Kiln-Formed Glass 101: Fusing, Slumping, and Kiln Forming Explained July 14, 2026 17:21

If you've spent any time browsing the Red Hot Glass Instagram or wandering the website, you've probably come across terms like fusing, slumping and kiln forming without much explanation of what they actually mean. I get asked about this a lot, both by people new to glass art and by customers wanting to understand what they're buying. So here's the plain-English version – no jargon left unexplained!

Both fusing and slumping techniques fall under the umbrella of Kiln-Formed glass, sometimes called Warm Glass. Unlike glassblowing, where the glass is worked hot and heated by the furnace, kiln-forming happens inside a kiln, heated slowly and predictably to specific temperatures, then cooled just as carefully. It's a quieter, more considered process - less about ‘drama’ and split-second reflexes of the blowing ‘hot’ shop and more about planning, patience and understanding your material.

Fusing

Fusing is the process of joining pieces of glass together using heat, and can be done without a mould. You place sheet glass, frit or other glass elements in your design, on a kiln shelf, then heat the kiln until the glass softens enough to bond the elements together, into a single piece.

How far you take the heat changes the result. A gentle warm can ‘tack’ pieces together while keeping texture and separate layers visible. Push further and the glass fully fuses into one smooth, flat sheet. This is often where COE comes in – CEO is the coefficient of expansion, a measure of how much a glass type expands and contracts with heat (expansion is one of many changes to glass under heat). 

Compatible glass (when the COE of each piece is the same) needs to be used together, or the finished piece can crack, sometimes weeks or months after it comes out of the kiln, as the mismatched glass creates stress which keeps pushing against itself even after it comes out of the kiln.

Fusing is the foundation technique a lot of ‘warm’ or kilnforming glass artists start with, and it's also where a lot of foundation design for the piece starts for example: the colour and pattern work of layering frit, powder and sheet glass to build an image before putting in the kiln for the heat work.

Slumping

Slumping usually happens, after fusing, in its own dedicated run of the kiln (or ‘firing’ as we call it). Slumping is a generalised term to cover the process of heating glass just until it softens enough to bend.  This delicate process is used to form the glass ‘over’ or ‘into’ a mould, to form that mould's shape. Think of a flat fused piece being ‘slumped’ into a shallow bowl, or ‘draped’ over a form to create a gentle curve. 

Slumping uses lower temperatures than fusing, because you're not trying to melt glass together, just soften it enough to move under its own weight (or with a very gentle assist from gravity and time). This is where mould preparation really matters - a well-prepared mould, properly kiln-washed or lined, is the difference between a clean release and a piece that's stuck fast to the mould or picked up texture you didn't want. 

It's also where materials like SilkeMat® Lite really earn their keep, as it can be used to create fine mould that hold detail.

Kiln-wash, that’s a specially formulated wash used to paint (or spray) onto a mould to prevent sticking.  The difference between Silkemat and kiln wash is that with Silkemat you create a permanent mould, where kiln wash is used to as a surface barrier which burns out on firing.  Both are necessary in a busy studio! 

Kiln Forming

Kiln forming covers a vast range of techniques used in the kiln – of which fusing and slumping are just two.  Most of the techniques take the basics of fusing and slumping and push them in to far more technically challenging spaces.  One of the artists using kiln forming techniques to produce astonishing work is Kirsty Rae – I recommend you take a peak at the skill and control to produce her work!  I admire her a lot!

Casting

Casting is my personal favourite for how much shape and texture it offers. Rather than working with sheets of glass, you're working with glass in large rectangles (called bullets) or smaller broken pieces called frit – frit comes in graded sizes, all the way down to powder and is used in both fusing and casting.  The larger sizes used for casting while the smaller sizes in fusing.  Casting is a huge other process which I will go into in another blog post soon!

Fusing and Slumping, Kiln Forming

Understanding the difference isn't just useful vocabulary. It changes how you plan a piece, what temperatures and schedules you're working towards, and what can go wrong at each stage.

If you're just starting out, my advice is: pick one type of CEO - get comfortable with it (and your kiln).  It won’t be long before you’re diving into other techniques (if you want – you don’t have to!). Fusing and slumping are the natural starting points for a lot of people. And before long you’ve fallen in love with glass too!

Made by hand. Made to keep.

This piece is slumped over a piece of SilkeMat then when the SilkeMat is removed, fired again to tack fuse it to the white square of glass underneath.  This tack fuse was made more complicated as I didn't want the heart shaped to soften.