Oyster Shells In Low Fire Glaze On Earthenware

[tanjag]

New to ceramics and just learning about glazes. 

 

I am using found clay to make earthenware sculptures that I am firing at 06 (and which can withstand temperatures up to 02 before melting).

 

I want to use ground up oyster shells (essentially CaCo3, or whiting) in the glaze for these pieces. The use of the oyster shells is conceptually related to the project i.e. they must me an ingredient in the glaze.

 

I would like the glaze color to be light grey to white in color and preferably on the glossier side. Translucent to opaque is fine.

 

I am not terribly concerned with perfection in the glaze (crazing, bubbles, unevenness are almost desirable) as long as the glaze adheres to the earthenware and doesn't easily flake/crack off.

 

Where would I begin finding/creating a recipe for such a glaze? 

 

I have done my reading research on glazes but have never mixed a glaze myself.

 

Any and all advice much appreciated. Thank you!

 

 

[neilestrick]

Since you don't have any glaze mixing or formulating experience, I would start with just replacing the whiting in  a glaze with the oyster shells. How they behave will depend greatly on how finely they are ground. You could also just try adding them to a glaze and see what happens. Make some catch plates to put under your test tiles just in case they run a lot.

[Dick White]

I don't do low fire work, but there is an article over in the CeramicArtsDaily section on some easy low fire glazes, one of which includes whiting:

 

Ferro Frit 3195 - 43%

EPK kaolin - 14%

Whiting - 43%

 

For opacity, you might start with 5-10% zircopax.

 

For the grey color, you might add around 2% manganese dioxide, or select a grey mason stain.

 

Test test test samples before committing to your final project.

[Marcia Selsor]

I don't know about shells in glaze but shells in clay become calcined  much like lime kilns making plaster. In Europe some places soak clay after it comes out of the kiln to rehydrate the calcium from the shells in the clay bodies. Otherwise the clay crumbles. I have seen this happen in Spain and Italy. 

After a few days , if the clay wasn't soaked in water, the pieces will fall apart. can't say what it will do in a glaze, but let us know.

 

 

Marcia

[tanjag]

Dick,

 

Thanks -- that is a really useful starting place for me -- a much simpler recipe than I have found on my own. 

Question: If I added the zircopax, what ingredient should I start off trying to substitute (Ferro Frit 3195, EPK kaolin, or

Whiting)?

 

Yes -- will do many tests!

 

 

 

 

 

I don't do low fire work, but there is an article over in the CeramicArtsDaily section on some easy low fire glazes, one of which includes whiting:

 

Ferro Frit 3195 - 43%

EPK kaolin - 14%

Whiting - 43%

 

For opacity, you might start with 5-10% zircopax.

 

For the grey color, you might add around 2% manganese dioxide, or select a grey mason stain.

 

Test test test samples before committing to your final project.

[tanjag]

Marcia, this is also very useful info for me as I am experimenting with using the shells as temper in the clay body. I had not heard about the soaking post-kiln and will try that. 

 

Using preheated/roasted shells in the clay body apparently has quite a bit of historical precedence, but it is unclear to me how essential the preheating is, unless the temp is very high (at 500-600C the CaCo3 releases its CO2 and leaves CaO or calcium oxide/lime). Still looking into that one.

 

More tests!

 

 

I don't know about shells in glaze but shells in clay become calcined  much like lime kilns making plaster. In Europe some places soak clay after it comes out of the kiln to rehydrate the calcium from the shells in the clay bodies. Otherwise the clay crumbles. I have seen this happen in Spain and Italy. 

After a few days , if the clay wasn't soaked in water, the pieces will fall apart. can't say what it will do in a glaze, but let us know.

 

 

Marcia

[Fred Sweet]

Tanjag-

The zircopax is an addition to the base glaze, not a substitute for part of any of the other ingredients.

If you were doing a 100 gram test of the base with an addition of 5% zircopax, you would have 100 grams of the base plus 5 grams of zircopax, giving you 105 grams of dry minerals in your container.

Does this help?

Regards,

Fred

[Dick White]

Yes, what Fred said...

[tanjag]

Tanjag-

The zircopax is an addition to the base glaze, not a substitute for part of any of the other ingredients.

If you were doing a 100 gram test of the base with an addition of 5% zircopax, you would have 100 grams of the base plus 5 grams of zircopax, giving you 105 grams of dry minerals in your container.

Does this help?

Regards,

Fred

 

Yes -- very helpful -- thanks!

[tanjag]

I don't know about shells in glaze but shells in clay become calcined  much like lime kilns making plaster. In Europe some places soak clay after it comes out of the kiln to rehydrate the calcium from the shells in the clay bodies. Otherwise the clay crumbles. I have seen this happen in Spain and Italy. 

After a few days , if the clay wasn't soaked in water, the pieces will fall apart. can't say what it will do in a glaze, but let us know.

 

 

Marcia

 

Just found an incredibly helpful paper investigating historical shell tempering techniques, with tests using crushed oyster  shells (pre fired and not) in the clay body, with variables including reduced atmosphere firing and varying temperatures. Verdict: pre-firing shells not important for integrity of final vessel, but reduced atmosphere firing OR keeping firing temp below 650C/1200F are important for a serviceable vessel. Seemed worth including here should anyone in the future go down this rabbit hole. https://www.jstor.org/stable/40713455