Cold porcelain is a non-edible air-drying modeling material. Although various brands are available commercially premixed or in dry form, variants of cold porcelain are fairly easy to make at home. Made with cornstarch / cornflower and PVA (white) glue, cold porcelain is usually mixed roughly one to one (1 cup cornstarch to 1 cup pva) with a small amount of glycerine, baby or cooking oil or facial cream added to the mixture at a ratio of roughly 2 teaspoons oil to 1 cup of cornstarch. These oils are added to make the mixture smoother and to try to reduce cracking as the paste dries. Some recipes suggest the inclusion of stearic acid (available from candle making suppliers), which will help thicken the cold porcelain as well as help it hold its shape.
The cold porcelain mixture is made by heating the ingredients either in a microwave (bit by bit!) or on a stove (using utensils which will never be used for food!) until it reaches the consistency of stiff mashed potatoes and can be gathered easily into a ball. Once it reaches this stage it is allowed to cool, then kneaded and stretched until it forms a smooth elastic paste. It is best set aside for 1 day, carefully wrapped, before it is used. It should not be refrigerated. A range of different recipes are available, the finished paste can be highly variable depending on the type of glue used (use a good quality pva glue) and the humidity of your working area.
Barrowdene Miniatures has a great cold porcelain recipe and tips on handling and coloring the material from miniature flower specialist Diane Gould. You can find Diane Gould ( Harfield) cutters (useful for 1:12 scale cold porcelain and polymer clay flowers) at her daughter's site, The Pedlar's Tray.
You can also find a range of recipes for cold porcelain on the Cold Porcelain Tutorials Site as well linked from The Glass Attic. Cynthia Howe Miniatures has a recipe and instructions for making cold porcelain as well.
How Long Does Cold Porcelain Last?
Cold porcelain paste is often combined with preservatives to try to extend its working shelf life (and make sure it doesn't mold), which is at least 1 week without preservatives under normal household temperatures if the material is properly wrapped. Preservatives in recipes include witch hazel, lemon juice, citric acid powder, and oil of cloves. Sodium Benzoate is used in some recipes. Some earlier recipes include formalin/formaldehyde as a preservative, which should be avoided as it is hazardous to health. Check commercial porcelain clay mixes for the preservative ingredients before purchasing!
Cold porcelain should be wrapped in cling film and stored in an air tight plastic bag/container in a cool area after mixing. I should not be stored in the fridge or freezer, as this may affect the PVA glue. Some recipes which contain glycerine will store in the freezer for reportedly up to 3 years. This may be affected by the brand of PVA glue you use, so experiment with freezing your favorite recipe.
Remove only small amounts from your storage container as needed and work color into the amount you have removed. Excess of any color mixture can be wrapped in cling film and stored. Some coloring agents can reduce the shelf life of cold porcelain by drying it out faster, so avoid mixing in color (including white) until you are ready to work with your cold porcelain mix.
Coloring Cold Porcelain
When it dries, cold porcelain is a semi-transparent material, so white acrylic (tube acrylic) or a white dry paint pigment (titanium white) should always be added to any clay you want to be more opaque. The mixed clay can be colored with paste or powder food colorings, watercolor paints, acrylic paints, powdered paint pigments or pastels/chalk dust. The use of oil paints as tints is possible, but not advisable for material which will be handled without gloves for modeling.
One of the delights of working with cold porcelain is that it shades beautifully, so mix shaded colors and apply dry brushed tints to almost dry clay (pan pastels work well) to give items delicate shading.
Once thoroughly dry, cold porcelain can also be painted.
Working with Cold Porcelain
Cold porcelain resembles sugar pastes and fondants in its working characteristics. It is fairly soft and does not hold sharp detail although it can be shaped into very thin sections (flower petals) with some transparency. It 'frills' and waves easily along fine edges so for larger flower miniatures it works well. There are some miniaturists who use this material for small scale flower work (Diane Gould (Harfield) is probably the best known This is not a material which can be layered for 'caning' like polymer clay. Most items made from cold porcelain are fairly small as cracking becomes a problem with larger models. The material is easy to layer and assemble using pva glue.
When working with cold porcelain you will want to have on hand standard modeling tools, as well as texturing tools, a container of cornstarch/cornflower to dust your tools to prevent sticking, and some hand cream or baby oil, to coat your hands or tools to prevent sticking. To blend successive layers of cold porcelain or 'join' parts you may need to wet the main section with a bit of water or sand it slightly to give it tooth (if it has dried) before you can add on the additional layers.
Cold porcelain requires at least 24 hours to air dry. Items which are thicker than 1/4 inch (6mm) may require much longer or may be subject to cracking.
All forms of cold porcelain clay will shrink, usually between 15 and 20 % as they dry. Test your mixture to determine how much shrinkage you can expect before you work on larger pieces or you may run into cracking as the clay dries faster on the outside.
Storing and Displaying Cold Porcelain Models
Although cold porcelain is very hard and sturdy when dry (depending on the thickness of the pieces) there are some storage and display considerations. As cold porcelain is based on starch, it may be attractive to insects and should be kept away from any other art pieces which could suffer insect damage. The preservatives used to keep the clay from going moldy in its working state will not protect it against insect infestations.
Take care how and where you store or display cold porcelain items. As cold porcelain will absorb slight amounts of moisture and is not entirely waterproof, it may need to be protected from moisture or humidity once thoroughly dry using some type of waterproof varnish (acrylic varnish). A thorough coating of acrylic paint over the surface may also act as a protectant. Cold porcelain models should not be subjected to humid or wet environments. If possible, display cold porcelain items in protective glass or acrylic cases, which include a small drying agent package of silica gel to absorb any humidity.