Converting Blue Prints into Black Prints.

Manufacturer and Builder ?, 1893

The Revué de Chimie Industrielle says that the prints should be first passed through water acidulated with nitric acid, and thence into -
Carbonate of soda ... 50 grammes.
Water ... 1 liter.

In this the picture is changed to an orange tone, when it is removed and placed in -
Gallic acid ... 50 grammes.
Water ... 1 liter.

Being subsequently washed in water acidulated with HCl.

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