Back when I was using a regular clay body, any cracks would spell doom (not to mention, heartache) to my piece. Sometimes I can fix the crack if I catch it in time when the clay is still in the leather hard stage or slightly past it. Forget about it if your piece is bone dry. Sometimes you hope for the best through the bisque fire and then hope even more that the glaze will "seal" the crack. Most of often than not, you find it does not work that way.
One big advantage of paper clay over regular clay is that any cracks that develop in your piece can be fixed, even at the bone dry stage. Let me first clarify the different types of cracks and why cracks happen in the first place when making dry to dry joins.
Why cracks happen when joining?
This problem can always be attributed to insufficient moisture in your pieces to be joined using dry to dry joining techniques. Remember bone dry pieces are very, very thirsty and will soak up water very easily and quickly. Beginners usually do not have enough moisture in their pieces to be joined, or your mushy paper clay (which constitute the main "glue" that holds everything together) is not mushy enough (not enough water) or the paper clay slip is too thick (not enough water).
How do you get moisture to the pieces to be joined?
The simplest way is to soak the area in water that needs to be joined. Keep an eye on the part so that it does not revert back to its leather hard stage, or worse still, become mushy and soft. For a piece that is about 1/4" thick, approx. 30 seconds to a minute. I frequently check the water content of the part by taking it out of the water soak and see how fast the water is re-absorbed into the surrounding paper clay. If the water disappears too fast, then I know the piece still needs more moisture and back it goes into the water soak.
If my piece is too large to fit inside a soaking container, I've used wet paper towels pressed against the area to be joined. The dry paper clay will suck readily moisture from the wet paper towels so keep the paper towels wet with a sprayer or a sponge.
Types of cracks.
There are 2 types of cracks. The first kind is the surface crack. This is mostly seen at the very feathered edges of a join caused by the difference in water content between the pieces to be joined and the mushy paper clay/paper clay slip used as "glue." This kind of crack can also happen AT the join. If your dry to dry joining technique is good, this crack can be repaired without any trouble simply by adding more paper clay slip to the crack to "seal" it.
Otherwise, this is a sign of the second kind of crack which is a structural crack. This is no good. Again, the main culprit is insufficient moisture content in the pieces to be joined. Joins like this can easily be pulled apart and will fail very nicely and cleanly. This is a tell tale sign that means your dry pieces need to be soaked longer in water. The thicker the piece, the longer it will take. A lot of it comes from experience working with paper clay.
A good test of your dry to dry joining technique.
- Make a dry to dry join.
- Let it dry completely to bone dry.
- Pull it apart.
- If the pieces come apart cleanly at the join, then you know your joining techniques need more practice.
- If the join holds and your piece breaks away from the join, then you have made a good strong join.
With practice and a good understanding of the physical properties of paper clay, one can begin to understand why one join fails and another survives. Paper clay affords us this training and removes this "fear" element of "Oh, no. I have a crack in my piece." (The above exercise is a good one for beginners to go through. The paper clay can always be recycled)
'Cracks, huh? No problem. I can fix it."
Also see earlier related post on "Paperclay and Cracking."