Granite & ash glaze - Carl Gray Ceramics

A couple of years ago I discovered a quarry in the Limousin region of France where granite rubble is mined.  This is passed through sieves to separate the different size particles.  On the verges of the road close to the quarry are piles of fine granite sand.  I brought a bucket of this back to England with the thought of developing a glaze with it.

I would like to thank Mike Dodd and Jim Malone for imparting their knowledge about granite and showing me examples of their work with granite based glazes.  Jim suggested I try a simple line blend test using the granite sand and wood ash.

The test was fired in a gas kiln to about 1,260°c.  The most promising result came from the 40% granite, 60% ash mix.  I then went on to conduct further tests adding china clay and silica and settled on the following formula:

32% Granite
48% Ash
10% Flint
8%   China clay
2%   Bentonite

While this recipe sometimes produced excellent results I found that the sucessful firing temperature range was very narrow: too low a temperature and a matt finish resulted, too high and the glaze tended to run off the pots.  The glaze also tended to craze heavily on most clay bodies.

After many more experiments I settled on the following recipe which I fire to cone 9:

Granite 28%
Ash 36%
Flint 14%
China clay 12%
Bentonite 2%
Spodumene 8%

1% black iron oxide

The higher clay & flint content has increased the successful firing temperature range and made the glaze more stable.  The spodumene has reduced the crazing (spodumene being a member of the feldspar family that contains lithium which has a low expansion coefficient).