In some cases it is preferable to collect sublimates on the surface of a Plaster of Paris tablet rather than on charcoal. Such tablets can be easily made by spreading a thing layer of the wet plaster upon a glass plate, the surface of which has been oiled. While the plaster is still moist it should be cut into rectangular strips measuring about one and one-half by four inches. After the plaster has hardened these can be broken out into the desired tablets. The material to be tested is placed in a small depression made near one end of the tablet and the heated before the blowpipe exactly as in the case with charcoal. The iodide coatings are especially marked on the plaster tablet. The important test are summarized in the table below.
Composition of coating
Color and character of coating on plaster tablet
Remarks.
Selenium Oxide. SeO2.
Red to crimson.
Volatile giving reddish fumes and characteristic odor.
Tellurium Oxide. TeO2.
Dark brown.
Volatile.
Cadmium Oxide. CdO.
Greenish yellow with brown both toward assay and at distance.
Nonvolatile.
Lead Iodide PbI2.
Chrome – yellow
Bismuth Iodide BiI3.
Chocolate – brown with underlying red.
Subjected to ammonia fumes coating becomes first orange – yellow then red.
