fully vented.

3. A dip in the shelf can cause bubbles, if you put a straight edge across your shelf you should not see any gaps, try this on both sides of the shelf and fire on the flattest side. Ensure that your kiln shelf has no dips in it or just flip it and use the other side.

Why am I getting black bubbles in my glass?

black bubbles in glassBubbles with sooty deposits are usually caused by glue residue. The black is carbon left from the glue burning out. The bubble may not be where the glue originally was as it will track to the thinnest point. It’s best to avoid using glue or use it very sparingly. When glue is used, use it on the edge of the piece as this will allow it to burn away cleanly.

How to I create a grid of small bubbles in my work?

grid of bubbles in glassYou can cross-hatch stringers between layers of glass to create a pattern of small uniform bubbles. Choose a transparent glass colour, and use the same colour of stringer. Lay the stringers in a row along the base glass, and then do the same with the cap glass. Sandwich the stringers together at 90 degrees to each other before firing. This creates an elegant grid of bubbles

stress and cracks in glass

Stress and Cracks:

Tips to avoid stress in your glass

Bubbles in fused glass

Cracks are usually the result of stress in the glass. Stress can be caused by a number of factors, like heating the glass too fast or not annealing correctly. 

Why has my glass cracked during firing?

cracked glass after firingThere are three reasons for glass cracking in the kiln:

Incompatible glass – This will only happen if you use glass from different manufacturers and is identified by the crack running around a piece of glass in the design.
Thermal shock – This is due to heating too fast or cooling too fast. Soft edges in the break indicates heating too fast and sharp edges on the break indicates cooling too fast.
Poor Annealing – Annealing should be calculated according to thickness (about 1hr for 6mm). If your design has different thicknesses the annealing hold should be doubled

For our recommended firing schedules, see our Kiln Schedules page.

My casting has cracked, can I fix it?

Some cracks are fixable and some are not. It depends on the crack, how severe it is and where it is on the piece. If the crack is very large or on the interior of the piece, the only thing you can really do is recast the piece, as no repair will make the crack disappear sufficiently. This is the same for thermal shock, as the edges of the crack will have rounded off, and also if your piece has stuck to the mould making material. In all of these cases, before recasting you should review your casting method and make improvements to stop the piece cracking again. If the crack is not on the interior of the piece and gluing seems appropriate, we would recommend a two-part epoxy glue such as E4005 Structural Epoxy or DP460 Epoxy Glue.

Is glass frost-proof?

All glass will take very low as well as very high temperatures. However, as soon as glass is made into something certain things will influence how frost proof they are, especially when there are rapid changes in temperature. Things influencing the strength of glass objects are:  

  • Design, variations in thickness (like a wine glass) reduces an objects ability to cope with rapid heat change.
  • Poor annealing will leave permanent stress in the glass resulting in weakness which will cause the glass to crack.

If a design is even thicknesses and well annealed then it should be frost proof. To test to failure we put things in a plastic bag, freeze them overnight and then run them in the dishwasher. If they tolerate this they should survive outdoors without a problem.

Why did my piece crack when slumping it in an s-curve mould using the kiln's factory setting slump cycle? Was my piece too thick?

The factory cycle of your plug-in kiln may be a bit harsh if the piece was thick. Also, the piece may have gripped the mould. If the edges are sharp then the glass cracked in cooling and this would indicate either cooling too fast or gripping the mould. If the edges of the cracks are soft, then the glass had stress in it before it was fired, it was heated too fast or it was too close to the elements.  Take a look at our Successful Slumping in a Plug In Kiln tipsheet for more information, and always use the firing schedule appropriate to the specific mould, which if you bought it from us, can be downloaded from the product page for the mould.

grinding marks on glass

Surface Flaws:

Get the finish you want

Bubbles in fused glass

Surface flaws can result from a number of different issues. The most common cause is devitrification. This is a change in the chemical structure of the glass from being amorphous to crystalline, causing the surface to lose the glossy appearance that is characteristic of glass. 

