The dehydration of an opal specimen was investigated
by thermogravimetric analysis (TG) in powder and bulk forms. The change in
geometry resulted in a significant difference in the temperature range in
which dehydration occurred with peak temperatures in the differential TG (DTG)
curve for the hand ground opal at 203°C and for the bulk opal at 340°C.
This difference was attributed to time taken for diffusion of free water in
the bulk opal to the specimen surface prior to evolution as a registered mass
loss. A model was proposed to account for the diffusion of water and was used
to estimate the diffusion coefficient.