Interesting to read about Numi's experience with boiling
polymer clay. There's actually been a lot of discussion (and controversy
--and hard feelings) about doing that in the clay community over the years.
The main concern is that all
the clay in a piece won't get completely
polymerized during the curing period. That means it could leach out unpolymerized oily plasticizer over time, getting onto furniture or other porous items, or could get sucked on by a toddler, etc., and also that it could result in a weaker
Originally the brand of European polymer clay called Cernit used to come with instructions for boiling so Scandinavians and some other Europeans did that, especially for their beads (not commonly known in the US).
There are a lot of factors that have come to light though, or that have changed over time, that can make a final answer more problematic.
First, how fast a thermosetting
plastic (the type of plastic that polymer clay is) will polymerize has to do with both the temperature and
the length of curing (they're interdependent). Iow, polymer clays can be thoroughly cured at lower temps than suggested, but they will take longer
to do it (sometimes a lot longer)...there's actually a formula for that relationship between time and temp required.
Second, the exact types of plasticizers/etc that are used in a particular brand or line of polymer clay, as well as which formulation
is currently being sold--which has changed for some clays several times-- also affects the rate of polymerization.
Those also affect the degree
of heat that a clay can take without darkening or scorching.
Boiling has one good advantage that way --because it keeps the temp lower, the clay will
darken less, but that also results in a lower temperature than has been recommended by manufacturers for some of the clays (265-275).
formulations of the 2 Fimos and Cernit have much lower recommended temps than the versions of those clays that may still be in US stores, but those newer versions are also weaker than their previous incarnations after curing.
Boiling may be better for small items
than larger ones though because of the amount of time needed for the polymerization to be complete all the way to the center of the piece. The Scandinavians generally boiled their beads
, at least back then (those were small items though, and they were also rounded
items since projections could be distorted with this method).
Boiling larger items though, especially for shorter times, can result in the clay feeling
hard, and even being completely polymerized on the outside
, but the items won't be polymerized all the way through unless boiled for a lot longer (and over time they'll probably have the problems I mentioned at the beginning).
Garie Sim (polymer clayer and experimenter) has also shown that it's quite possible to boil polymer clay in a microwave under water
...(though never try to cure
polymer clay in a microwave without the water!
(For anyone interested in more details on all these things, there's more info on this page at my site:http://www.glassattic.com/polymer/baking.htm
...look under the category Other Ways to Cure Polymer Clay