Important: Safety Warning
This page describes trade processes that use dangerously toxic materials such as cyanide solutions, mercury amalgams, or strong acids. These processes require training, equipment, and precautions far beyond what the text describes.
This information has been left in for historical purposes but should not be acted upon.
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Indigo Blue
Dissolve indigo in sulphuric acid, and dilute with pure water to the required shade of color; filter as directed in No. 2648.