I had been using crayola air-dry clay for some time to make sculptures. However, it seems after a few years the clay begins to disintegrate, if you will, with the clay gradually breaking out into a white powder. This has been quite a disappointment for me, as I used to love sculpting with clay but tired of seeing my creations disintegrate before my eyes. The degradation process is hard for me to describe, so I'll add a picture of a sculpture I did six years ago for a school project which is now falling apart. I was wondering if anyone has any idea as to why this would happen and could help me avoid it in the future? I would really love to start using clay again in my projects but don't want to have to fire it. Are there any really good, quality air-dry clay brands that are known to last over time? I am curious about paper clay as well and am wondering if that would be a better choice to work with. Thank you very much! I'm hoping you can help me figure this out. Margaret
Crayola Air Dry Clay Composition
Crayola Air Dry Clay is a proprietary formulation and Crayola is very secretive about the composition. They say it is a natural earth clay. From the MSDS, it's clear that it contains some amount of organic material that might be added to control handling characteristics or act as a binder. They say not to try to bake it or fire it, so the composition isn't something that will stay intact under heat or fuse together like pottery clay. That probably means that it contains more than a trivial amount of non-clay material and relies on something other than the earth clay as a binder.
Why it disintegrates
Earth clay is composed of tiny, plate-shaped particles that form weak bonds with each other. When the clay is wet, a molecular film of water between the plates lets the plates slide around, allowing you to shape the clay. When the clay dries out, that film evaporates and the plates can't slide, making the clay rigid. The bonds between the particles are strong enough to hold the clay together and retain the shape it's been given, but the clay is brittle and fragile.
With pottery clay, if you raise it to a very high temperature, the clay particles melt and fuse together like glass, making it one solid object rather than stuck-together particles. An air dry clay based mainly on earth clay remains just stuck-together particles. It needs to contain some other ingredient to act as a binder or the result would be too fragile to hold up to handling.
With earth clay, as the water evaporates, it leaves pockets and micro-cracks throughout. Once dry, the clay expands and contracts through changes in temperature and humidity. It's also exposed to environmental vibration. These put additional stress on the cracks and they spread, weakening the sculpture. If cracks around an area meet, chunks fall off.
So if the Crayola product behaved like almost pure earth clay and started to deteriorate, what falls off would be chunks and flakes. It wouldn't break down into powder. The frilly nature of the damage in the picture also isn't characteristic of earth clay disintegration. It looks like the product relies on another ingredient as a binder, and that is breaking down. Or as Elmy mentions in a comment, the product may contain mineral salts that are recrystalizing and causing or contributing to the clay breaking down.
I suspect the objective of the Crayola product is a kid-safe material for play that doesn't need to last forever. In that sense, it may be a poor material for serious crafting of objects you want to last. Many other air dry clays have compositions that should last a very long time.
You can buy pre-made air dry clay, but there are endless recipes online for clays you can make from inexpensive household ingredients and readily available materials that are at least as good, customizable in their characteristics, and used by professional artists. Some of the best air dry clay recipes I've found are at Ultimate Paper Mache, which would be a good starting point.
There are a number of general types of DIY air dry clay. Some are basically "dough", based predominantly on flour or starch and using the stickiness of these materials when wet as the binder. These generally don't hold up well over time. Another type uses glue as a binder and combines various ingredients to fine-tune the characteristics. This provides much more control over the nature of the clay, and the result lasts a very long time. So this type is generally what's used for more serious art and I'll focus on that.
The question asks about paper clay as a distinction from air dry clay. This isn't really a useful distinction. With "glue-based" clays, there are many different types of ingredients that can be included (and in varying proportions), to fine-tune the characteristics of the clay. Some amount of paper pulp is often included because the fibers add a lot of strength and resilience. But if it's made right, you probably wouldn't know from handling the clay that it contains paper fibers.
Paper pulp alone can be molded into objects. Adding glue creates a kind of moldable paper mache clay. You can make that paper mache clay more clay-like by adding a thickener like talc or starch. You can reduce shrinkage and make the result harder by adding something like calcium carbonate powder or drywall compound. You can make the air dry clay predominantly out of the other ingredients and include a little paper pulp to add strength. Paper is just one of the optional ingredients you can include to give the clay the characteristics you want.
