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Glass Firing

Glass Separator or Kiln Wash and Fiber Paper

      In this section, we will explain the difference between using kiln wash or fiber paper. It is important to understand kiln wash for glass is very different than kiln wash you put on your regular big kiln shelves for ceramics. People in the glass world may call it kiln wash, but those doing ceramics and glass would do well to differentiate them. Kiln wash is for coating your kiln shelves for ceramics. Glass separator, either in powder or pre-mixed form, is for glass kiln shelves and glass molds. For the purpose of this article, we will use them interchangeably, but remember, we always mean glass separator unless specifically designated otherwise.

     Glass will stick to most surfaces when it gets hot and to keep it from melting onto kiln shelves and molds, we treat these surfaces with glass separator. Kiln shelves can be prepared for glass by either applying the separator to the top of the shelf or by cutting a piece of specially made paper, called thin fire fiber paper, and placing it on the shelf and putting the glass ON the paper. Even most molds that are used for either frit casting, sagging, draping or slumping, must be coated before they have glass put in or on them and fired in the kiln.

     When you buy a glass kiln, the furniture kit usually comes with a bag of powder glass separator. It is mixed with water to form a thin creamy liquid, or it can also be bought pre-mixed. The glass separator is applied to the shelf with a special brush called a haike brush although most people use a simple decent paint brush. Just don’t use one of your good painting brushes and don’t use a brush that has ever been used for something else. If there are any particles left behind in the ferule of the brush and gets mixed in with the separator, you could potentially have some kind of toxic emission. Three or four coats of glass separator are usually sufficient when each coat is brushed on in a different direction than the previous one. When it dries, it keeps the glass from sticking directly to the kiln shelf. When using frit or dam molds and especially the Colour De Verre Frit Molds, we recommend Hotline Primo Primer - 4 thin coats. Some people prefer Boron Nitride spray instead of brushed on glass separator.

     You don’t have to reapply glass separator on your kiln shelves every single time, only when the wash begin to comes off either from sticking to a piece of glass or just wear and tear. Reapply Primo Primer or Boron Nitride, a special glass separator, every time you fire a Dam Mold or Little Fritters or a Colour De Verre frit mold to a full fuse because the mold, glass and separator are being exposed to high temps for a long period of time and the glass is becoming a sticky fluid.  All glass molds must be treated before being used. Primo Primer and Boron Nitride are NOT for the kiln shelves.

Removing Glass Separator

     If you have jut brushed on your glass separator and it is not dry and you put glass on it and go ahead and fire, don’t be surprised if your piece comes out of the kiln with wash on it. And you may have a very hard time getting it off of the piece. To prevent this from happening, apply fresh wash often and temper the shelf with the fresh wash by firing your kiln up to 300 degrees to make sure all the moisture is dried out of it. You will use this same procedure when you prep your slump molds.

Firing Process Definitions and Temperature Ranges

The below pre- programmed firing schedules can be used in the Glass Fire Mode. Anneal from 1000 F to 900 F, which should be adequate for most glass. By incorporating such a broad range the risk of breakage is limited. Opening the lid too soon can cause pieces to break. I have bolded the firing schedules that you will be using most of the time if you are a beginner or intermediate glass enthusiast


Process Temp Range (degrees Fahrenheit)

Definition of Process       



Shaping glass by heating it until it bends over a mold under its own weight

Fire Polishing


Heating glass to the point where the edges round off and are left with a shiny appearance



Shaping glass by heating it until it stretches and drops (slumps) into a mold

Tack fusing


Heating the glass to the point where the individual pieces begin to stick together with each piece retaining its shape and character

Full Fusing


Merge two or more layers of glass by heating and temperature soaking until the glass is fully combined

Frit Casting


Small crushed pieces of glass (frit) are placed inside a dam mold to control the shape and fired to full fuse

Pate de Verre


Finely ground glass is emulsified, placed into a special mold and kiln fired to form a solid glass sculptural shape



Glass is softened to almost molten state and is manipulated by “raking” a metal tool across the surface

Glass Casting


Glass is melted in a crucible to a liquid state then it is poured into a specially prepared mold

     Although the above article does give you some guidelines for firing projects, we know that depending on the kiln you are using, the kind of glass you are using, the thickness of the project and the kind of firing you are attempting, there can be MANY variations to your firing schedules. We offer one-on-one technical support to our customers that buy glass kilns and/or glass startup packages from us for their test fires, setting up their glass displays, how-to projects and how-to price and on-going technical support.

Ready for some firing schedules? Click here for some good basic schedules for most of your projects.

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