How do I avoid devitrification?

devitrificationDevitrification or devit shows as a scummy white haze on the surface of glass after firing.  It is usually caused by contamination such as finger prints, sticker residue, grinding marks and dust. To avoid devit, clean the glass using glass cleaner and paper towel, protect the glass from dust before firing and use our basic full fuse firing schedules. If you still get devit after following these steps, try firing onto shelf primer rather than thinfire (this is essential if you have been grinding the glass).

If devitrification does occur, you can use clear powder to remove it. Take a look at this tip sheet for more information: How to Fix Surface Flaws.

I used 3mm fibre paper to line my kiln shelf. After a full fuse the fibre left a rough white consistency on the underside of the glass which does not come off even after brushing under running water. What went wrong?

3mm Fibre is not suitable as a separator for fusing as it will stick to glass in certain circumstances. Fibre is used instead of a shelf by some people and when used in slumping will release, it can also be used in casting to form a barrier; generally opal glass will stick to fibre more than transparent glass.  The specific preparations for separating glass from the shelf are Thinfire Paper and Bullseye shelf primer.

Bullseye tip sheet about fibre paper
Bullseye Thinfire paper

How do I prevent my cast piece from having sharp edges? If it does get sharp edges how can I remove them?

Sharp edges occur on cast pieces when the glass scrapes down the sides of a mould during firing. To prevent this make sure that glass is positioned in the mould so that it flows out to meet the edges of the mould, rather than scraping down the sides of the mould and getting caught.

Sharp edges can be removed using various cold-working methods, ranging from using simple hand tools to using dedicated cold-working machines. We offer a range of tools suitable for all kinds of cold-work. See our hand tools here, and our range of cold-working machines here.

How can I prevent suckers?

Suckers are unsightly depressions or wrinkles that can appear when casting. They are caused by differences in temperature during the cooling process, as thinner areas will cool more quickly than the rest of the piece.

To prevent suckers, keep your glass at a uniform temperature during cooling by soaking/holding the glass at 677°C to thoroughly unify the temperature, then cooling gradually to the annealing phase.

How can I prevent band saw cuts from causing devitrification on 3mm thick glass?

devitrificationIf you grind glass prior to firing, it can cause the cut edge to devitrify on firing. This can be reduced by keeping the piece wet when grinding and cleaning the cut with a toothbrush under a running tap, this gets all the loose bits out of the cut. Then fire the piece onto shelf primer rather than thinfire as the smoke from the thinfire can help to kick off devitrification. If you fire at 333C per hour between 677C and 804C this will also help.

When firing to a full fuse, my shelf primer adhered to the glass. I needed to use a diamond hand pad to remove it. What went wrong?

Shelf primer usually only sticks if it has been fired before, it is too thick, or if the primer is very old. Outside of this, primer can stick when the firing is too high. Take a look at our Kiln Schedules page for tried and tested schedules for use with Bullseye glass. If none of these factors are the case, you can only resolve the issue by using Thinfire paper rather than shelf primer.

Inclusions in glass

Inclusions:

Using inclusions and reactions in glass

Bubbles in fused glass

Inclusions are an excellent way of adding a new dimension to your glass work. Metal foils, wire and meshes, mica and Glassline pens and paper can all be used to add interest, as well as enamels, decals and stencils.

How do I apply decals and how do I fire them?

decals on glassDecals are a simple and effective way to add images to your glass work. They work particularly well in jewellery pieces. The firing schedule you use depends on the type of decal you have. As a general rule, decals should be applied to your finished, fired piece, and re-fired at a lower temperature to fix them permanently to the glass. Check the instructions that came with the decal or take a look at the product page for the decal you bought for instructions on how to apply and fire your particular decal.

