Pros and Cons of Homemade Clays

Pasta Francesa and Cold Porcelain Clay vs Commercial Polymer Clays

Cornstarch Based Clays

Marie: Thanks for the [cornstarch clay] recipes. I had been planning to look for a recipe to make this sort of clay and stumbled upon this one by chance. I am so glad you and the others have shared this. I can't wait to try it!

Eileen: Wow Marie, you’ve put me through my paces with your questions. But you’ve asked some really good ones and I enjoyed researching them.

I had to contact my friend Raquel in Uruguay for an update on her work with vinegar as a substitute for formaldehyde in pasta Francesa as well as consult several chemists regarding what makes a polymer a polymer. LOL.

Pasta Francesa (PF)

Marie: I have read the third recipe called pasta Francesa.

I would like to know how much vinegar to use in the recipe?

What kind of vinegar is used in recipe?

Will the clay still turn to polymer with vinegar?

Eileen: To answer these questions, I asked my chemist friends to analyze both recipes #1 (Fiona’s Cold Porcelain Clay or CPC) and #3 (PF). I asked for a comparative analysis because CPC already includes an acidic ingredient, lemon juice.

I found their analysis very interesting.

Both CPC and PF include 1 cup of cornstarch for body and 1 cup of PVA glue as plasticizer (or polymerizer). So both recipes create true polymer clay.

CPC includes 2 tablespoons oil for creaminess; while PF uses1 ½ tablespoons cold cream for this purpose. Close.

The small amounts of glycerin and formalin in PF are additional plasticizers (formalin being an especially strong one).

Formalin is a solution of about 40% formaldehyde in water.

[Interestingly, formaldehyde is the plasticizer used to make Bakelite, the very first synthetic plastic invented - and still in use today.]

Leaving the formalin out of PF reduces its tendency to polymerize.

The lemon juice in CPC is a catalyst that triggers polymerization.

QED, if you reduce the tendency to polymerize by leaving out formalin, add something acidic to offset the reduction by triggering polymerization.

Lemon juice is up to 8% citric acid. White vinegar is about 5% acetic acid, so you need 1 ½ tablespoons of white vinegar for the same effect.

One final note: the PF recipe includes ½ cup water which allows you to make less stiff dough but requires more time to cook.

Marie: How long does the clay last?

Eileen: The shelf life varies from one week to one month depending on the ingredients, storage temperature and humidity. Store it in a cool dark place.

Marie: How to cure it?

Eileen: All the clays in my list of Cornstarch Clay Recipes air cure, but drying can be accelerated in a warm oven.

Marie: How to add color to it?

Eileen: You may add oil paint either when making the PF clay or when you are ready to model it, but adding color before storing reduces the clay’s shelf life slightly.

Adding too much paint alters the recipe and could cause the dough to dry out or crack as it dries - if you want intense color, reduce the amount of cold cream.

Projects may also be made with uncolored clay, then painted after curing with acrylic or oil paints.

Marie: And have you tried the recipe using the vinegar?

Eileen: Yes I have - just for you, Marie ;-)

I’m no Fiona, but here is my first effort.

The formula (substituting 1 ½ tablespoons vinegar for the formalin) worked. The clay dried to a nice translucent plastic. That’s the good news.

The not-so-good news is that there was 35% shrinkage (by weight) as the figure cured. The shrinkage caused some cracks to appear. I filled those with Liquitex Super Heavy Gel PVA.

Victorian Salt Clay

Marie: Also is the second recipe for the [Victorian Salt] clay similar to Cernit? I asked because I saw Cernit on Fiona’s site of the beautiful [CPC] figurines.

Eileen: Not at all. Cernit is a brand of commercial polymer clay, comparable to Kato. Cernit and Kato are the stiffest and strongest polymer clays. They are about as far from salt clay as hard candy is from sponge cake.

Marie: About Kato and Cernit - Kato is stronger than Sculpey and Fimo?

Eileen: Yes, Kato and Cernit are stronger than Sculpey and Fimo.

Marie: And does this clay have the same shrinkage as the others above?

Eileen: Cernit and Kato have less than 2% shrinkage. You can expect salt clay to shrink about 40%.

Refer to my Clay Shrinkage Chart here.

Marie: How close is this recipe to the actual polymer clay that is sold in craft stores?

Eileen: Here I go again, but cornstarch salt clay is about as close to commercial polymer clay as a slice of bread is to a nylon jacket.

Commercial polymer clays are comprised primarily of Polyvinyl Chloride (PVC) and Phthalate plasticizers. Other examples of synthetic polymers are Bakelite (which is a phenol-formaldehyde resin), Kevlar, Mylar, Nylon, Orlon and Teflon.

Examples of natural polymers are DNA, RNA and cellulose.

Marie: How close is pasta Francesa to the actual polymer clay that is sold in craft stores?

Eileen: Pasta Francesa and the cold porcelain clay are chemically quite similar, especially when you make the pasta Francesa with vinegar rather than formalin. They shrink a lot by weight and that causes cracks on larger sculptures.

The polymer clay in craft stores is made of PVC and phthalate plasticizers. Phthalate plasticizers do such a good job of plasticizing the PVC that there is very little shrinkage - no more than 2%.

The shrinkage with home made polymer clays is the main problem. Consistent consistency is another.

If you undercook the clay, it is soft and easy to work but shrinks more.

If you overcook it, the clay is stiff and hard to work but may shrink as little as 20% compared to 35 to 40%.

Marie: I believe the homemade polymer clays are probably better because they’re homemade. I think most all things are better homemade.

Eileen: It depends on what you’re making.

The homemade clays are fun and great to do with kids (no formalin please!).

If your projects are simple and/or small, like Kay Davies bread clay beads, homemade clays are fine and save you money.

I prefer the commercial clays for consistency, less shrinkage and durability. They’re much better for detailed pieces like molded shapes, sculptures and millefiore canes.

Marie: Well of course we can't make everything at home but when we can we should.

Thank you. I appreciate the interested you've taken in my questions. Tell the chemists thanks and everyone that helped with the questions I had.

Have a great day! And thanks again!

Eileen: It was a pleasure "chatting" with you, Marie.

Be sure to read my other articles about Home Cured Modeling Clays