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sophenine (the precipitate has an olive-brown shade), chrysamine (gives a brown precipitate), and diamine-blue B (a fine shade). Sulphate of alumina does not precipitate so many colouring matters as lead acetate or barium chloride, probably on account of its great acidity, which keeps the colour-lakes in solution to a large extent; by neutralising this acidity with soda or ammonia some colours can be precipitated, but the addition of alkali has to be made with great care, or the shade of the colour-lake may be affected. Freshly-precipitated and washed hydroxide of alumina, Al, H. Os, has a strong affinity for many of the coal-tar colours of all classes, and this may be taken advantage of in preparing lake-pigments (see below).
The remarks appended to group a regarding the so-called Congo colours, are applicable to the precipitates obtained with barium chloride and alumina sulphate.
These lists do not pretend to be exhaustive, but they include all the colouring matters which are most useful for this particular purpose. It is well to point out that the same name is given by different makers to different dyestuffs, so that it is quite possible for, say, the scarlet G of one maker to give a precipitate with alumina sulphate, while the scarlet G of another maker will not give a precipitate.
Of the three precipitating agents, barium chloride gives the best and most satisfactory results; although both lead acetate and alumina sulphate give useful colour-lakes; but barium chloride is a more universal precipitant than either of the others, and the colour-lakes it gives are not affected by admixture with sulphur pigments, as is the case with lead acetate colourlakes, which are apt to go brownish if mixed with pigments containing sulphur or if exposed to the action of sulphur gases.
Lake-pigments made with either barium chloride, lead acetate, or alumina sulphate, are very satisfactory in use, and are serviceable either as oil- or water-colours.
3. Adjective Colouring Matters. This class of colouring matters was named by Bancroft adjective dyestuffs ;* their peculiarity is that they are usually not coloured of themselves, and, used alone, are not capable of imparting any colour to textile fibres, but that they require the aid of a second substance, which, in dyeing, is called the mordant, to develop the colour and to fix it on the fibre; further than that the colour which is so fixed on the fibre is largely influenced by the character of the mordant, and with different mordants different colours are produced; thus alizarine, with alumina gives a bright red, with iron a violet, with tin
* Of late the tendency is to speak of them as mordant-dyeing colouring matters.
a scarlet, and so on. The dyestuffs belonging to this group are capable of yielding lake - pigments, which are characterised by their permanence.
Lakes from them are rather difficult to prepare in anything like brilliant hues.
For fuller information relating to the coal-tar colours, the student may consult Benedikt and Knecht's “Chemistry of the Coal-tar Colours," or the author's “ Dictionary of the Coal-tar Colours."
2nd. THE PRECIPITATING AGENT.-The nature of the precipitating agents used in preparing lake-pigments has been partly considered while dealing with the coal-tar colours themselves, and very little more needs to be said. Tannic acid is the precipitant for the basic colours. This should be bought of good quality; in fact, for this particular purpose the purer it is the better; the common qualities of tannic acid are more or less adulterated with dextrine, &c., and are slightly coloured, which qualities have an injurious influence on the brilliancy of the pigment thrown down by it. In addition to tannic acid, tartar emetic is used; this adds considerably to the permanence of the pigment, probably for two reasons; one is that the antimony of the tartar emetic combines with the tannic acid to form an insoluble tannate of antimony, and with this the colouring matter combines, thus forming a more insoluble and therefore more permanent pigment; then, again, most of the colouring matters are sold in the form of hydrochlorides of the colour-base, and in the process of making the lake the hydrochloric acid is set free, and may tend to prevent proper precipitation of the lake. When tartar emetic is used the hydrochloric acid acts upon this and combines with the alkali, liberating tartaric acid, which has no action on the colour-lake formed. The addition of sodium acetate has been proposed for the purpose of preventing this formation of free hydrochloric acid.
As regards the precipitating agents used with the acid dyestuffs, viz., barium chloride, lead acetate, alumina sulphate, &c., these should be of good quality, and contain neither insoluble matter nor free acid ; this last remark applies more particularly to alumina sulphate, which is apt to contain free acid, whereby the formation of the colour-lake is prevented; in using this agent the addition of sodium acetate to neutralise any free sulphuric acid which may be present, or which may be formed in the process of precipitation, is to be recommended.
For the adjective dyestuffs the acetates of alumina, chrome, &c., will be found to give good results. Many processes for the preparation of pigments from these colouring matters are based
on the formation of the hydroxides by precipitation with sodium carbonate or sodium hydroxide, and then combining this with the colouring matter. This is not a satisfactory method, because the alkali has a tendency to affect the shade.
3rd. THE BASE.—The base or carrier exerts a most important influence on the value of the lake as a pigment'; the body or covering power almost entirely depends upon the base, while this also modities the tint or shade of the pigment very greatly. The base used is commonly one or other of the white pigments (which have already been described), but in one or two lake-pigments other bases, such as red- or orange-lead, is present.
The following bases are used in making lake-pigments : barytes, whiting, china clay, gypsum, French chalk, zinc-white. For details as to the composition, &c., of these reference is made to the chapter on white pigments.
Barytes is the base most commonly used; the common qualities are apt to be gritty, a fault which must be avoided ; further, in making what are called pulp-colours for paper-stainers, barytes is not very useful, as its absorbent properties for water are not sufficiently strong; these colours are required to contain about 50 per cent. of water, while barytes will not take up more than 25 per cent. The artificial barytes, blanc fixe, is a good base, and the method of preparing the pigment may be so devised as to cause the formation of this form of barytes during the process of making the lake, and then a pigment of great brilliancy of hue and covering power is obtained; but this method of working is more costly than using the natural barytes.