Can you use the Glassline pens like paints and build up layers or will they crack?

painting with Glassline pensGlassline pens are great to use like paints, you can build up lots of layers into your pieces. They can crack when they are drying out but this cracking will not affect the glass, just the look of your piece. To avoid such cracking apply the paints in thin layers. Paint one layer and let it dry then paint another layer on top. Layering up the paints also gives a good covering of colour, if you use them too thinly they can look a bit washed out after you fire them.  A good way of telling if you have enough paint down is to hold your piece up to the light and if you can see lots of light coming through it you probably need to add a bit more.

Why does silver wire cause a golden glow around it when used as an inclusion?

The golden glow around the silver is silver oxide. This will always be present in silver but if you clean the silver with vinegar before use and try Crystal Clear 1401 as your clear cap you will reduce the possibility of this happening. Naturally, if you want you can get impressive reactions with glass such as French Vanilla and the reactive glasses such as Reactive Ice Clear.

I love the reaction of fusing French Vanilla with silver. Really beautiful. Are there other colours in the Bullseye range which react with silver?

silver reactions in glassThe best silver reactions happen with Red Opal, Reactive Ice and Reactive Cloud, but you can expect interesting things to happen with any glass containing sulphur as well. To find out which glasses contain sulphur just look at the check the Reactive Glass Chart here.

I would like to make my own bails with silver wire and fuse them in between the glass. Which size would be better, 0.7mm or 1.0mm? Will it tarnish in the kiln?

Both the 0.7mm and the 1mm can be used between glass and it really depends on what thickness will look best with your piece. All silver will tarnish a bit and this can be removed with Silvo or vinegar. The silver wire we sell is 999 pure and therefore will tarnish less than sterling silver. Please be aware that silver will react with some glass types, see our How To... Kilnforming Guides section for more information.

Do silver and gold flakes have to be used between layers like mica powder?

Using silver and gold flakes in glassYes, the silver and gold flakes need to be used between layers of glass; they are made of mica and will not stick to the glass otherwise. They are a bit tricky in that they can trap a lot of bubbles. Try using them sparingly to add a little sparkle here and there.

I have Sunshine Enamels and the water based medium. Should the piece be pre-fired before topping? Does it need to be completely dry before firing?

The Enamels we sell mature at 750 - 810C and therefore can be used in a full range of kiln work. There is no need to pre-fire before applying enamels, however the lower you fire the enamels (within the maturing range) the stronger they will come out. Pre firing layers before topping is not essential but will help to reduce potential bubbles.  Enamels do need to dry before firing or they will blister badly. Sifting a very thin layer (2 grains thick) of clear glass powder over the enamels can help achieve a smooth glossy finish and will also reduce bubbles if sifted between layers.

How do I use Glassline paper?

Using glassline paperGlassline paper can be fused between glass layers to add interest to your work. There is a tipsheet on how to use it here and you can look at some images of students' work using the paper on our 'Get Going with Glassline' course in our gallery here.

Please can you give me advice on how to use Millefiori?  On top? In between glass layers? 

orange millefiori sampleMillefiori can be used on top or between layers of a fused piece. It's good to start by making some jewellery pieces and work up as you get used to the way the millefiori works with heat and class thickness. If you are placing it on top it is good to sieve a very thin layer of clear powder over the millifiori to prevent devitrification. If you are placing it between layers use our super bubble squeeze program and ensure that the design will not trap air.

glass studio tips

Tools and Supplies:

Using glass tools and studio equipment

Bubbles in fused glass

Get the most out of your glass studio tools and equipment.

What is the difference between each of the shelf primers you stock?

Shelf primer (or kiln wash) is used to prevent your glass from sticking to the kiln shelf or mould during firing. We sell an number of different types and each have their particular uses:

Bullseye Shelf Primer: Our most popular kiln wash, this is a good general purpose primer. It has a helpful pink tint so you can see exactly where you have applied it (the tint burns out on firing). It also dries quickly between layers and is easy to apply evenly with a soft haik brush. Another benefit is that if you fire under 706C, then you will not need to re-apply the primer for another firing. A new coating should be applied after a full fuse (766C or higher). 

