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Technical Support

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.

  • Problem with Black Bubbles 

    bubbles in glass with black depositsQ:   My fused tiles have bubbles in the after firing. The bubbles are surrounded by a black spidery inclusion.  They were fired on thinfire paper.  I have noticed bubbles in my glass work from time to time without the black surround, so I assume this is something different from run of the mill bubbles?

    A:  Bubbles 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 and when it is used; use it on the edge of the piece as this will allow it to burn away cleanly.


  • Successful Slumping

    Q:  I slumped my piece after a full fuse in an S curve mould, sadly the piece cracked although the kiln was on factory settings on slump cycle. Why? Was my piece too thick?

    A:  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.


  • Using Glassline Pens 

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

    A:  Glassline 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.


  • Discolouration using silver wire

    Q:  Why does silver wire cause a golden glow around it when used as an inclusion? I've tried it with and without glue to fix and both ways it still leaves a pronounced yellow hue in the surrounding area. 

    A:  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.


  • Using Enamels with Glass

    Q:  I have Sunshine Enamels and the water based medium. I am unsure if the piece should be pre-fired before topping. Also does it need to be completely dry before firing?

    A:  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.


  • Copper Inclusions 

    copper inclusions in glassQ:  Any suggestions how to keep copper bright when fired between Bullseye sheets?

    A:  Copper doesn’t really ever stay copper coloured, it will turn reddish when fired between layers, and if the copper is exposed to more air during the firing it will turn a darker blueish colour; so to make sure the copper stays as bright as possible use it towards the centre of your piece. There will always be some colour change though.


  • Broken Mould

    Q:  My earring pod mould broke on firing. I used Primo primer (4 coats) and put the mould on a kiln shelf, the kiln was cold to start off. I set the kiln to 870C full speed for 10 minutes, switched off kiln, opened door leave to cool to 550C, then shut door and left it until it reached room temperature. The break had occurred in the first ten minutes. Why?

    A:  The speed of the firing has caused the pod mould to crack through thermal shock. The 'full speed' setting is really only appropriate for small pieces of jewellery and whilst the glass could have taken the heat, the mould could not. You can download an appropriate firing schedule from the product page for any mould we stock.


  • Screen printing on glass

    Q:  Can you help with screen printing on glass? I am using Sunshine enamels and the oil based mixing medium. Despite mixing different consistencies the mix becomes almost solid on the screen and requires strong chemicals to remove it. Using it too frequently would I think end with damage to the screen. Should I be using the water based medium instead?

    A:  We use the water based medium in the studio for exactly the reason you suggest. Whilst the consistency of the oil based is slightly nicer to work with, the clean up is very time consuming and strong chemicals are required. The water based medium just washes off with water and a bit of detergent. I would use oil based medium for painting and water based medium for printing.


  • Reactions with Silver 

    Bullseye red opal glass reacting with silverQ:  I love the reaction of fusing French Vanilla with silver. Really beautiful. Are there other colours in the Bullseye range which react with silver?

    A:  The 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.


  • Glueing glass to glass

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

    A:  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.


  • Strength of Bullseye glass

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

    A:  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.


  • Bubble Patterns 

    uniform bubbles with glass stringerQ:  How to I create a grid of small bubbles in my work?

    A:  You 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.


  • Using silver wire for bails

    Q:  I would like to make my own bails and fuse them in between the glass.  I have looked at the silver wire and there are two sizes, 0.7mm and 1.00mm. Can you let me know which would be better and whether it will tarnish in the kiln?

    A:  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.


  • Choosing glass cutters

    Q:  Which cutter is best, pistol grip or pencil grip?

    A: it 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.


  • Firing Temperature of Bullseye Glass

    Q:  What is the best temperature to fire Bullseye glass to?

    A: We generally fire a full fuse to 804C - full details of firing schedules are accessed from the Knowledge Base.


  • Using Glassline Paper 

    using Glassline paperQ:  How do I use Glassline paper?

    A:  Glassline 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.


  • Choosing a Glass Kiln

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

    A: 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.


  • Firing Glass in a Ceramics Kiln

    Q:  Can I use a ceramics kiln to fuse glass?

    A:  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.


  • Huge Bubbles 

    huge bubbles in glassQ:  My piece had a really huge bubble in it after firing. It has ruined the piece. What went wrong?

    A: Some 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.

    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. An 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.


  • Microwave Kilns

    Q:  Why don't you stock microwave kilns?

    A:  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.


  • Spikes When Using Pod Moulds 

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

    A:  Jewellery 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.


  • Shelf Primers

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

    A:  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


  • Silver and Gold Flakes 

    Gold flakes for glassQ:  Do silver and gold flakes have to be used between layers like mica powder?

    A:  Yes, 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.


  • Mould Making

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

    A:  The best product for multiple glass casting is Castalot Casting Plaster.


  • Using drop out moulds

    Q:  Do I really need to flash vent when using a drop out ring? All the research I've done talks about flash venting if you do a deep drop. Do you have an example of a firing schedule? Otherwise I was going to start with the one on the website linked to the mould.

    A:  The dropout schedule on the site will work, but to be honest a simple 167C to 650-700C with a long hold will work just as well.  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.


  • Preventing devit from grinding

    grinding marks on glassQ:  How can I prevent band saw cuts from causing devitrification on 3mm thick glass? I tried Spray but it still leaves a line? 

    A:  If 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 helps to kick off devitrification. If you fire at 333C per hour between 677C and 804C this will also help.


  • Kiln Bead Door

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

    A:  The 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.


  • Cracked Glass 

    cracked glassQ:   Why has my glass cracked during firing?

    A: There 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 means heating too fast and sharp edges on the break means 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.


  • Using fibre paper

    Q:  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?

    A:  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 tipsheet about fibre paper
    Bullseye thinfire paper


  • Frit and Bubbles 

    trapped bubbles using glass fritQ:   When I use frit in between layers of glass I get a problem with trapped bubbles. How can I avoid this?

    A:  When using frit it’s always a good idea to run your frit 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.


  • Fibre hardener

    Q:  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'?

    A:  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.


  • Using a Kiln Bung

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

    A:  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.


  • Avoiding Devitrification 

    devitrification on glassQ:   How do I avoid devitrification?

    A: Devitrification or devit is 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 Tipsheet for more information: How to Fix Surface Flaws.


  • Keeping Glass in Shape 

    Q:   When I fire a piece to a full fuse, sometimes it draws it, causing the piece to be mis-shapen. Other times it spreads out. How can I get my piece to keep its shape?

    A: This is a very common problem and it is caused by the thickness of your design. Glass likes to be 6mm thick. If your design is less than 6mm then the glass will draw in when molten, causing the familiar 'dogbone' effect.  If your piece is more than 6mm then the glass will flow out on firing, unless you use dams or a mould to contain it. Try basing your design around two sheets of 3mm glass to avoid this issue. This property can be used to good effect when making Frit Balls, where the frit draws into a perfect sphere in its attempt to become 6mm thick.


  • Firing Schedules for Moulds 

    Q:   When I fired my piece in a slumping mould, it pulled in at the edges and became distorted. What went wrong?

    A: It is important to use the correct firing schedule for the mould in question to avoid problems like distorted edges, bubbles or the glass sliding to the base of the mould. Moulds vary greatly and there is no 'one size fits all' schedule. We publish a suitable firing schedule on the product page of each of our moulds (for use with Bullseye Glass). Don't forget to prime your mould with a shelf primer to avoid the glass sticking to the mould. We recommend Bullseye Shelf Primer for simple moulds, and Primo Primer for intricate moulds such as casting moulds. See our video on choosing primers for more information.


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