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that if the shade or tint that is being produced is too yellow 88 compared with a standard sample, then, by throwing a little more blue into the mill, the fault can be remedied at once; but against this advantage must be set off the disadvantage that the tints of green are not so fine as those which are obtainable by wet methods. The following proportions can be taken as guides in making the various shades of Brunswick green by this dry method :
Pale Brunswick Green.-1 cwt. of barytes, 11 lbs. of Prussian blue, and 35 lbs. of chrome-yellow.
Middle Brunswick Green.--1 cwt. of barytes, 21 lbs. of Prussian blue, and 35 lbs. of chrome-yellow.
Deep Brunswick Green.-1 cwt. of barytes, 5 lbs. of Prussian blue, and 35 lbs. of chrome.yellow.
Extra Deep Brunswick Green.-1 cwt. of barytes, 8 lbs. of Prussian blue, and 35 lbs. of chrome-yellow.
The quality and tint of the chrome-yellow used will be found to have some influence upon the tint of the green produced ; for the pale shades lemon-chromes should be used; while for the deep shades the middle shades of chrome can be used. Before making up a large batch of green with a new batch of either chrome-yellow or Prussian blue a small trial lot should be made to see if the two pigments will produce the required shade, as experience shows that different makes of chrome-yellows, even if of the same shade, do not always give the same results in making these greens; while the difference between two makes of Prussian blue as green-producers is greater than that between two makes of crome-yellow.
:'or making the palest shades of Brunswick greens, only the berty and brightest Prussian blues should be used; for the darker shides of green the quality of the blue is not of so much consequence.
The barytes may be replaced with gypsum, if thought fit; less gypsum is required to produce a given tint of green than barytes, the proportion being about 1 cwt. of gypsum to 2} cwts. of barytes.
WET METHODS.—The wet methods are those commonly adopted by makers of Brunswick greens, partly because they are the oldest (for colour makers are so conservative that it is difficult to induce them to alter their methods), and, partly, because the wet methods produce the finest greens; but they are very much more troublesome to carry out and require no little practical experience on the part of a colour maker to produce the best results. From the recipes which have been given above it will be seen that barytes is the principal ingredient in these greens; hence the principle which should underlie all wet pro
cesses is to precipitate the yellow and the blue constituents on the barytes simultaneously. This is by no means easy to do, and yet much of the brilliancy of the green depends upon this being done as successfully as possible. The best way to ensure this result would be to mix solutions of the acetates of lead and iron or of the nitrates of those metals together and to add the barytes and precipitate with a mixture of the bichromate and ferrocyanide of potash, but, unfortunately, this course is not at present available, for the reason that while the acetate or nitrate of lead can be purchased on a commercial scale of sufficient purity, the acetate or nitrate of iron is not so purchasable; the common iron liquor is too impure for use in making greens, while the nitrate of iron so-called is of too variable a composition to be recommended for use in colour making.
The materials which are used in the preparation of the greens are copperas (ferrous sulphate), which should be used as pure and as fresh as possible, the variety known as green copperas is the one required, acetate of lead, bichromate of potash, ferrocyanide of potassium (yellow prussiate of potash), and barytes. The red prussiate of potash would give rather better results than the yellow, only that its extra cost is against its use for making greens for common use, but when a good price is obtainable its use is to be recommended, as the green is much easier to make with it than with the yellow prussiate.
For producing the various shades of Brunswick greens the following proportions may be used which, as well as those given above, may be varied so as to sạit the special requirements of each individual maker. The following points are, however, well worth attending to in making alterations in the proportions. ist. That equal weights of prussiate and copperas must be used. 2nd. That the proportion of acetate of lead to the bichromate of potash should be, as nearly as possible, 10 to 37.
Pale Brunswick Green.—1 cwt. of barytes, 13 lbs. of acetate of lead, 1 lb. of copperas, 1 lb. of yellow prussiate of potash, and 4 lbs. of bichromate of potash.
Middle Brunswick Green.—1 cwt. of barytes, 13} lbs. of acetate of lead, 11 lbs. of copperas, 11 lbs. of yellow prussiate of potash, and 44 lbs. of bichromate of potash.
Deep Brunswick Green.-1 cwt. of barytes, 14 lbs. of à cetate of lead, 2 lbs. of copperas, 2 lbs. of yellow prussiate of potash, and 41 lbs. of bichromate of potash.
Extra Deep Brunswick Green.—1 cwt. of barytes, 16 lbs. of acetate of lead, 4 lbs. of copperas, 4 lbs. of yellow prussiate of potash, and 5 lbs. of bichromate of potash.
Instead of the bichromate of potash the bichromate of soda may be used with some advantage, on account of its greater solubility and less cost.
Instead of barytes, any white pigment may be used, when it will be found that a smaller quantity will be required to give a green of the same depth of colour; or, to put it in another way, an equal weight of another white base will require a greater quantity of the other ingredients to produce the same shade of colour; with gypsum the proportion will be about 2} times as much, with china clay about 3 or 4 times as much, and with zinc white from 4 to 5 times as much; it is very rarely that any other pigment than barytes is used in making these greens.