Hi-Fire Shelf Primer: An excellent shelf primer, this primer is specifically designed to perform well at high temperatures for techniques such as raking. It is also good for bead release and seems to work well at lower firing temperatures as well, making it a good all-round primer. It also has a pink tint to help show where it has been applied, and goes on nicely in even layers. It cleans from the shelf and glass easily after use.

Primo Primer: We recommend this primer for use with casting moulds, such as the Colour de Verre dragonfly mould or nano bead mould.  Although slightly more difficult to mix and apply (Tip: leave it for an hour after adding the water before mixing again and applying), it is truly excellent at releasing work from these complex moulds, leaving great detail and a smooth finish. It also needs minimal clean up afterwards.  

Boron Nitride Spray: It is expensive, but a little goes a long way and it is ideal for stainless steel moulds such as the floral former because, unlike other primers, you do not have to heat the mould for the primer to adhere. It also gives a fantastically smooth finish to your glass, minimising cold work. Not recommended for firing temperatures above product.

Bullseye Thinfire Paper: This product removes the need to prepare and apply a primer. A more expensive option than primers because the paper only lasts for one firing, but preparation times are almost zero. Just put the paper on the kiln shelf and place your work on top.  Bullseye Thinfire leaves a very smooth finish too. We use this extensively in our studio.

For more information, see the product pages or watch our YouTube video: Help Me Choose: Shelf Primers.

How can I learn to cut glass?

If you've never cut glass before, it's a good idea to take a class to teach you the basics of cutting glass, as well as the best tools and techniques to use. At Warm Glass UK we offer a range of classes suitable for beginners to advanced glass artists, so there's something for everyone. See our current class list here.

What is the Rule of Halves?

The Rule of Halves refers to cutting glass. A cut is much more likely to run smoothly if there are equal amounts of glass on each side of the score, hence the Rule of 'Halves'.

How do I coldwork a piece that I've cast?

The methods used to coldwork a kilncast piece can vary depending on exactly what needs doing to the piece. If a reservoir needs removing, this can be done using a tile saw with a diamond blade, by using rotary tools, or by hand lapping with loose grit. Diamond hand tools can be used to remove small imperfections on the surface of your casting, such as flashing - areas where glass has flowed into small cracks in the mould.
Please note: A casting should be at room temperature for 24 hours before any cold work is carried out. This is because even if a piece feels cold on the outside, it could be considerably warmer in the centre, so can still be subject to thermal shock.

Can I cast glass in a slumping mould?

Yes. Just remember to reapply primer after each firing.

Can I cast glass and clay together?

Glass and ceramic clay are not compatible. It may be possible to formulate a clay that is compatible with Bullseye glass, but as of yet this has not been achieved. Some ceramicists will incorporate glass experimentally in their pieces to create interesting effects.

Can I firepolish my casting?

Firepolishing happens at slumping temperatures, so flat pieces can be firepolished, but more upright pieces would be affected by the heat so are not suitable to be firepolished.

Can I make a casting mould that I can use again?

This is possible, as long as you make the correct investment mix. Castalot is an example of a great mix for creating reusable moulds.

I would like to glue glass to glass. What glue do you recommend that dries clear?

There are several options for glueing glass to glass, the simplest is silicone with aquarium silicone being the clearest. Single part silicone is good for glueing large areas together. Alternatively, professional laminating glues are available from Bohle and you can either use 2 part silicone glue or ultra violet curing glue, both are very strong and very expensive.

What product would you recommend to create moulds in which to cast glass? I would like to use it several times.

castalot for glass casting mouldsThe best product for multiple glass casting is Castalot Casting Plaster.

I have made some place mats using two layers of Bullseye fused glass. How tough and heat resistant will they be?

Bullseye glass is as strong as most other glasses but is not toughened or heat resistant like Pyrex. Assuming that the glass has been annealed properly Bullseye glass can be used for placemats, plates, bowls and coasters without issue.

