The
clear liquor only is used. A piece of cotton dipped in it, and exposed
to the air, quickly turns blue by absorbing oxygen, and is thus dyed.
The best proportions for the indigo-vat are, for cloth dyeing, 4000
parts of water, 40 of indigo, 60 to 80 of copperas crystals, and 50 to
100 of dry slaked lime. The usual plan is to put in the water first,
then add the indigo and copperas, which should be dissolved first, and
finally to add the milk of lime, stirring all the time. Artificial
indigo has been made from coal-tar products. The raw material is a
coal-tar naphtha called toluene or toluol, which is also the raw
material for saccharin, a sweetening agent made from coal-tar. This
artificial indigo is proving a formidable rival to the natural product.
Orchil paste, orchil extract, and cudbear are obtained by exposing the
plants (species of lichens) containing the colouring principle, called
_Orcin_, itself a colourless substance, to the joint action of ammonia
and air, when the oxygen of the air changes that orcin by oxidising it
into _Orcein_, which is the true red colouring matter contained in the
preparations named.
Pages:
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149