S Cracks in Large Thrown Pieces - How to Avoid Them
by Jim Robinson
After I throw large platters, as the piece is drying, I am suddenly getting a large S shaped crack on the base. Any suggestions for me to help avoid this annoying and frustrating problem????
Yes, I have answers.
I have tried many, many common "cures" for the issue of S Cracks in the bottom of a pot, including the two most recommended to me:
- Compressing the floor of the pot with a rib
- Coning the clay before centering (coning is where you squeeze the clay in and upward while centering to pull the clay up into a tall cone to mix the clay and align the clay particles. You do this several times and then press it back down and finish centering like normal)
However, while they are good practice, they never helped me with S cracks. S Cracks are all about water and shrinkage.
The only way to prevent S cracks for me is:
1- Allow as little water into the bottom of the platter as possible while throwing (some is unavoidable, but keep sponging it out as you throw. Never let a puddle build up down there and dry it out really well before removing from the wheel)
The reason for this is that if the walls are dryer and the base is more wet, they will dry at different rates causing them to shrink at different rates, causing S Cracks.
2 - Be sure to cut the platter free from the wheel right away. As soon as it is dry enough to remove from the bat or wheel head, place another bat or board on top of the platter and flip it.
Remove the bottom bat. This allows the pot, again, to dry more evenly and to shrink without resistance.
If your pot dries on the bat, stuck to wet clay, as it shrinks, the clay underneath won't let it move freely and it promotes S Cracks.
3) Finally, dry your platter VERY SLOWLY. This is just one more fail safe for preventing the shrinking, which always occurs during drying and firing, from creating cracks.
Well, that was long-winded, but it's a frustrating issue and one worth covering in detail. Hope that helps.