What can I use fibre paper and fibre rope for?

using fibre for kilncastingFibre paper and fibre rope are very versatile, and different thicknesses lend themselves to different tasks:

1mm fibre paper is useful for lining a box casting or for lining the base of your kiln. Some people use this fibre as a separator paper in place of kiln wash, however does not give as smooth a finish as Thinfire paper. Opal glasses tend to stick to fibre papers more than clear and transparent coloured glasses, for this reason we do not recommend fibre paper is used as a shelf separator unless it is used on combination with another separator such as Thinfire paper.

 3mm Fibre Paper is great as a separator in box casting, lining the box or protecting dams. Just cut some strips and arrange them inside the box. You can also use 3mm fibre paper to add texture to glass in a simple process known as kiln carving. It can also be used to insulate against thermal shock and improve annealing, or you can cut and construct with it for bas relief design.

6mm Fibre Paper is also very versatile. As well as kiln carving, you can cut and construct with it for bas relief design or use it to form low profile dams when making pieces using frit. Use with fibre hardener to create rigid moulds that can withstand more than one firing. 

We don’t recommend firing onto any of these fibres above 804C (for kiln carving) unless you are planning to do some coldworking.

Fibre rope can be used to create lateral holes in pendants. try using two strands of the rope to achieve a hole large enough for a necklace chain. Fibre rope can also be used in kiln carving to create shapes in glass.

To get started with kiln carving, try our Kilncarving Kit.

The fibre we sell is Biosoluble Fibre Paper. This is not the same as Ceramic Fibre (which we have never sold). The fibre paper we sell is made from an Alkaline-earth Silicate Wool, unlike Ceramic Fibre, AES Wool is bio-soluble and if inhaled does not accumulate in the lungs. This is an excellent alternative to ceramic fibre but it can be itchy so we recommend using a FFP3 Dust Mask when using this product, and cleaning up using a vacuum with a HEPA filter.

Which cutter is best, pistol grip or pencil grip?

toyo glass cutterIt is simply a matter of personal taste. Some prefer the pencil grip while others like the pistol grip. We use both types regularly here at Warm Glass. For more information, take a look at our YouTube video: Help Me Choose... Glass Cutters.

I have been using fibre paper as a mould. I cut shapes then use my 'waste' glass (the very small pieces) in it.  This works well but obviously I can only get one firing per piece of paper.  I have noticed the fibre hardener on your site.  If I was to soak the fibre paper in this product, would I get more than one firing with each 'mould'?

Yes the hardener will make the fibre hard enough to use for repeated firings. However, you will need to use Primo primer or Bullseye shelf primer on the form to allow the glass to cleanly release. You can also use iridised glass against the mould for a clean release.

When I use the Candle Bridge mould, the glass pulls in at the edges on slumping. How do I prevent this?

To prevent the glass pulling in at the edges when it slumps into the top of the mould, make sure your glass is slightly wider (no more than 5mm) than the edge of the mould. This will stop the glass pulling in at the sides.

glass firing tips

Firing Tips:

Firing your glass correctly

Bubbles in fused glass

Having problems with your firing? Here we answer the most commonly asked questions about firing glass in a kiln.

What is the best temperature to fire Bullseye glass to?

We generally fire a full fuse to 804C - full details of firing schedules are accessed from Kiln Schedules page in the Knowledge Base. Firing schedules for using Bullseye glass in our casting and slumping moulds can be found on the product page for each individual mould.

Will my glass change colour when I use it for casting?

Glass can change colour when it is used for casting, but it depends on the conditions inside the kiln and the shape of the object that you are casting.
Colours with Bullseye codes starting with 11-- or 14-- may appear black if used in certain thicknesses, as they are already extremely colour-saturated.
Transparent glasses that rely on cadmium, selenium or sulphur to create their colours are more sensitive to heat, and may turn opalescent if exposed to excessive heatwork.
The type of glass used also affects colour - a piece made with billet will have much better colour clarity compared with a piece made from frit, as when using a billet there is no space for air bubbles to form between glass particles. See Tipsheet 8: Basic Lost Wax Kilncasting for more information on selecting appropriate glasses for casting.

