[Cécileanne]

Hi everyone,

I dug some clay in a river a couple of years ago. After sieving it we decided to test it and we cooked a tile in an electric oven around 1000 C for biscuit. it worked very well.
we did some low temperature glazing. The first result was fine but a couple of day later the tile cracked by itself. Inside was some white lump.
Does anyone have an idea of what is the problem and if it can be fixed?

Thanks very much

[neilestrick]

That white lump could be just about anything. How fine a sieve did you use?

[Cécileanne]

neilestrick, on 15 May 2012 - 08:05 PM, said:

That white lump could be just about anything. How fine a sieve did you use?

Thanks for your answer. It is the first time I use a non commercial clay.
The sieve was not that fine, but there were quite many of them and more like a flour lump, something that probably agregate during the cooking. My teacher was also surprised and had no idea of what could have happen or what could be the chemical provoking a crack a couple of days after cooking

[neilestrick]

Limestone/calcium impurities in clay can pop out weeks or months after firing. If they were large enough, they could cause a piece to crack. I recommend slaking down the raw clay into a slurry and running it through a 30 mesh sieve, or finer if the clay isn't too rough, to get the chunks out and disperse the small stuff.

[timbo_heff]

...Also, add some grog if you are making tiles ... it easily could solve the dunting (cracking) problem by simply controlling the shrinkage.

[Cécileanne]

neilestrick, on 15 May 2012 - 11:23 PM, said:

Limestone/calcium impurities in clay can pop out weeks or months after firing. If they were large enough, they could cause a piece to crack. I recommend slaking down the raw clay into a slurry and running it through a 30 mesh sieve, or finer if the clay isn't too rough, to get the chunks out and disperse the small stuff.

thanks, I will try

[Cécileanne]

timbo_heff, on 16 May 2012 - 01:50 AM, said:

...Also, add some grog if you are making tiles ... it easily could solve the dunting (cracking) problem by simply controlling the shrinkage.

Thanks for the tip! This tile though was only for fire testing the clay.