Firing pieces made with mixed types of clay

[JanetMermaid]

So happy to find this group! Being a ceramicist can be a lonely craft. I moved to Spain in September so am just now getting my studio set up and running again. I found some wonderful new low-fire clay colors but they also introduce new challenges. I want to experiment with making pieces out of multiple colors, but they have different firing temperatures. Where I used to live I only had the choice of red or white clay, and they had the same firing temperature (cone 04). Here I have black, blue, dark brown, red, and white -- well, I have even more choice than that, but those were all my budget would allow for now!

I'd like to create a piece using these three colors -- I've noted the firing temperature (in C) for each, as specified by the local manufacturer:

* Black: 970-1025
* Red: 980 
* White: 1020

The temperatures aren't drastically different, but there's enough that I was unsure what temperature/cone do I fire the piece on so that all clays harden and none crack? My kiln is a Skutt KM-822, if that makes a difference.

Thanks for your help!
 

[Babs]

what would cause them t crack would not be temp reached imo, rather % shrinkage.

I'd make a bar if each marked out, measure the unfired dry clay then fire to say, 980 , which covers the middle, then measure again when cool.

that way you will know what to expect.

if any marked difference in shrinkage would make me rethink using them willynilly on same piece...

[JanetMermaid]

Thank you Babs. I hadn't even thought about firing shrink-test bars. Excellent idea. I've just never had to worry about that before. I'll make strips today of each of my new colors. That way if one of the three I want to layer shrinks at too different a rate, I can perhaps choose one of the other colors to include instead.

[Babs]

Do you think it is the same clay body which has been coloured or three different clay bodies?

Those temperatures are pretty close... Cone 06 is around 1000deg.C so maybe, if compatible shrinkage, go for that??

Maybe ask your supplier a few more questions in this field after you see the shrinkage rate.

May pay to fire pretty slowly till 600deg C

Drying slowly too  as different clays often hold different amounts of water..

[JanetMermaid]

Spent the day making a bunch of different layered pieces and also identically sized pieces of each clay. I don't know if they are the same clay body that's been colored, or different clay bodies. And my Spanish is still pretty limited so I'm not sure I could ask that and they'd know what I meant. I'll give it a try, though. In any case, I have a real-world test going on.

[Babs]

let's know how it all goes:-)

[Rae Reich]

On 3/22/2018 at 9:18 AM, JanetMermaid said:

Spent the day making a bunch of different layered pieces and also identically sized pieces of each clay. I don't know if they are the same clay body that's been colored, or different clay bodies. And my Spanish is still pretty limited so I'm not sure I could ask that and they'd know what I meant. I'll give it a try, though. In any case, I have a real-world test going on.

When I make test bars, I make them 6" long and mark them like rulers in 1/4 inches and inches. Makes it easier to quantify shrinkage and comes in handy, too, when you need to make matching lids and rims, etc. If you need to make a lid to, say, replace a broken sugar bowl lid for a finished piece, measure the bowl's rim with the fired ruler and you will know how wide to make the replacement.

Also, do your best to mark all your test bars when the clay is at its freshest, to be the most equivalent to freshly thrown clay thru drying and shrinking (but don't cut out till leather hard).

Remember to put a hole in one end for hanging or stringing.