By varying the proportions a great variety of green shades can be produced.
It is mainly owing to the variable proportions used that the middle green, say, of one maker so seldom has exactly the same tint or depth of colour as that of another maker.
The following is the best method of working, in order to obtain good bright pigments with the ingredients given above. The iron salt is dissolved in a tank of cold water, the lead salt is similarly dissolved in another tank, while the two potash salts can be dissolved together in one tank. The barytes is thoroughly mixed with water in another tank, and, when properly mixed, the iron solution is run in with constant stirring; and then the lead salt is run in. Some of the lead will be precipitated as the sulphate, owing to double decomposition taking place between the two salts, but this cannot be avoided, so that allowance should be made for it in all recipes for making Brunswick green by increasing the amount of acetate of lead as the quantity of copperas is increased ; every extra pound of the latter will require 1 lb. 3 oz. of acetate of lead to be added in addition to that required to form chrome-yellow with the bichromate. After the lead has been run in and mixed with the rest of the ingredients, the whole is kept stirred while the potash salts are run in; the green soon forms and is allowed to settle; the clear liquor at the top is run off and the pigment washed by running in water and stirring well, again allowing to settle and running off the wash waters; this washing should be repeated once or twice. Then the colour is taken out, thrown on filters to drain, and, finally, dried at a gentle heat. Various other methods of manipulating the preparations of these greens are in use among the various makers, but it is not necessary to describe them.
A method of working, which has been used by the author with very good results as to ease and quality of the colour produced,
is to grind all the ingredients together, in the proportions given above, in an edge-runner mill, and when they are properly mixed, to put them into the tub and run water on to them, with constant stirring; the green is rapidly developed, and is allowed to settle; the clear top liquor is then run off, and fresh water run on to wash the pigment, after which it is finished as usual.
These greens are sold under a variety of names -Brunswick green (which is the commonest), Chrome-green, Victoria green, Prussian green, &c.
COMPOSITION AND PROPERTIES OF BRUNSWICK
These pigments are compounds of barytes, chrome-yellow, Prussian blues, with occasionally small quantities of lead sulphate, gypsum, and other bodies. The following analyses made by the author of different shades of these greens, of a good make, will show the average composition of the pigments :
They are good pigments, and work well both in oil and water, especially the former; their opacity is good, and, therefore, they have good body or covering power, in this respect surpassing all other green pigments. They can be mixed with other pigments, with but few exceptions, without any change being brought about by interaction, these exceptions being those pigments containing sulphur, which would act upon the chrome-yellow and darken the green, by the production of black lead sulphide and highly basic colours, like whiting or lime, which would act
both upon the chrome-yellow and the blue, turning the green into a red.
They are fairly permanent when exposed to light and air, for, although not quite permanent, they are so for all practical purposes; exposure to light causes the yellow constituent to fade first, as a rule, so that, especially in the dark shades, the green has a tendency to turn blue, but in this respect the blue is very variable; in some makes the yellow goes first, in others the blue, much probably depends upon the composition of the particular green and the circumstances under which it is placed.
Acids turn the colour bluer, owing to their dissolving out the chrome-yellow; on the other hand, alkalies turn it orange, owing to their combined action both on the blue (turning this of a reddish brown) and on the yellow (which they turn orange), as is noted in describing the blue and the yellow in their respective places. Sulphuretted hydrogen darkens the tint considerably.
ASSAY AND ANALYSIS OF BRUNSWICK GREENS. -Brunswick greens require assaying for colour or tint, covering power or body, brilliance, &c., by the usual methods. Since, as already stated, the pale shade of one maker may not exactly agree with the pale shade of another maker, the different makes should always be compared together for the various properties just named ; as a rule, it will be found that different batches of the same maker's green will match one another very closely.
It is rarely that an analysis of these greens is required; but, if so, the following method, described by Brown,* may be followed :
1. For Chrome Green.–Weigh out 2 grammes of the green, treat with 28 to 30 cc. of strong hydrochloric acid, at the boil, for about 10 minutes, then filter while still hot, and wash well with boiling water, adding the wash waters to the filtrate.
The residue, consisting of barytes and Prussian blue, is strongly heated until the blue is decomposed, and, after cooling, the residue is weighed; this gives the weight of oxide of iron and barytes in the green.
Treat the residue with a mixture of nitric and hydrochloric acids, boil well, then dilute with water and filter; dry, ignite, and weigh the residue, which is the barytes; deduct this weight from the original weight of barytes plus oxide of iron ; the difference is the amount of oxide of iron, which, multiplied by 2.212, gives the amount of Prussian blue
in the green.
Filtrate.- Nearly neutralise by the addition of ammonia, then
Brown, Chemical News, December 31, 1886.