Gypsum forms a very good base for lakes. It is lighter than barytes; hence it does not make the pigment feel so heavy, and has less tendency to separate by subsidence when made into a paint; it takes up rather more water than barytes, and is therefore better for pulp-colours. In covering power it is about equal to barytes. Precipitated calcium sulphate may now be obtained as a bye-product in the manufacture of many chemical products ; this form would be found useful as a base, and better, in fact, than the natural gypsum. Gypsum, weight for weight, takes rather more colouring matter to produce a given shade of lake than does barytes.
China clay makes a good base for these lake-pigments, being quite inert in all its properties, as well as of good covering power and colour.
It is largely employed in making pulpcolours, owing to its great absorbent properties for water, in which respect it is superior to either barytes or gypsum ; on the other hand, unless a large proportion of dyestuff is used the
resulting lake is apt to appear chalky, on which account china clay is not suitable for making pale-tinted lakes.
French chalk would make a good base, so far as its chemical properties are concerned; but it does not work well as a paint, being apt to be slimy.
Whiting is a fairly good base for lakes, and is largely used for this purpose; although, owing to its somewhat alkaline properties, it does not suit all colouring matters. Like china clay, it is liable to make the lake appear chalky in tone, and, therefore, can only be used in making dark tints. It does not suit lakes made from basic colours, owing to the action of tannic acid on it.
Zinc-white makes a good lake, but its cost is against its meeting with an extensive use for this purpose.
To produce a given shade of lake, barytes takes less colouring matter than any of the other bases noted, and, consequently, a barytes-lake costs a trifle less than does a lake with other bases next to barytes comes gypsum, which takes from one-and-a-half to twice as much dyestuff in proportion to barytes, while china clay takes rather more than twice as much. Pulp-colours, such as are used by paper-stainers, are made so as to contain about 50 per cent. of water, and, in making these, barytes cannot well be used, as it will only take up about 25 per cent.; the best base for such is a mixture of barytes, china clay, and gypsum, in about equal proportions.
In any case the base should be of a good white.colour, and free from any trace of grit. Before using, it should be ground with water, and then sieved, so as to obtain it of as fine a consistence as possible.
PROCESS OF MANUFACTURE.—The method of making pigments from the coal-tar colours is comparatively simple, and does not necessitate the use of any special plant; the only requisites are tanks for dissolving the various colouring matters and precipitating agents, and a tank (like those shown in Fig. 18, p. 126 ) for precipitating the lake in.
The colouring matter is dissolved in a vat or tub, by simply mixing the dyestuff with water. In the case of those colouring matters which, like some brands of violets and magenta, are sold more or less impure, and in a cake form, it is best to dissolve them in boiling water. A good proportion is 1 lb. of colouring matter to 10 gallons of water. It is advisable to strain the solution before using it, so as to free it from particles of undissolved dyestuff and grit.
The precipitating agent is dissolved in a separate vat and the solution filtered to free it from the dirt and other insoluble matter which would deteriorate the lake.
Into a third vat is placed a quantity of water, which is heated to from 120° to 150° F., and then the base (barytes, china clay, or whatever is used) is thoroughly mixed or diffused through the water, special care being taken to break down all lumps, because these would give the lake a speckled appearance, a thing which is to be avoided. Next, the solution of the colouring matter is run in, thoroughly mixed with the base, and, after heating the whole to the temperatures given, the precipitating agent (in solution) is run in slowly with thorough agitation; when all has been run in, the lake formed is allowed to settle, and the top liquor run off; this should be colourless, or nearly so; if it be strongly coloured, the precipitation of colouring matter has not been conplete and more precipitant is added. The lake is now washed by adding clean water and then finished, as may be required, by the usual methods. When required for pulp-colours the lake simply requires filtering off; if required to be dried then, after filtering, it must be dried in the stove at a temperature below that by which the colour or tint of the lake would be affected.
The process here given is generally applicable to all the coaltar colours, but, in some cases, modification of the details are required, which will be pointed out when dealing with the pigments themselves.
LAKES FROM BASIC COAL-TAR COLOURS.Magenta lake.—100 lbs. of barytes, 1 lb. of magenta, 1 lbs. of tartar emetic, and 13 lbs. of tannic acid make a lake of a deep crimson colour.
Bluish-pink lake.—100 lbs. of barytes, 3 lbs. of rhodamine. 3 lbs. of tartar emetic, and 3 lbs. of tannic acid. This makes a lake of a peculiar shade of bluish-pink, which is fairly resistant to exposure to light and air.
Pale crimson lake.—100 lbs. of barytes, 2 lbs. of safranine prima, 2 lbs. of tartar emetic, and 3 lbs. of tannic acid. The lake obtained is a fine shade of crimson.
Violet lake.—100 lbs. of barytes, 1 lb. of aniline-violet, 1 lb. of tartar emetic, and 13 lbs. of tannic acid. The shade of this lake will depend entirely upon the shade of the violet used, which may vary from a violet-red (violet 3 R) to a pure violet (violet 5 B). Either methyl-violet, or Hofmann's violet, or Paris violet may
be used. Blue-green lake.—100 lbs. of barytes, 1 lb. of brilliant green, 1 lb. of tartar emetic, and 11 lbs. of tannic acid. This gives a very deep bluish-green lake.
Yellow-green lake.—100 lbs. of barytes, 1 lb. of brilliantgreen, 1 lb. of auramine, 11 lbs. of tartar emetic, and 24 lbs. of