Magnesium Matte Glaze Tests

Tim Carlson

August 3, 2011

Conventions in these notes:

Here, as in all my notes, the units used are by weight unless spec-ified otherwise (e.g. 1tsp Sodium Silicate). Some may complain thatthe recipes included are in parts by weight which have not been nor-malized. A problem I find is that in practice, all one needs to repro-duce a specific glaze is the correct proportion of parts and the samekiln and firing schedule. Ha! Well, at least the correct proportionsand a close approximation to the same firing schedule are achievable.Another problem I have is: what normalization do I use? Normaliza-tion to 100% overall by weight? Or normalization of the halides to amolecular proportion of 1?1

For all the glazes listed in my notes, we fired to cone 10 (∆10) inreduction using a Bailey Downdraft 10ft3 kiln, though to be honest,the firing is more of a soft ∆11.2 The schedule which we fired isapproximately given by: neutral up to a Shino/body reduction, firestrong reduction through to ∆9, tapering the reduction to neutral at∆10 with a neutral soak for 20 minutes. The humidity we fire inis fairly low, and is mentioned here since it has a lesser effect on thereduction environment in the kiln (I apologize that the actual humidity%, nor the barometric pressure are recorded here for reproductionefforts).

1‘Some day,’ when I feel the urge to do a more complete analysis, I will provide amolecular proportion analysis as an appendix; currently, I am much more focused on thereproduction of colors and textures which requires the simplicity of testing, testing, testing.

2I would list peak temperature, but the major factor in maturity of the clay body aswell as the glaze is the amount of heat energy which is applied to the materials insidethe kiln; here is the main reason for using pyrometric cones. The cones are a simple,reasonably accurate measure of the heat energy which has been supplied to the materialsin the kiln.

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1 First Magnesium mattes

Since I am a sucker for simplicity, especially with numbers, a cone 10 (∆10)magnesium matte glaze I found on the web really appealed to me. The oneI found is:

Kona Spar 20Soda Spar 20Whiting 20Dolomite 20Kaolin 20

Very simple numerically, and low in complexity of ingredients, but itseemed to be a little low in magnesium for my tastes3. So, I tried testing thisas a base glaze, but I just couldn’t help myself with the question regarding;”What if....”

In the first magnesium matte base glaze I attempted, I replaced all thesoda spar with Kona spar (a potassium spar), cut the Whiting in half andsubstituted Strontium Carbonate instead. Since the glaze seemed a little lowin magnesium and especially since I was using a more active (lower temper-ature melting) feldspar, I thought that the addition of 10 parts magnesiumcarbonate would stiffen the glaze and add matte-ness to the glaze. Whoknows?

So, the base glaze I first tried is:Mag Matte Base 1Kona Spar 40Whiting 10Strontium Carbonate 10Dolomite 20Kaolin 20Magnesium Carb 10

3Though I should try it in the future for completeness

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In the picture to the left is a sam-ple of the base glaze. It turnedslightly purple-pink where thick;note the mottling as well. Pos-sibly from incomplete mixing, butalso maybe from the introduction ofstrontium carbonate. Replace withwhiting? Also, since the strontiumwas an impromptu replacement ofsome whiting, possibly reducing thestrontium in half is more accurateaccording to the molar mass com-parison with whiting.

So my thinking here (quite erroneously, as I found), was that cobaltand chrome oxide had minimal color interaction with the melt they werein, that is, they remained blue and green respectively with only slight colormodifications due to the solvent they were in. So I tested the base glaze,cobalt carbonate, and chrome oxide.

1. Cobalt colorant.

Mag Matte Base 1 100Kona Spar 40

With the addition of 1.5% CobaltCarbonate, the magnesium-cobaltinteraction produces a sparklymatte dark purple.

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2. Chrome colorant.

Mag Matte Base 1 100Kona Spar 40

In this test, .8% Chrome oxide wasintroduced, producing the mottledpuke-green / brown shown.

A question I’m interested in is; could the replacement of 10 whiting forthe 10 strontium reduce the mottling effect?

2 More Magnesium Mattes

These tests addressed the question above about the substitution of StrontiumCarbonate in place of Whiting. Notice that the new base glaze recipe belowis the same except for the lack of use of Strontium.

All these glazes were striking in that they behaved as a fake ash glaze. Inorder to produce a magnesium matte, the viscosity needs to be increased aswell as a slight reduction in the surface tension.

Mag Matte Base 2Kona Spar 40Whiting 20Dolomite 20Kaolin 20Magnesium Carb 10

I apologize that there is no test tile of the base glaze for the MagnesiumMatte Base 2.

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1. Iron colorant. Test tile C.

Mag Matte Base 2 100Iron Oxide 2

Surprisingly, the 2 parts iron oxide pro-duced a pleasant yellow with dark greencrystals in areas of thicker buildup.

2. Iron colorant. Test tile D.

Mag Matte Base 2 100Iron Oxide 10

Here, the 10 parts iron oxide produced asexpected a dark brown, almost temmokuappearance fake-ash style glaze.

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3. Rutile colorant. Test tile E.

Mag Matte Base 2 100Rutile 3

This tile produced a yellow fake-ash witha grey/white background.

4. Copper colorant. Test tile F.

Mag Matte Base 2 100Copper Carbonate 2

Not too surprisingly, the 1.5 parts ofcopper carb produced a dark green glaze,though it is a fake-ash.

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5. Iron Chromate colorant. Test tile G.

Mag Matte Base 2 100Iron Chromate 2

Here, the 2 parts iron chromate producedan interesting mottling of brown/greywhich seemed slightly less runny than theother tests.

6. Cobalt colorant. Test tile H.

Mag Matte Base 2 100Cobalt Carbonate 2

The 2 parts Cobalt Carbonate was used tolook for contrast with the previous1 Magmatte cobalt test. Here, this one produceda subtle purple/lavender fake-ash glaze,which is strikingly different from the pre-vious.

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3 Open Questions:

1. What would Nickel do as a colorant in either of these two base glazes?

2. Would more mag carb ( 20 total) make this (Mag Matte Base 2) amatte glaze rather than an ash glaze?

3. Are the crystals which were produced in the first mag matte a conse-quence of the firing or the use of strontium or some other unknown?Need more tests...

4. What would the replacement of Whiting with Strontium Carb do inthese base glazes?

5. What would the introduction of titanium do to either of these two baseglazes?

6. Increase the Kaolin to stiffen the glaze?

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