One of my biggest challenges in sculpting this last year has been to prevent cracking.Most of my initial sculptures cracked.
First–why do water-based sculptures crack? I don’t know all of the reasons, but I’m pretty confident that I know some of them after having it happen so many times in my first year of learning to sculpt.
Many sculptors just skip the whole problem of clay sculptures, but water-based clay is cheap and molds are not. So for the frugal, mastering this problem is a great money saver. I’m not at the point where my sculptures require molds and casts. I also want to keep a lot of my originals for sentimental reasons. So sculpting in wax-based clay won’t work in that situation.
Wax-based clay is perfect for practice and for skilled sculptors who plan to make a mold and casts off of their original. When I get to that stage, I’ll do it. 🙂
Heat : The sphere within a cube cracked because I left it in a hot car. There was no fixing it after that! If the outside of a sculpture dries faster than the inside, you will certainly end up with cracks.
I solved this problem by following the advice of one of my sculpture books to place your sculptures in a cold dark corner somewhere and WAIT. I was way too impatient. I wanted to see my amazing or not so amazing creations. Haha! I’ve learned to wait and had much better success. I no longer leave sculptures up on a table or desk to dry… especially near a sunny window!
Armature being too close to the surface: This happened a lot and particularly on my first abstract. In fact, all of the sculptures I made with armature cracked. Mostly, this seemed to be when my armature wire was too close to the surface.
I think in the case of the donut that not only was the armature too close to the surface, but also the wood dowel rod probably soaked up the moisture out of the clay. The lessons from the NMA, assume you are using wax-based clay. That’s a smart idea because they are just practice lessons. You aren’t supposed to necessarily be saving those for posterity. 🙂 However, I wanted to save them as teaching tools later on, since I’m pursuing an art education degree.
Uneven Drying: Even if you avoid the problems above, you still have the very obvious problem of thick sculptures drying unevenly. By some miracle, my bell pepper never cracked. I’m so happy about that as I think I would have been discouraged after all the labor I put into it. I believe I sealed it before it actually fully dried, which may have prevented it from drying too fast on the surface.
However, in order to get my ‘Trojan Women’ to not crack, I finally tried several things:
- Dry the sculpture in a cool, dry, dark place. It will of course crack if it gets too hot and dry on the outside before the inside has cooled.
- Sculpt small parts of the sculpture last, so they don’t dry before the main body. That was part of the problem with my Mother and Baby sculpture. I did the hands right away before I’d figured out the rest of the sculpture. They became extremely fragile and cracked immediately. Save hands and feet for last. 🙂
- Hollow out the sculpture. This is obviously necessary if you are going to fire a piece, but I think it also allows the sculpture to dry evenly if it’s a uniform thickness on all sides. I now hollow out every sculpture I can, once it’s ‘leather dry’. That means solid, but still retaining moisture.
- Use a soft armature inside for things on a stand i.e. my large Helen of Troy statue, or the sphere I hollowed out as practice. I don’t mean that the material you build your armature out of should actually be soft. You want it to be very firm in fact. I only mean not rock hard. As the statue dries, it shrinks. If there is some ‘give’ inside, then I think it’s less likely to crack. I stuff heavy paper tightly around my armature support to build up the general shape. Be sure to cover your paper with a plastic bag and tape in place tightly, so the paper doesn’t soak all the moisture out of your sculpture prematurely. I wish I’d pulled out the armature paper and bags out of my Helen of Troy while she was only ‘leather hard’. She now has stuff permanently stuck in the upper part of her. Since I’m not firing her, that’s not a big deal.
- Remove Armatures by slicing large sculptures open i.e. Busts I watched a video on how to sculpt a bust out of water based clay. They actually cut the sculpture open before it dried and pulled out all of the paper and plastic bags, then scored it and glued it back together with slip. This is a requirement for firing.
- Use Armature wire only as necessary, and be sure it’s deeply buried beneath the surface. Also make sure it’s thin enough to be in proportion to the sculpture. The wire I used for my Mother and Child sculpture was overkill. Armature wire makes sense when you are sculpting with wax-based clay. It never hardens. But if you can just sculpt without it, you will find you are much more successful with the the drying process. Obviously, many sculptures require an armature, so don’t skip it if you need it.
- Of course, use the right kind of clay for the project. There is a place for water-based clay and a place for wax-based. My donut would have not cracked if I’d just used the right kind of clay for a ‘lesson project’.
My first two figurines of the ‘Trojan Women’ cracked and had to be repaired. That’s when I finally tried hollowing them out!
The sculpture above is 2 feet tall and made of water based clay. She ‘almost’ cracked, but she was built on a tall armature of paper and plastic bags. At the end I removed most of the stuffing. She’s completely dry!
As you can see, my armature of heavy paper covered in plastic bags and taped, was not quite even, so there are some splits internally (not cracks), but I do see one crack. It was not external but it certainly is internal.
My guess is that without the support of the armature stand, it simply was too much weight. I’m still learning! My plan, for the rest of the larger replicas of the Trojan Women set, is to use self-hardening clay. (Not air-dry clay) It is almost as hard as if it was fired, and that’s probably something I need to consider for really large statues.
I probably haven’t solved all of your problems with cracking sculptures, but these methods will help a lot!