ROGER BOURLAND on sat 22 jun 96
Thanks to all who responded to my granite request.
I found my granite powder source listed under "monuments" in the yellow
pages. Called them and they said "come on down. Bring a jar."
There was a beautiful pile of the stuff. Fine as face powder, but heavy.
The fellow filled up my jar and told me that another guy came in about
once a year, an organic gardener, (which I am too.) and said that it was
a wonderful source of phosphate and even kept harmful insects from
bothering your plants.
As I understand, the granite powder 50/50 with wood ashes makes a neat
glaze. Shouldn't some clay be added to this? I will search out a source
for the Grebanier book and post it if and when I find it (Chinese
Stoneware Glazes) will share the news.
Thanks again and I will report on the graanite glaze results.
Roger Bourland
Don Curtis on sat 22 jun 96
Grebanier book can be found at the Jones library in Amherst, Ma. Hope this
helps.
Richard Tarrant on sun 23 jun 96
>As I understand, the granite powder 50/50 with wood ashes makes a neat
>glaze. Shouldn't some clay be added to this?
Roger,
I've found that about 10% - 15% ball clay works well, although granite
glazes have a *fearsome* tendency to settle in the glaze bucket. Try to
give your granite glazes a good stir at least every week.
A good starting point for a celadon style glaze would be(depending on the
iron content of the granite) :
granite 70
whiting 15
ball clay 15
You could also think of granite as an iron - bearing cornwall stone, so, in
any recipe you have (or can find) that contains cornwall (or cornish) stone
you could simply substitute your granite directly (1 - 1) for the cornwall
stone.
"Granites" vary widely in composition so the above can only be a starting
point. Also, granites are only one of a very large family of intrusive
igneous rocks. There is no way of knowing (short of getting it analysed!)
the analysis of your sample.
But there are some general tendencies. Granites are mostly composed of
feldpsars, silica and mica. In addition they contain a whole raft of lesser
quantities of other minerals (accessory minerals). A rough and ready
analysis of granite ( and pretending that mica is a mix of silica, feldspar
and iron oxide) would be something like:
feldspar 50 - 78%
silica 20 - 40%
iron oxide 2 - 10%
Almost a cone 10 glaze in itself. All that's needed is some calcium
(whiting, wood ash, calcined sea shells, marble dust, etc) and some ball
clay to make the mix behave.
A final point:
>Unfortunately they also do marble work on the same slabs
>so the sump tank is a mix of all kinds of granite and marble. Oh well.
Richard,
1. Provided there is not too much marble dust mixed with the granite it is
no bad thing. About 20% marble dust would do nicely. Even if there was
more, the mix would still be a useful glaze ingredient, check it out.
2. Calcining to about 1000ŚC, especially if done a couple of times makes
the granite lumps much easier to crush down to individual crystals of
quartz and felspar. However it still needs ball - milling or hammer milling
to get it fine enough for glazes. Not something to do by hand.
Its probably easier to develop glazes based on the granite marble dust mix.
Cheers,
Richard (coughing and spluttering after a vile dose of 'flu)
PS. Anyone making the trip to OZ for the Ceramics Conference?
Richard Tarrant (rtarrant@extro.ucc.su.OZ.AU)
Sydney College of the Arts
The University of Sydney
Ph +61 2 810 2890 Fax +61 2 810 2890 (by request, phone before faxing)
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