A successful glass fusing project is decided well before the kiln reaches temperature. The glass must be compatible, the layers clean and thoughtfully arranged, and the firing schedule matched to the thickness and effect you want. A beautiful sheet of glass can become a cracked, cloudy or distorted piece if one of those basics is overlooked.
For makers moving from stained glass or mosaics into kiln work, fusing can seem deceptively simple: cut glass, stack it, fire it. The process is approachable, but heat changes more than the shape of the glass. It affects colour, surface texture, bubbles, edges and internal stress. Understanding those changes gives you far more control at the kiln.
What glass fusing actually does
Glass fusing joins compatible pieces of glass through controlled kiln firing. As the temperature rises, glass softens and begins to bond where pieces touch. How far it flows depends on the top temperature, the time held at that temperature, the thickness of the glass and the kiln’s characteristics.
A tack fuse leaves individual pieces recognisable, with raised texture and defined edges. It suits work where you want the assembled pattern to remain tactile. A full fuse takes the glass further, generally creating a smoother, more level surface as layers flow together. Neither is automatically better. A detailed floral panel may benefit from a tack fuse, while a coaster blank or jewellery component often needs the cleaner finish of a full fuse.
Slumping is a related process rather than fusing itself. Once a fused blank has been properly annealed, it can be reheated over or into a mould to create a bowl, platter or curved form. Each firing places fresh demands on the glass, so good record keeping becomes valuable very quickly.
Choose compatible glass before colour
The first rule of kiln-formed work is to use glass designed to fuse together. Compatibility is commonly identified by coefficient of expansion, or COE, but COE is not a complete guarantee. It describes how much a glass expands and contracts with temperature change, yet glasses with the same stated COE may still behave differently because of their formula, colourant or production method.
For dependable results, use products from the same tested fusing range and manufacturer. Bullseye 90 glass, for example, is formulated and tested as a compatible system when used within its range. Oceanside 96 compatible glass is another system, with its own sheet glass, frits, powders, stringers and accessory materials. Keep each system separate unless you have tested a specific combination.
This matters especially when using offcuts. A scrap box is useful only when every piece is clearly identified. Cathedral glass, window glass, stained glass offcuts and decorative glass intended for another process may look tempting, but are not reliable choices for fusing with tested art glass. Even if a piece survives one firing, it can retain stress that causes cracking days or weeks later.
A quick compatibility test
When you are unsure about two glasses, make a small test tile rather than risk a larger project. Place strips of one glass over a base of the other, fire them to a full fuse, then inspect the result after cooling. A stressed combination may show a crack, a sharp break when pressure is applied, or a visible line at the join. Polarised film can reveal stress more clearly, but a test firing is still only evidence for that particular combination and schedule.
For production work or a valued commission, tested-system glass remains the sensible choice.
Build clean, deliberate layers
Clean glass is easier to fuse well. Fingerprints, cutting oil, dust and fragments of kiln wash can create marks that are difficult or impossible to remove after firing. Wash and dry the glass before assembly, then handle it by the edges where practical. A clean cutting surface also prevents tiny chips becoming trapped between layers.
Consider the combined thickness as you build. Many full-fuse projects are designed around approximately 6 mm, often two layers of standard sheet glass. At this thickness, the glass is more likely to spread in a predictable way. A thinner stack may pull inward and develop a rounded, irregular edge. A thicker stack may spread outward, particularly during a long full-fuse hold.
That does not mean every piece must be 6 mm. Deliberately thin work can produce elegant organic edges, and thicker inclusions can create relief and visual weight. The trade-off is that you need to allow for movement. Leave space around a design, use dams where appropriate, and avoid assuming that a carefully cut outline will look identical after firing.
Opaque, transparent and opalescent glasses also respond differently to light. Transparent colours can deepen dramatically when layered, particularly over dark bases. Opalescent glass can conceal the lines beneath it and give a more graphic result. Before committing to a large panel, fire a small sample with the same layer order and background colour.
Heatwork matters more than one temperature
Makers often ask for the correct temperature for a full fuse. The useful answer is that firing is about heatwork: the combination of temperature and time. A brief hold at a higher temperature can produce a similar effect to a longer hold at a lower temperature, but not always with identical surface quality, bubble behaviour or colour response.
Your kiln also matters. The thermocouple position, element placement, shelf load and how closely the lid fits all influence actual heatwork. A schedule that produces a smooth full fuse in one kiln may leave texture in another. Start with the glass manufacturer’s recommended schedules, then make measured adjustments based on your own kiln records.
A typical firing includes a controlled ramp-up, one or more holds, a controlled cool through the annealing range, and a final descent to room temperature. The slow cooling portion is not optional. It allows the glass to equalise internally and release stress. Opening the kiln too early can undo hours of careful work, especially with thicker pieces.
Avoid common firing faults
Bubbles usually form when air is trapped between layers, often in enclosed spaces or beneath large pieces fired too quickly. A slower early ramp and a lower-temperature bubble squeeze hold can help air escape before the glass seals. Clean surfaces and flatter construction also reduce the problem.
Devitrification appears as a dull, scummy or crystalline surface. It can be caused by contamination, excessive heatwork, repeated firings or certain reactive glasses. Start with clean glass, avoid unnecessarily long hot holds and consider whether a particular effect really needs another firing. Some artists use devitrification sprays, but prevention is generally preferable.
Cracks after firing commonly point to incompatibility, poor annealing or a piece that is thicker than the schedule allowed for. Check whether the crack follows a join between two colours, which can suggest compatibility issues. If it runs through the body of a compatible piece, review the anneal soak and cooling rate. Thicker work needs more time for the centre to catch up with the surface.
Kiln furniture is part of the finish
The kiln shelf is not merely a support. It affects the underside of your work and must be prepared properly. Kiln paper offers a smooth, disposable separator for many flat fusing projects. Shelf primer or kiln wash is often used directly on shelves, while moulds need their own suitable release product and preparation method.
Choose the release material for the firing temperature and the surface you need. Kiln paper can leave a subtle texture underneath glass, whereas a well-prepared smooth shelf may give a cleaner finish. Do not reuse contaminated kiln paper or fire directly onto an unprotected shelf. Molten glass bonded to kiln furniture is an expensive lesson.
When slumping, make sure the fused blank is slightly smaller than the mould’s usable area unless the design calls for an overhang. Glass can bridge across a mould rather than settle into it if the firing is too cool or too short. Too much heat, on the other hand, can soften details and leave the glass overly thin at the rim.
Make your first projects repeatable
The best early projects are small, flat and made from a single compatible glass system: sample tiles, pendants, coaster blanks or simple suncatchers. They teach cutting, layout, bubble control and annealing without using a large amount of glass. Label each firing with the glass range, thickness, schedule, shelf preparation and outcome.
Keep notes on effects as well as temperatures. Record whether a particular transparent colour darkened over white, whether a stringer stayed raised, or whether a frit blend spread further than expected. These observations become your own practical reference, built around your kiln rather than somebody else’s.
Quality materials make that learning process clearer. Art Glass Imports specialises in high-quality art glass and kiln-forming supplies, with practical advice for makers choosing compatible sheet glass, frit, stringers, kiln paper and related materials.
Glass fusing rewards patience because the kiln always tells the truth. Start with compatible glass, test ideas on a small scale, and let each firing teach you what the next piece needs.