How large can my casting be?

A casting can be as large as the interior of your kiln will allow, but as a general rule of thumb the interior of the kiln should be twice the height of the final casting. Something important to bear in mind, especially for large castings, is that the piece must be heated and cooled uniformly to minimise the risk of suckers or thermal shock.

What firing schedule should I use for casting?

Firing schedules for castings vary greatly depending on the mould type you're using, the process you're using and the glass you're using. Bullseye TipSheets are very helpful for casting advice. Find them in the 'Glass Techniques' section on our How To... Kilnforming Guide page.

What is the best temperature for firepolishing?

Firepolishing can happen anywhere from 593C to 804C, but the right temperature for different pieces depends on things like glass colour, rate of heating, and how much the surface has been coldworked. Firepolishing can also be achieved during slumping.

What temperature should I use to slump glass?

There is no specific slumping temperature, as each glass project varies so much and has so many contributing factors. Generally, slumping temperatures are quite low and are held for a longer length of time than other processes in order to allow the glass to slowly sink into the mould. Bullseye have created many 'TipSheets' which offer very comprehensive advice for slumping all kinds of shapes - find the one you're looking for here.

How do I know what temperature I should use for different kilnforming techniques?

There are no specific temperatures for any kilnforming techniques, as different pieces will have completely different properties, and will therefore require completely different firing schedules. Please follow the links below to access Bullseye's 'TipSheets' for each topic which will provide basic information about each different technique:

Slumping
TechNote 4: Heat & Glass
TipSheet 7: Platemaking Tips

Casting
TipSheet 5: Box Casting TipSheet 8: Basic Lost Wax Kilncasting
 

TechNote 4: Heat & Glass provides the basics of fusing, tack-fusing and slumping glass.

 

What is the best temperature to fire Bullseye glass to?

We generally fire a full fuse to 804C - full details of firing schedules are accessed from Kiln Schedules page in the Knowledge Base. Firing schedules for using Bullseye glass in our casting and slumping moulds can be found on the product page for each individual mould.

What is the best temperature to fire Bullseye glass to?

We generally fire a full fuse to 804C - full details of firing schedules are accessed from Kiln Schedules page in the Knowledge Base. Firing schedules for using Bullseye glass in our casting and slumping moulds can be found on the product page for each individual mould.

What is the best temperature to fire Bullseye glass to?

We generally fire a full fuse to 804C - full details of firing schedules are accessed from Kiln Schedules page in the Knowledge Base. Firing schedules for using Bullseye glass in our casting and slumping moulds can be found on the product page for each individual mould.

When using my pendant pod mould, each pendant has come out with spikes of glass round the edges. How can I prevent this?

glass pendant mould spikes after firingJewellery and pendant moulds can make great pieces even using scrap glass. The main trouble people have is the glass spiking on the edges of the mould. The spikes are caused by the glass getting caught on the side of the mould as it melts. This can be solved either by using fine frit as the filler or piling the glass up like a pyramid in the centre so that it melts from the centre outwards.

Can you give me any tips on using a drop-out mould? Do I need to flash vent the kiln?

You do need to cool the glass rapidly to stop it dropping, this is done when you can see the glass has dropped to the point you were hoping.

Our Top Tips when using drop outs:

The blank should cover the outside diameter of the ring but not hang over the edge.

For drops under 10cm the optimum thickness of the blank is 6mm to 9mm.

For drops over 10cm the optimum thickness of the blank is 9mm to 12mm .

Fire the blank on a basic full fuse and anneal correctly for the thickness of the glass.

Use Bullseye separator on the dropout ring and also 1mm fibre, cut 3mm back from the ring edge.

