[Clayart] Crystals

Robert Harris robertgharris at gmail.com
Tue Sep 28 02:48:42 UTC 2021

Deb, the silica we buy in the US IS ground up quartz. In the UK you can
still buy flint. Flint is amorphous silica (that is, it is in a glassy, non
crystalline state). Flint sometimes still contains a little flux (mostly
sodium, I think), and because of its amorphous state, melts much better and
more easily than silica sourced from quartz sand.

Amethyst gets its colour from a complex lattice structure. Gemstone
Amethyst are single large (i.e. visible to naked eye) crystals, whereas
glazes, for the most part, are glasses (even micro-crystalline mattes have
a glassy matrix). Amethyst is created by the replacement of some silicon
atoms for iron atoms in the crystal, by (natural) irradiation. If you heat
amethyst you essentially get Citrine. Putting it in a glaze, sadly, would
do nothing. Most brightly coloured silica minerals cannot be replicated by
us potters, because their growth is so slow. Artificial amethyst can be
grown, but it is done at relatively low temperatures (by our standards) in
a high pressure solution over many days and weeks.

On Mon, 27 Sept 2021 at 19:49, Deborah Thuman <debthuman at zianet.com> wrote:

> I got to thinking. I do that now and again. I took a geology class a year
> ago and learned that fast cooling makes micro crystals and slow cooling
> makes macro crystals. Quartz is one of those fascinating igneous minerals
> that does wild stuff. Amethyst is quartz. Quartz can be clear. Quartz can
> be opaque…. Anyone here ever experimented with using quartz in a glaze?
> Not just silicon dioxide, but ground up quartz? What happens if you use
> ground up amethyst in a glaze?
> Deb Thuman
> debthuman at zianet.com
> DebThumanArt.com
> https://www.spoonflower.com/profiles/deb_thuman
> You measure a democracy by the freedom it gives its dissidents, not the
> freedom it gives its assimilated conformists.
> Abbie Hoffman
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