Bakelite

In answer to Mary Todd Glaser's query on Bakelite, this plastic, a
phenolic compound, was the first or one of the first thermosetting
resins (plastics) and was developed soon after the discovery of
celluloid (cellulose nitrate) which is a thermoplastic resin.  The
difference is that thermoplastic resins become soft and melt when
exposed to heat, while thermosetting plastics do not generally
become soft or melt.  Rather, they will char or burn before melting.
The classification "thermosetting" refers to the fact that these
compounds liberate heat during curing or polymerization.

The import of this information is that early 20th Century
collectible plastic jewelry was typically made from either celluloid
or bakelite.  A hot needle can usually be used to differentiate the
two if you can tolerate a small spot of damage on the reverse, in a
crevice, or in the string hole of a bead.  When a hot needle is
applied (hold it in a pin vise and heat over a match or gas burner),
both will smell of "burning plastic" but the bakelite will smell
more like phenol.  Many opaque bakelite objects, however,
particularly those used for electrical parts or cookware handles,
were filled with wood flour or other fillers.  The characteristic
odor of phenol given off by these mixtures can easily be confused by
the smell of the burning filler.

The real test is that celluloid will melt around the needle but
bakelite will not melt, although it may sizzle, smoke, and a cavity
or depression may be formed.

Another useful test that is somewhat less controllable is the use of
a solvent such as acetone.  Wetting a small area of a celluloid
object with acetone will soften, dissolve, or make the surface
tacky.  The effect of acetone on bakelite will be minimal, either
none or a clouding of the surface.  Immersing a small scraping or
chip in acetone will also tell you if the resin is soluble.

There are some phenolics that are somewhat thermoplastic in that
they will become more flexible when heated.  They do not melt,
however.  An example of these is the types of Formica or other
high-pressure laminates that can be heated to make a curved splash
and front edge for counter tops.

Other plastic resins that were introduced later may confuse the
issue. Early "Lucite" was a styrene resin, a thermoplastic which
behaves much like celluloid in the tests described above.  Other
thermoplastic resins include the acrylics (plexiglass, B-72, etc.),
PVA and many others that we use as adhesives because they dissolve
in solvents. Other thermosetting resins include epoxies and some
2-part acrylics.

There are so many formulations of plastic resins that these simple
tests are reliable only with other supporting historic information.
Good Luck.

Geoffrey Brown
Curator of Conservation
Kelsey Museum
University of Michigan
313-747-0439

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                  Conservation DistList Instance 9:14
                   Distributed: Friday, July 28, 1995
                        Message Id: cdl-9-14-002
                                  ***