The optimum top temperature is 650C to 700C depending on your kiln, glass thickness and how much you want to drop.

In plugin kilns remove the shelf for longer drops but remember to protect the base of the kiln with fibre paper.

glass kiln advice

Glass Kiln Advice:

Help on choosing and using kilns

Bubbles in fused glass

Top tips about choosing and using a glass kiln.

What should I consider when buying a kiln for glass fusing?

We stock many different kilns suitable for glass fusing.  When choosing a kiln, consider the following:

Size:  If you are primarily focussed on jewellery making, or smaller pieces, then there’s no point in spending extra on a bigger kiln that will take up more room and cost more to fire. For jewellery makers, a small kiln like the Paragon SC2 would suffice unless planning to fire large numbers of pieces at once, and it also comes with an optional bead door. If you want to make large plates or bowls or bigger decorative pieces, then you need a kiln big enough to fit the work into.  Also consider the volume of work you wish to fire. If you want to fire more than a few pieces each day then you will need a chamber big enough to take the work. Remember that pieces will be in the kiln for the best part of a day. Lastly, consider the logistics. Will it fit where you want it to go? Can you get it through the door? If you are looking at a large 3 phase kiln, you will need to make sure you can get it wired into your power supply.

Controller:  Look for a controller that has pre-set firing schedules to get things started, but with the option of adding your own schedules.

Heating elements:  Glass requires even heat across its surface to fire correctly. Particularly with larger kilns, look for one with elements in the top, allowing for the heat to distribute evenly over the surface of the glass. If the kiln is deep to allow for three dimensional work, then it should ideally also have side elements to help the heat penetrate lower into the chamber.

Build quality:  We recommended only buying a kiln built by a quality manufacturer with an established history such as Kilncare, Skutt, Naberthem or Paragon (that’s why we stock them).  Prices of kilns vary, but generally the more you pay, the higher the build quality, the bigger the chamber and the more ‘extras’ you get like quartz elements or lid opening mechanisms. 

After sales service:  If you buy a kiln from us then we are on hand to talk you through getting started with your kiln, programming it and looking after it, for as long as you need it and at no extra cost. Everyone who buys a kiln from us also gets a 5% or 10% discount on products they buy from us in the future.

For more information, take a look at our YouTube video: Help Me Choose... Glass Kilns.

Can I use a ceramics kiln to fire glass?

 A ceramic kiln can be programmed to fire glass, especially if you are doing glass casting but we would not generally recommend it. You may get issues like devitrification if you are fusing or slumping glass because ceramic kilns heat from the sides rather than the top, making the important even heating of glass difficult.  Also, ceramic work leaves pollutants in the kiln which can taint glass work causing problems such as devitrification or discolouration. If your only choice is to use a kiln which is also used for ceramics work, then we recommended using a separate ‘glass only’ kiln shelf to try to reduce this problem.

I have seen that the Paragon SC2 kiln can come with a bead door. What is the bead door for?

paragon sc2 glass kiln with bead doorThe bead door on the Paragon SC2 is so that after making beads using a flame you can anneal the beads in the kiln without opening the door every time. It is perfectly possible to anneal beads in a kiln without a bead door, but the specially designed door does make it easier to keep the heat in the kiln when you open it.

Why don't you stock microwave kilns?

We don't stock microwave kilns because we don't generally recommend using them to fuse glass. Glass needs to be heated and cooled evenly and in a controlled manner or it introduces stresses into the piece. In our experience,  microwave kilns heat glass the same way ordinary microwaves heat food: unevenly, which could make your work weak and more liable to break during or after firing.

When should I remove my kiln bung? With every firing?

The bung is used to cover the vent hole in the kiln and the kiln can fire with or without the bung in place. Some firing schedules will recommend venting the kiln and if you want to vent the kiln you remove the bung, if you do not want to vent the kiln then the bung can be left in the hole.

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