This entry discusses my glaze test results from week 15, which I’m in the process of writing up, but I couldn’t help myself as I was thinking of this as I was documenting the photos. I tested the following Magnesia Matt recipe and removed the Magnesia component, so to speak. The test ring pictured above is the result.

Magnesia Matt (without dolomite):
Potash Feldspar 18
Whiting 16
China Clay 33
Silica (Quartz) 22

This recipe is supposed to include Dolomite 12.

Here, it struck me that a most curious thing is happening. I always thought that the “mattness” in a magnesia matt was caused by the Magnesium Carbonate in Dolomite. Yet the glaze is matt without the dolomite! Looking at the base recipe without the dolomite, I would have expected a clear glaze, judging by the other tests I’ve done so far!

I checked the chemical compositions of the materials I was using in “The Craft and Art of Clay” by Susan Peterson.

(Sorry I can’t do subscript, you’ll have to make do with my slightly sub-standard representation of chemical formulas. Look them up in Susan Peterson’s book to see the proper notation).

Dolomite: CaCO3•MgCO3
(that’s Calcium Carbonate – same as whiting – and Magnesium Carbonate in equal amounts).

Whiting: CaCO3 – Calcium Carbonate, lime, limestone or Calcite. (It has many names but smells just as sweet… no don’t sniff it!)

Silica: SiO2 – Silicon dioxide – glass former

China Clay: Al2O3•2SiO2•2H2O – Alumina Oxide (Alumina – stiffener, Silica – glass former and water + obviously something else to make it plastic)

Potash Feldspar: K2O•Na2O•Al2O3•SiO2 (also maybe CaO) apparently each feldspar is different and you have to check the manufacturer’s specs, which I don’t have. Potassium Oxide, Sodium Oxide, Alumina Oxide, Silicon Oxide, Calcium Oxide… But no Magnesium Carbonate at any rate.

So what is making my glaze matt? When I look at the recipes of the previously tested clear glazes, they all either have a very large proportion of feldspar or silica forms about a third of the total recipe:

WK/L3, L4 & L5:
Any Feldspar 85
Whiting 15

CSA/F5:
Potash Feldspar 40
Silica 30
Whiting 20
China Clay 10

CSA/F4:
Potash Feldspar 55
Silica 21
Whiting 17
China Clay 7

CSA/F3:
Potash Feldspar 28
Silica 32
Whiting 20
China Clay 20

CSA/F2:
Potash Feldspar 62
Silica 25
Whiting 8
China Clay 5

JDB/01:
Nepheline Syenite 60
Silica 30
Whiting 10

(for glaze sources please see week 13 and week 15 glaze test results).

Without the dolomite in the mix, the magnesia matt recipe should be more correctly represented thus:

Potash Feldspar 20.2
Silica 24.8
Whiting 18
China Clay 37

(I worked this out for Potash Feldspar for instance by saying 18 parts out of 89 is how many parts out of 100? and so on)

It seems like the China Clay is the most dominant component, so perhaps I’ve simply made a vitreous slip? Or since there is a lot of calcium carbonate in the glaze (from the feldspar and whiting) I’m getting a kind of calcium matt as well?

At this point I think it would be interesting to do a quadraxial blend (if I could get my head around such a thing) to work out at which point glazes turn from glossy to matt – in terms of proportions of feldspar to silica to whiting to china clay. This might be an interesting project if I could be a student for ever – but – must get on with things and be firm with myself! This is a project for later – or if someone has done it already, would you share the result?

Anyway, the conclusion seems to be that I can get quite a passable buttery matt glaze on bone china at cone 10 and I don’t even need dolomite!

This entry was posted on Sunday, October 28th, 2007 at 12:34 am and is filed under Ceramics, MPhil. You can follow any responses to this entry through the RSS 2.0 feed. Responses are currently closed, but you can trackback from your own site.