Throwing Larger Pieces And Cracks Everywhere!

[moh]

Hi all,

I've been scaling up size in my pieces and running into massive 90% crack rate.

These are 10-15lbs in terms of wet weight.

Do you recognize any patterns here? I'd love to get some tips/pointers from the seasoned large throwers on what I can do to remove these issues.

 

Thank you!

[Min]

Underside of bowl cracks I would say those cracks were there before you glazed the pot. The glaze edges are smooth. How fast are you cooling your bisque firing and are you stacking bowls inside of bowls?  Other crack pattern I've never seen before. Is this on greenware? Just a hunch but I'm thinking the base is thinner than the walls which set up a drying stress crack. I'ld break it open and compare the base (inside the foot-ring) to the wall thickness.

[Marcia Selsor]

I have seen it before. I taught for decades and have seen this. I think the cause is stress and too thin or weak above the foot. 

Try putting a short thumb tack inside your pot down where those cracks appear. Maybe use several. You will hit the point of the thumb tack and know the thickness. Practice this until get learn the feel of the thickness down there. 

Marcia

[neilestrick]

It's a thickness/unevenness issue. Everything is compounded as you go larger, so you have to be really diligent about keeping things even.

 

Based on the size comparison with your hand, I'd say those pots are pretty thick for their size if you used 10-15 pounds. I make sinks with 16 pounds, and those come out 16 inches in diameter glazed and done with porcelain.

[Pres]

I agree with both Neil and Marcia. I think that your pots are too heavy for their size (looking at your thumb in comparison to the bowl). I have been throwing what I call mixing bowls out of 8# of clay, these have a 14-16" diameter and rise about 8 inches, and will handle single large cookie batch. I believe that you would benefit from working with half the amount of clay you are pushing here, maybe 6 to seven pounds and see how they work out. Also be very particular in the trimming of the foot and foot ring with gradual taper into the bowl walls-there needs to be enough clay in the lower walls to support the upper walls at the angle outward as the bowl is more of a hyperbola than parabola in form.

 

 

best,

Pres

[GEP]

Just to echo what is being said above: when I throw an 8 lb bowl it will be 15 inches wide when wet, maybe 4 or 5 inches tall. Therefore the bowl in your hand is too small for an 10-15 lb bowl. Which leads me to believe it is very thick, too thick to dry it safely. 

 

I would also advise you to break open the finished cracked pots, just to see their thickness and even-ness. And I think Pres's advice for going forward is spot on. 

[Marcia Selsor]

Mea is right. You need to examine the walls. break them open and learn what the problem is. Good lesson.

Marcia

[Mark C.]

I see these as thickness compression issues-soft clay will make it even worse.

I think what Pres and Mea said will get you on the right track .