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Deciphering correct firing temperature for unknown clay


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Hello wizards!

I’ve been given a load of red terracotta clay which is wetting down. I am just browsing my glaze recipe book for a nice earthenware glaze recipe and see there are two sections Temps 1050-1100 and 1100-1150. Is any temp within this range ok? I am guessing it just effects the darkness of the clay? I am firing in a fast fire gas kiln. Havn’t used or fired earthenware before. Will be making functional ware. How do you chose which temp to fire to? I have always just gone with the temp of the glaze i like when firing stoneware, is there anything else to it? 

Many thanks

sarah

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29 minutes ago, Housefull of pots said:

Hello wizards!

I’ve been given a load of red terracotta clay which is wetting down. I am just browsing my glaze recipe book for a nice earthenware glaze recipe and see there are two sections Temps 1050-1100 and 1100-1150. Is any temp within this range ok? I am guessing it just effects the darkness of the clay? I am firing in a fast fire gas kiln. Havn’t used or fired earthenware before. Will be making functional ware. How do you chose which temp to fire to? I have always just gone with the temp of the glaze i like when firing stoneware, is there anything else to it? 

Many thanks

sarah

I guess I would test in a test kiln to see where it’s limits are. I assume these are degrees C and you are describing a range roughly between 05 and cone 1ish which is  pretty broad for glazes. The clay will be porous so you may run into some functional issues.

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To decipher the correct firing temperature of an unknown clay, my recommendation is to make some "cones" [in size and shape similar to the pyrometric cones]  and place them in a pad of your normal studio clay body and fire them to your usual temperature to see what happens.  If you like the results, then make some forms.  If the tested clay melts, then you know you need to lower the firing temperature for that clay material, or use it as a decorative coating on your regular clay body.  

When I first started in ceramics, the professor had us using low fire (cone 05) red and white clay bodies for ware we fired to cone 3 oxidation in a gas kiln.  The ware was fully mature, did not slump, nor did melt.  Slumping began around cone 5.  Later on we   switched to cone 10 reduction, and the low fire clay bodies were usable as decorative glaze.  

I use commercial red and white terra cotta clay bodies as decorative coatings on bowls, containers, and sculpture for my cone 10 reduction ware. The red clay becomes a dark reddish black semi-gloss coating and the white clay  becomes a matte white coating. The thickness of the coating is a significant variable for the outcome. 

LT

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