flexible glue
Generally, an ordinary animal glue containing GLYCEROL or SORBITOL , the
softening effect of the latter supposedly lasting
longer than the former. These softening agents, or
"humectants," are hygroscopic and may, under
certain conditions, absorb so much moisture as to
promote the growth of mold; consequently a mold
inhibitor is generally included in the plasticizer.
The origin of "flexible" animal glues is unknown,
but they were certainly in use since early in the
20th century. Their use in bookbinding, however,
has declined sharply in recent years in favor ofINTERNALLY
PLASTICIZED synthetic adhesives, such as the
RESINOUS ADHESIVES
. (198 , 236 )
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