Some of these diagrams before you this evening show clearly the
fastness to light generally of the lakes formed with copper mordant.
This peculiarity of the copper compounds has not escaped the notice of
other observers. Dr. Schunck, for example, during the progress of his
research on chlorophyl, noticed the very permanent green dye which
this otherwise fugitive coloring matter gives in combination with
copper.
Then there is the assertion of practical dyers, that the use of copper
sulphate in dyeing catechu brown on cotton assists materially in
rendering this color fast to light.
The use of copper mordant with phenolic coloring matters is perfectly
natural. Some time ago, however, it was successfully applied, for the
purpose of rendering more permanent, to certain of the Congo colors on
cotton, e.g., benzo-azurine, etc., in the application of which,
metallic salts had not hitherto been deemed necessary.
Noelting and Herzberg have also observed that the fastness to light,
even of basic colors, e.g., magenta, methyl violet, malachite green,
etc., is increased by a subsequent treatment of the dyed fabric with
copper sulphate solution, although in many cases the color is much
soiled thereby.
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