Paper Fiber in Clay
You can also add paper fibers to your clay body or use paper clay. This Porcelain Paper Clay helps to prevent the clay from cracking when drying.
If you want to make your clay water-resistant I always recommend using either a clear polyurethane varnish or an acrylic sealer. Both of these create a non-porous transparent layer that seals your clay project. They can be brushed or sprayed on and usually come in either a matt, satin or gloss finish.
Cracking is normal in air dry clays: it's caused by shrinkage because of the loss of the water inside the clay body. Cracking in air dry clay is typically caused by sculpting over an armature or using a lot of water, either to mix the clay or to help it adhere onto a previous layer.
In general, cracks result from stresses in the clay. There is always some stress in clay because of the fact that it shrinks as it dries and when it is fired, and it also expands and contracts during firing. Sometimes the stress is too much for the clay to handle and it cracks.
Vinegar is also used in clay bodies to increase acidity to improve plasticity. The acid works to neutralize sodium ions (from water, leaching feldspars) that tend to deflocculate the clay. Excessive acid may tend to dissolve more feldspar or nepheline syenite negating the effect.
This can cause the clay to dry out, crack, and shrink. To prevent this, you need to hydrate the clay regularly, either by spraying it with water or using a clay conditioner. You should also cover your clay model with a damp cloth or plastic wrap when you are not working on it, to prevent it from drying out.
Once you place your clay in the oven, maintain a constant temperature to prevent cracks from forming. Many countertop ovens lack proper seals that ensure a consistent temperature. The absence of seals can cause your oven's temperature to swing wildly, with your oven's temperature dipping down and then spiking up.
For more problematic cracks or broken pieces, put one part toilet paper to three parts clay in a bucket or container, cover it with water and let it soak overnight. Pour the mixture into a blender and mix it up thoroughly. Allow the mixture to dry to the consistency of heavy slip, and use it to repair the broken clay.
In a bowl, mix a ratio of 80% PVA glue to 20% water to create a creamy consistency, and then coat your fully dried clay piece, and allow the sealer mixture to dry. Once you have applied your PVA glue mixture or acrylic medium, you can then apply 3 coats of acrylic paint to seal your work.
The short answer is no. It's not necessary to seal polymer clay pieces because polymer clay is both durable and water resistant. Depending on the look you want to create, you may only want to sand and buff your polymer clay pieces. Only when you use additional effects such as resin is it necessary to seal the piece.
Once you are done painting the clay, you need to seal it to protect it and make it waterproof. If you do not seal the paint, it will eventually begin to peel off, or if you get water onto it, the paint can be washed off.
Petroleum Jelly
The second polymer clay tip, vaseline does miracles to clay.
Vaseline (petroleum jelly) is a much better option to soften hard polymer clay! Put a small amount of Vaseline on your fingertips and rub it into a small ball of clay. Cut the clay into smaller pieces and then squash the pieces together. Add more Vaseline, flatten the clay by rolling it out, then knead it again.
Make Sure That the Surface of Your Clay Is Smooth and Dust-Free. Once your clay has dried, it may still have some bumps and cracks along its surface. Before you get to painting, you should smooth these imperfections out with low-grit sandpaper.
Between 480–700ºC chemical water (“water smoke”) is driven off. Upon cooling, cristobalite, a crystalline form of silica found in all clay bodies, shrinks suddenly at 220ºC. Fast cooling at this temperature causes ware to crack.
Oven-bake clay can be painted after curing. We recommend using water based acrylic paints. Before painting the piece – Test a hidden section of the project first. Apply paint and allow to dry completely.
No, you don't need to use a sealer with polymer clay. Baked clay is water-resistant and durable, even without a glaze. For a polished finish, sand and buff the cured clay using our wet/dry Sculpey Tools™ Sandpaper Variety Pack.
You need gap-filling adhesive/touch-up glaze, an emery board, and soap and water. Wash and dry chipped surfaces. Apply glue. Complying with the instructions in the product information, fill in chips or cracks and rebuild any lost material to protect the mug or object from further chipping.
Most pottery that was broken can be fixed! If you have all the pieces and a high-quality adhesive, you can put it back together. Repairing pottery requires high-quality ceramics glue. In cases with valuable pottery, you may want to take it to a professional.
If your crack is very very small, you may be able to hide it by filling it in with glaze. However, the glaze firing is hotter than the bisque firing, so there's also a good chance your crack will continue to expand.
Crazing Is a Network of Hairline Cracks in The Glaze of Your Pottery. These Cracks Appear After Firing, And They Occur When the Thermal Expansion of Your Glaze Is Not High or Low Enough for Your Clay Body. Applying A Thinner Coat of Glaze and Cooling Your Piece More Slowly Are a Couple Of Ways To Prevent Crazing.
Some clay suppliers add hydrous magnesium sulfate (Epsom salts) to their clay mixes to improve plasticity and stabilize bodies against the thixotropic and spontaneous softening effects of certain soluble compounds in the mix (e.g. from soda feldspar, nepheline syenite).