Short clay

[Retha Matthee]

@glazenerd I use a local South African white midrange stoneware clay which fires to cone 6.  It is extremely short and I read that adding a 50/50 ball clay to bentonite mix will improve the plasticity. They recommend to add 50g to a kilogram of clay.

Will this influence the cone my clay fires to? At first I thought it was short because it was freshly made, but I'm 6 months down the line and it's still short. It even cracks when I wedge and it's not dry. I think it might have neph sye in it because if I wedge normally it cracks like crazy, but when I stack and slam it becomes pudding like. 

[Babs]

May need to edit title and personal message Glazenerd.

[Retha Matthee]

5 hours ago, Babs said:

May need to edit title and personal message Glazenerd.

Thanks for your help, I will try to do so, technology is not my strong point ☺️

[glazenerd]

Retha: Welcome to the forums. 

If you are starting from dry powder premis: do one 500 gram test batch with 50 grams of ball clay (10%), and a second 500 gram test batch with 75 grams (15%) ball clay. If you have moist clay; then calculate dry weight by subtracting 20% for water. 1000 grams moist minus 20% water (200 grams) = 800 grams dry weight x 10% ball clay = 80 grams added to moist clay. Ball clay comes in low, medium, and high plasticity; so your experience with local materials will have to guide you. White stoneware has low iron and magnesium; which will also guide your ball clay selection. When hand mixing or slaking premix clay; it will take 5-7 days for plasticity to develop; 3 days if using a deairing pugger. 

Bentonite can be used as a plasticizer; more commonly used in porcelain. 3-5% addition would be a good starting point. Important that you throughly blend in bentonite before adding water. Bentonite does not take kindly to being wedged into moist clay; I would avoid it. 500 gram test batches are more than adequate to test plasticity. Do not overdo ball clay additions because they do absorb water; excess can cause slumping or folding when thrown- in addition to higher shrinkage rates if handbuilding or doing slabs. 

Nerd

[liambesaw]

When mixing clay from scratch I have found that bentonite and veegum-t both do much better if sifted into the water as the first material and allowed to sink on its own as it absorbs water.  I guess this would be considered slaking.

I just measure out the veegum (which I have found to be the supreme master of plasticity) and then put a sifter over my bucket and just put it all into the sifter, then give it a shake every time the veegum sinks into the water.  It stops it from gumming up way better than dry mixing it.  Even when dry mixing the bentonite I have found that it clumps when the dry mix is added to the water.  Very annoying to remove from the bucket and even more annoying to sieve.

This was for a porcelain recipe, both slipcast and clay body.  Original recipe calls for bentonite at 5% but I substituted for veegum-t at 3% because I have it, and I have found it superior to bentonite.

[Retha Matthee]

On 5/8/2020 at 10:18 PM, glazenerd said:

Retha: Welcome to the forums. 

If you are starting from dry powder premis: do one 500 gram test batch with 50 grams of ball clay (10%), and a second 500 gram test batch with 75 grams (15%) ball clay. If you have moist clay; then calculate dry weight by subtracting 20% for water. 1000 grams moist minus 20% water (200 grams) = 800 grams dry weight x 10% ball clay = 80 grams added to moist clay. Ball clay comes in low, medium, and high plasticity; so your experience with local materials will have to guide you. White stoneware has low iron and magnesium; which will also guide your ball clay selection. When hand mixing or slaking premix clay; it will take 5-7 days for plasticity to develop; 3 days if using a deairing pugger. 

Bentonite can be used as a plasticizer; more commonly used in porcelain. 3-5% addition would be a good starting point. Important that you throughly blend in bentonite before adding water. Bentonite does not take kindly to being wedged into moist clay; I would avoid it. 500 gram test batches are more than adequate to test plasticity. Do not overdo ball clay additions because they do absorb water; excess can cause slumping or folding when thrown- in addition to higher shrinkage rates if handbuilding or doing slabs. 

Nerd

Thank you so much for your detailed explanation, I'm going to test with the ball clay, my clay is wet so your wet calculation helps alot! I'll stay clear of the bentonite. 

[Retha Matthee]

On 5/9/2020 at 1:03 AM, liambesaw said:

When mixing clay from scratch I have found that bentonite and veegum-t both do much better if sifted into the water as the first material and allowed to sink on its own as it absorbs water.  I guess this would be considered slaking.

I just measure out the veegum (which I have found to be the supreme master of plasticity) and then put a sifter over my bucket and just put it all into the sifter, then give it a shake every time the veegum sinks into the water.  It stops it from gumming up way better than dry mixing it.  Even when dry mixing the bentonite I have found that it clumps when the dry mix is added to the water.  Very annoying to remove from the bucket and even more annoying to sieve.

This was for a porcelain recipe, both slipcast and clay body.  Original recipe calls for bentonite at 5% but I substituted for veegum-t at 3% because I have it, and I have found it superior to bentonite.

Thanks I will search for veegum, hopefully after lockdown has eased I will be able to source some to try!