• Materials for Ceramics

Glass Cullet

Alternate Names: Powdered Glass, Cullet

Notes

A wide variety of cullets (glass powders) are available, they are normally made from window glass, container or consumer glass and industrial glass. These glasses are commonly high in Na₂O and CaO and thus referred to as "soda lime glass". Since glass powders have been pre-melted they behave like a frit, melting over a range of temperatures (rather than suddenly) and beginning to melt at much lower temperatures than raw mineral powders. In ceramics, soda lime glass can often treated as a high-sodium frit. Almost all ceramic glazes contain at least some Na₂O (and similar K₂O), these are normally supplied by feldspar. Potentially, glass powder would be a preferable source since it melts so much better. In addition, since it is a more concentrated source of alkali (having lower silica and alumina content than a feldspar) recipes employing it can source more alumina and silica from kaolin and ball clay (producing better slurry and drying properties). Powdered glass has even more potential for bodies. All bodies contain feldspar, the percentage determines the maturing temperature. The lower the desired temperature the more feldspar needed. To make a porcelain vitrify full at cone 6, 35% feldspar might be needed. At cone 4, 50%! That makes less room in the recipe for clay and results in bodies of low plasticity and dry strength. But if a concentrated source of alkali was used it would mean more plastic bodies and lower firing temperatures. Frits are such potent melter that 25% is enough to vitrify a porcelain at cone 04! Potentially glass cullet could do the same.

The previous being said, we have had reports of very strange behaviour of cullet in both plastic and casting bodies (e.g. very high water requirement, solidification and stiffness instability of pugged material over time).

For glass powder to work in ceramics it must be fine grained, 200 mesh or finer. While there are hundreds of companies grinding glass, very few have products at this size. One we have found is Vitro Minerals. They are worth mentioning since their website enables buying samples, even truckload quantities, with a few clicks. And even paying with PayPal!

This chemistry is similar to Ferro Frit 3110:

Related Information

How to make this glass pooling effect in pottery bowls

These are called "Little Dishes" and are sold in tourist areas. They are made in the USA. They appear to be fired at cone 10R (because of the appearance of the glazes and bare clay on the foot). After glaze application and pigment banding, a thick layer of glass powder (glass cullet) is poured in. Since this type of glass has a high CTE it crazes thoroughly during cooling. Of course, that weakens the piece, but since these are decorative the aesthetics are considered more important. This effect can be achieved at any temperature by just using frit powder, Ferro frit 3110 will melt and craze like this at cone 04 or even lower. Suspended bubbles will be a problem, use a firing schedule with a hold to give them time to surface. Also, use a body having low LOI so that it is not generating gas bubbles. Bisque firing pieces at a higher temperature will accomplish the same (for example, if you are making these at cone 04 then bisque fire at cone 03).

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