Sentence:  the borax <WSD>bead</WSD>;  the iron <WSD>bead</WSD>, etc. 
Meaning of the word marked with <WSD> tags:  A glassy drop of molten flux, as borax or microcosmic salt, used as a solvent and color test for several mineral earths and oxides, as of iron, manganese, etc., before the blowpipe.