The Dyeing of Cotton Fabrics | Page 9

point of view, but simply playing the part of a
base or foundation on which the colour lake may be formed. In the case
of the dyes of the fourth group, there being no chemical affinity of the
cotton known for them, these dyes cannot be used in a successful
manner; cotton will, if immersed in a bath containing them, more or
less mechanically take up some of the colour from the liquor, but such
colour can be almost completely washed out again, hence these dyes
are not used in cotton dyeing, although many attempts have been made
to render them available.
Indigo is a dye-stuff that stands by itself. Its combination with the
cotton fibre is chiefly of a physical rather than a chemical nature; it
does not form colour lakes in the same way as Alizarine and Magenta
do.
Cellulose can be dissolved by certain metallic solutions and

preparations:--
(1) =Zinc Chloride.=--When cotton or other form of cellulose is heated
with a strong solution, 40 to 50 per cent., it slowly dissolves to a syrupy
liquid. On diluting this liquid with water the cellulose is thrown down
in a gelatinous form, but more or less hydrated, and containing some
zinc oxide, 18 to 25 per cent., in combination.
(2) =Zinc Chloride and Hydrochloric Acid.=--When zinc chloride is
dissolved in hydrochloric acid a liquid is obtained which dissolves
cellulose; on dilution the cellulose is re-precipitated in a hydrated form.
It is worth noting that the solution is not a stable one: on keeping, the
cellulose changes its character and undergoes hydrolysis to a greater or
less extent.
(3) =Ammoniacal Copper.=--When ammonia is added to a solution of
copper sulphate, there is formed at first a pale blue precipitate of copper
hydroxide, which on adding excess of ammonia dissolves to a deep
blue solution--a reaction highly characteristic of copper. The
ammoniacal copper solution thus prepared has, as was first observed by
John Mercer, the property of dissolving cellulose fairly rapidly, even in
the cold.
If instead of preparing the ammoniacal copper solution in the manner
indicated above, which results in its containing a neutral ammonium
salt, the copper hydroxide be prepared separately and then dissolved in
ammonia a solution is obtained which is stronger in its action.
The cupra-ammonium solutions of cellulose are by no means stable but
change on keeping. When freshly prepared, the cellulose may be
precipitated from them almost unchanged by the addition of such
bodies as alcohol, sugar and solutions of neutral alkaline salts. On
keeping the cellulose undergoes more or less hydrolysis or even
oxidation, for it has been observed that oxycellulose is formed on
prolonged digestion of cellulose with cupra-ammonium solutions,
while there is formed a fairly large proportion of a nitrite.
On adding lead acetate to the cupra-ammonium solution of cellulose, a

compound of lead oxide and cellulose of somewhat variable
composition is precipitated. It is of interest also to note that on adding
metallic zinc to the cupra-ammonium solution the copper is thrown out
and a solution containing zinc is obtained.
This action of cupra-ammonium solutions on cellulose has been made
the basis for the production of the "Willesden" waterproof cloths.
Cotton cloths or paper are passed through these solutions of various
degrees of strength according to requirements, they are then passed
through rollers which causes the surface to become more compact.
There is formed on the surface of the goods a deposit of a gelatinous
nature which makes the surface more compact, and the fabric becomes
waterproof in character while the copper imparts to them a green colour
and acts as a preservative. The "Willesden" fabrics have been found
very useful for a variety of purposes.
CHAPTER II.
SCOURING AND BLEACHING OF COTTON.
Preparatory to the actual dyeing operations, it is necessary to treat
cotton in any condition--loose cotton, yarn, or piece--so that the dyeing
shall be properly done. Raw cotton contains many impurities,
mechanical and otherwise; cotton yarns accumulate dirt and impurities
of various kinds during the various spinning operations, while in
weaving a piece of cotton cloth it is practically impossible to keep it
clean and free from dirt, etc. Before the cotton can be dyed a perfectly
level and uniform shade, free from dark spots or light patches, these
impurities must be removed, and therefore the cotton is subjected to
various scouring or cleansing operations with the object of effecting
this end. Then again cotton naturally, especially Egyptian cotton,
contains a small quantity of a brown colouring matter, and this would
interfere with the purity of any pale tints of blue, rose, yellow, green,
etc., which may be dyed on the cotton, and so it becomes necessary to
remove this colour and render the cotton quite bright. This is
commonly called "bleaching". It is these preparatory processes that will
be