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Milling the glaze

Many of you are curious about the transition from rock to glaze.

Firstly to those of you wondering what glaze actually is, the glaze is the glassy like substance that coats a clay body, quite simply, its the colour on your coffee mug.

The intention of this project is to test the glaze qualities of a selection of rocks from throughout New Zealand using the publication "Rock Glazes of New Zealand" Minna Bondy (1960) as a field guide. Bondy's publication is in black and white and the original samples are missing, however, the book remains a fascination to many potters and to my knowledge the work has not been revisited as a whole. I will be extending Bondy's work by using wider testing methods. Sites and samples have also been recorded and will be documented together with the glaze results. I have endeavoured to keep the process as close as possible to the original methods. The samples will be glazed on a Temuka clay body and are ground using a ball mill.

I often explain glazing as like battering fish. The clay body is fired to 850-900 degrees which forms a biscuited pot. From there a glaze is applied much like battering fish. The pot is fired again this time to 1300 degrees to melt and bind the particles of the finished piece.

The process of milling the samples has been a relatively simple one as the ball mill does most of the work. The ball mill is a beautiful machine that I have been referring to as "high tech/low tech" as the mechanism is simple but the maths is nerdy. For those interested, I will direct you to Steve Harrison's website and books as he is a wealth of knowledge on the subject. The mill is essentially a set of rollers which a jar ( filled with the material/charge) sits upon and turns.

There are three components in the jar divided roughly into thirds, the charge; which is a slurry of rock and water, the porcelain balls, and the last third is air. The rolling mechanism turns at a specific speed which allows the balls to roll and drop within the jar, wearing at the charge till it is finely ground.

The mill runs overnight for a roughly a 24-hour cycle. The initial particle size of the material was quite large so it has needed this time to fully grind. After 24 hours the material is then sieved through a 50 mesh sieve and put aside to settle. As the material settles water is tipped off leaving the finely ground material, which is then dried to be weighed for the glaze recipe.

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