A good way to develop a recipe optimized to your own needs is to start with a few different popular recipes that are based on different ingredients and ingredients in different proportions. See what you like and don't like about each in terms of how they handle as clay and the hardened result. Then mix and match and substitute between the recipes, and adjust proportions, to find what works best for you. You can make small batches and test things side-by-side.
Elmy makes a good point in a comment that disintegration can stem from recrystalization of salts. There are a number of recipes for DIY air dry clay that use salt as a preservative. This is very common with recipes that are flour-based. Other soluable mineral salts, like baking soda, are used in significant proportions in some recipes. I've generally not liked these recipes because of their consistency or tendency to shrink a lot, as well as not holding up over time. If you need your work to last a long time, avoid recipes that contain a significant proportion of soluable mineral salts or that use salt as a preservative.
Degradation over time
Otherwise, with DIY air dry clay, the main way you get disintegration is if the clay contains a substantial proportion of organic material like paper pulp or corn starch, has no preservative and no protective paint coating, and is in a humid environment. In those conditions, mold or bacteria can eventually attack the organic ingredients.
DIY air dry clays, especially those that use glue as a binder, behave differently from earth clay. They can have a tendency to crack during hardening if certain methods aren't followed, which I'll cover below.
They usually contain a lot of water. As the water evaporates, the clay loses mass and shrinks, and it does that from the outside in. The outside shrinks faster than the inside, and your sculpture isn't a TARDIS that can be bigger on the inside than the outside.
If the clay recipe has high shrinkage and the outside shrinks too much faster than the inside (the thicker the clay, the more effect), the weakest area gives way. You end up with one or more big cracks. With earth clay, big cracks would be a sign that widespread serious deterioration has already occurred. But with DIY air dry clay, these are actually good in that they relieve the stress on everything else, making the sculpture stronger and avoiding damage elsewhere (and they are easily repairable).
The clay hardens in two stages. Most of the shrinkage is in the first stage, when the excess water that makes it pliable evaporates. When this stage is complete, the object will feel hard and solid but the glue will still be weak. In the second stage, there's much less shrinkage and the glue develops its strength.
There are a number of helpful methods for working with air dry clays.
- Use a recipe less prone to cracking. These typically contain less water, more non-shrinking filler, and more glue. And as mentioned above, avoid recipes that use salt as a preservative or other soluable mineral salts as a significant ingredient.
- Knead the clay thoroughly when you make it, let it rest overnight in a sealed container, then knead it again before you use it.
- Work in reasonably thin layers, and minimize the amount of clay you need by using armatures and filler cores for large pieces. Thick clay can take a very long time to dry and is more prone to cracking.
- Slow surface drying during the first stage of hardening so it dries and shrinks at a more even rate throughout. This can be done by keeping the finished object in a container that isn't air tight but keeps the humidity high, or keeping it covered with slightly damp towels.
- If you do get cracks, you can fill them with more of the same clay and the result is strong.
I have some suggestions, since I stopped using air dry clay for similar reasons to yours.
Paper clay, or even plaster embedded in cloth, is fun! These will need armatures since they drape and harden. Armatures could be made out of coated cardboard, aluminum foil, used plastic bottles, wire, something that won't mold on you when wet. You really just need a bowl of water, your medium, and your base.
For my paper clay bird sculpture, I used recycled materials that could hold their shape under weight like coated postcards from the mail, plastic forms from packaging, and electrical wiring in plastic cords that I took apart. The clay I bought, but there are recipes if you google "paper clay recipes". I took my time going from the center and out to keep the weight balanced. Since everything is wet, you have to think of what has to be propped up to keep its shape while things dry. And then remember there's an underside to coat as well. Like paper mache, drying thin coats between layers helps it be stronger. Once it's dry it's fairly lightweight. I wish I had kept my bird, but I had it for several years and nothing happened to its form during that time.
Just as a side note, you could try using regular clay. Your construction will have to compensate for the fragility of clay. I've done this a few times for different reasons.
Things that could be done: block carving, sculptures with thick bases, erosion/time commentary, environmental commentary, fragile things on purpose... Painting could be done with slip glazes, non-fired the colors will be soft and pastel and very easy to scratch, which could be a look. The unfired clay could be polished. It does dry out and become quite hard, like a dried out block would need a sledge hammer to break it.