On the other hand, there are some silicates which are only
attacked with difficulty even by fusion with alkaline carbonates;
consequently, it is always well to have the substance reduced to the
finest state of division by careful powdering, as this greatly assists
the subsequent action. With very hard silicates, the grinding away of
the mortar in this operation will be perceptible; the foreign matter
thus introduced must not be ignored. Previously igniting the substance
sometimes assists the powdering; but it is best to use a steel mortar.
The particles of steel can be removed by a magnet, or, where the nature
of the substance will allow it, by boiling with a little dilute
hydrochloric acid.
The dried and powdered material is intimately mixed with four times its
weight of "fusion mixture" in a platinum-crucible or dish. It is then
moderately heated over a Bunsen burner, and afterwards more strongly
fused over a blast, or enclosed in a clay crucible in the wind-furnace.
The action is continued until the fused mass is perfectly tranquil. With
very refractory substances, the action must be long continued at a high
temperature. When sufficiently cold, the crucible is examined to see
that no particles of foreign matter are adhering to its outer surface.
It is then transferred to a five- or six-inch evaporating-dish, where
its contents are acted upon with warm water for some time.
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