What's the chemistry of playdough?

December 16, 2006 8:02 PM

What's the chemistry of playdough? The recipe for basic playdough is: 2 cups flour, 2 cups water, 1 cup salt, 2 tsp cream of tartar. This plus a little oil and food coloruing + low heat and stir until congealed. What's the function of thee salt and the cream of tartar? The oil is obviously for lubrication the flour and water are what make the dough through the reaction of starch + heat.
posted by singingfish to science & nature (11 comments total)

Cream of tartar keeps the dough from crystallizing.
posted by iconomy at 8:11 PM on December 16, 2006

Oh, and the salt is used as a preservative to keep the dough from getting moldy.
posted by iconomy at 8:12 PM on December 16, 2006

The purpose of the salt is to form a matrix around the flour particles when it dries. It's the salt that gets hard.

It's fundamentally the same as what silicon dioxide does in clay when you fire it. There are particles of ceramic and particles of silicon dioxide all mixed together in clay in a suspension of water. When the clay dries, it is not held together very strongly. When you fire clay, you heat it up enough so that the silicon dioxide melts, and it cements all the particles of ceramic together.

Likewise, in Playdough, you've initially got a suspension of flour in saltwater. When the water dries out, it leaves the salt behind, which cements all the flour particles together.

(If I had to guess, I'd guess that the function of the cream of tartar is to make the salt water bond better to the surface of the flour particles.)
posted by Steven C. Den Beste at 8:31 PM on December 16, 2006

The cream of tartar might also make it so that the dried salt becomes amorphous instead of trying to crystallize.
posted by Steven C. Den Beste at 8:35 PM on December 16, 2006

If you mix flour and water together alone, and then let it dry out, you don't end up with powder, you end up with something relatively hard and a bit brittle (think dried pasta). If you take salt and mix it with water and let it dry out, you get a bunch of crystals, and it isn't particularly robust unless you end up with one big crystal (which you surely aren't when you make playdough.

I really doubt that the salt on its own adds particularly to the structural integrity of the mixture when it dries. I think it's more likely that the starch and gluten in the flour help glue the salt crystals together. It might influence interaction between protein chains in the gluten, but I think it's main function is preservative.

I imagine the oil helps a bit with the consistency.

I really don't know what cream of tartar does. It'll make things a bit acidic, which probably also has preservative effects, and might also modify the behavior of the gluten in the flour.
posted by Good Brain at 9:08 PM on December 16, 2006

The salt also serves as a deterrent to children who would otherwise eat raw or partially cooked dough. (This is actually an important factor.)
posted by acoutu at 9:35 PM on December 16, 2006

The amount of salt put into flour makes a difference to the texture because of how it influences the gluten development, so I'm assuming the same thing is happening in playdough.
posted by shelleycat at 9:50 PM on December 16, 2006

How much is a "little" bit of oil? I used to make this recipe for my students and wanted to share it with a teacher. But of course, over the years I have forgotten it. I know I am not responding to the post, but i would love to share this and I would need to know the amount of oil...
posted by cahlers at 6:09 AM on December 17, 2006

To answer cahlers' question, the first two Google hits for playdough recipe say 1 tablespoon of oil for 1c. flour/ 1c. water (so the above recipe would call for 2 T.)
posted by jessicak at 9:03 AM on December 17, 2006

Here's what Wikipedia says about the chemistry of the commercial product:

Play-Doh is a commercial modeling compound similar in texture to bread dough that has been sold as a children's toy around the world for a half century. Its exact makeup is a secret, but it is primarily a mixture of wheat flour, water, deodorized kerosene or another petroleum distillate (which provides the smooth texture), salt, a drying agent such as borax (which deters mold), an alum-based hardening agent, and colourings and perfume.

It is non-toxic, non-staining, and soluble in soapy water.

A perfume incorporating its smell has been created for its 50th anniversary.
posted by jamjam at 9:41 AM on December 17, 2006

The recipe my family has been using for >30 years calls for 2 TBSP of oil.
posted by acoutu at 9:45 AM on December 17, 2006