Hi Threehearts,
I work for an interactive science centre in Australia, and I've done Slime Shows (including
DCL style Flubber) for kids of all ages and abilities. The borax in the flubber forms stable cross-links with the PVA glue... basically, this means the borax pretty much stays put within the slime. After you have made the Flubber, you can even rinse it under the tap (faucet!) to remove any borax that HASN'T linked into the slime.
If you're still worried about the borax, another easier and completely non-toxic slime is easily made from cornflour (cornstarch) and water. I've just cut this from the Slime Show script I wrote last year...
Mix a whole box of cornflour with about 300ml of water; dont measure it, add it bit by bit as you mix (slowly, and using ones hand is best) with food colouring (a very small bit; you end up with terribly 80s pastel colours, but it means the slime doesnt stain hands (or other surfaces!) The slime is done (i.e., stop adding water) if your finger doesnt enter the slime when poked sharply, but sinks in when poked gently. The slime can be stored in an airtight container in the fridge, mix thoroughly when you take it out again as it tends to separate. Chuck it out when it starts smelling funny.
Cornflour choice depends on personal preference; some being made of actual corn, some being finely ground normal wheat flour, each with their own pros and cons. Usually you want to avoid the cheapest corn kind, which is sometimes even yellowish in colour and coarser ground than usual.
Water options: just not hot. I have experimented with chilled vs. tap vs. warm, but found little difference. Boiling hot water will cook the flour, denaturing the protein.
Food colouring options are also manifold - there are arguments both for and against the traditional green. Personally, show some originality! Yellow, for example, allows the slime to masquerade as custard, leading to amusing prop-eating gags. (Mmm, slime. Feel free to eat it; it will do thee no harm, being just flour and water. I feel eating a sample lends credence to the assertion it is made from what we say it is, i.e., no nasty chemicals.) Other colouring options exist, such as using a small amount of paint.
The science - As mentioned above, cornflour slime displays thixotropic characteristics. Microscopically, cornflour has large molecules of irregular, jagged shapes. These can flow over one another when water acts as a lubricant between them. When pressure is applied (stirring or poking or otherwise man-handling the slime) the water is forced out from between the molecules, reducing lubrication, increasing friction, and the molecules jam up.
Wow, this is a long reply! You can see this is my field, right?
