Matting Agents: MgO, CaO, Zo, Alumina. When to use one over the other

[mr_glazy_man]

I have been trying to create a really great matte glaze. I have a couple of contenders after my last post, and I’m excited to share the results.
 

In that thread, it was mentioned that good matting agents are calcium oxide, magnesium oxide, alumina, etc in addition to other particular chemistry requirements such as silica:alumina ratio.
 

This poses an interesting question for me: why use one particular oxide over the other to yield a matte surface finish?

[Min]

Feel of the glaze, colourant response plus cutlery marking are a few reasons. Magnesium mattes have a lovely buttery feel to them versus a strontium or barium matte which are very dry feeling, calcium mattes don't feel as buttery as magnesium mattes but can still be lovely. High alumina mattes (without too excessive an amount of one or more of the other matting oxides) are usually prone to cutlery marking, sometimes quite severe. Often mattes require slowing the kiln firing during cooling to develop the microcrystalline structure that results in matting as opposed to just under firing a gloss glaze. 

[Bill Kielb]

4 hours ago, mr_glazy_man said:

This poses an interesting question for me: why use one particular oxide over the other to yield a matte surface finish?

Color response can be a thing as well with use of certain fluxes. I would suggest (in a well melted glaze) a test blend by varying the Si:Al from 4:1 to  7:1 for a true matte just to get a feel of going from let’s say dry matte toward semi gloss or gloss is a worthwhile exercise to experience IMO.

Edited October 9, 2022 by Bill Kielb

[mr_glazy_man]

I’m excited to try these three recipes. My aim was multi fold:

- achieve a si:al ratio below 4:1

- ensure R2O is between .2-.3 (the CaO one doesnt  though )

- experiment with the B2O3 levels and ensure they’re above .45 for a cone 05-04 melt

- test the matte response difference between MgO and CaO

Any thoughts on the recipes? Anything I can improve?

 

 

 

 

 

[mr_glazy_man]

On seconds thought, perhaps adding some lithium carbonate or 3110 for the NaO will bring my R2O:R:O back above .2 for the CaO recipe (Matte 03)

Edited October 9, 2022 by mr_glazy_man

[Bill Kielb]

Experimentation is good, my thought is maybe start at 5:1  or high fours. Your alumina levels are relatively high, I get it, it’s easy to get your ratio that way but at some point it isn’t gonna melt. As an example: For cone six glazes I like to stay 0.58 Al or less, they generally are fairly stiff in this range in my experience. 0.45 boron is great at cone 04, increasing can create its own issues so if you follow the Katz research 0.45 for cone 04 is great, exceeding it, especially in excess often is not considered better. Those glazes have a large amount of clay as well so not entirely sure how they will mix up. Feldspars are a nice source of silica, alumina and fluxes as well, not sure if you have a pure preference to clay and Frits

In the end thoughtful experimentation is great though so note your results. You may be able to find some already made mattes on Glazy or elsewhere to experiment on as well than just creating one from scratch. Right now, you really don’t have glazes that require silica as it’s entirely sourced from the materials. You may be able to create simpler allowing for the addition of some silica.

Edited October 9, 2022 by Bill Kielb

[mr_glazy_man]

Great feedback. Thank you for that. Perhaps I will start with the above three, and then do line tests for each and gradually add 5% silica to replace the alumina until I get to mid five ratio?