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may be which is being dried, there is no condensation of steam on the bands, owing to each being provided with a separate set of steam coils, while all the water vapour which is driven off from the wet colour is removed by means of ventilators at the top of the machine.

The advantages of this machine are the non-handling of the material in the process of filling or drying and the consequent saving in labour. This has also a sanitary aspect when such poisonous materials as white lead are being dealt with. The danger in these arises from a handling to which they are subjected during the process of manufacture, such handling always resulting in the hands, clothes, &c., of the work-people becoming impregnated with it, sooner or later leading to lead-poisoning; if, then, the handling be reduced to a minimum, then this risk of poisoning is reduced to a minimum also, and this is what this new machine does in the operation of drying white lead.

In Fig. 34 is shown the plan of a drying stove devised by the Sutcliffe Engineering and Ventilating Co., of Manchester. It

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takes the form of a brickwork chamber divided into three compartments by the partition walls, E, E. In the smallest of these is fitted a steam heating apparatus, B, consisting of a cylindrical boiler through which pass pipes from end to end. Into the space surrounding the pipes steam is sent. By means of a fan, A, air can be sent through the pipes, and, naturally, this becomes heated. In the other two compartments are fixed racks, D, D,

on which the pans or trays of colour to be dried can be placed. These racks stand away from the partition walls, and so leave a space which is in connection with a flue to carry away the air and water vapour with which it becomes charged. The hot air from the heating apparatus, B, passes along the passages, C, C, C, C,

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as indicated by the arrows, over the pans of colour, drying the latter in so doing, and then away by the exit flue. This stove is very efficient and not expensive to construct or maintain in operation. For the purpose of drying such pigments as lakes, where the colour is liable to change if the heat applied be too great, there may be used the vacuum drying chamber shown in Fig. 35, and

made by Emil Passburg, of Berlin. In consists of an iron box in which are fitted a number of iron shelves, made hollow and containing pipes through which steam or hot water can be sent from the steam supply pipe shown at the right of the drawing. The chamber is in communication through the valve on the top of the chamber with an air pump, by means of which the air from the inside of the chamber along with the water vapour given off from the wet colour is continually drawn off. The heat maintained ranges from 95° F. to 120° F., and even at that low heat, which will not affect the tint or hue of any pigment, the drying is very rapid. The colour to be dried is placed in pans on the shelves. The charging of the chamber is easily done, and the working is clean. The temperature can be regulated by the valves on the steam pipes, and the extent of the vacuum, which can reach 26 to 28 inches of mercury, can be regulated by the working of the air pump. The chamber is made in various sizes, from one having 32 feet of heating surface to one having 854 feet.

In any stove the colours are best placed in earthenware pans of about 12 to 16 inches in diameter, and 3 to 6 inches in depth; smaller pans may be used, but it is not advisable to exceed the sizes just given. Pans made of galvanised iron have been used, but these are liable to rust and so lead to discolouration of the pigments dried in them; enamelled iron pans, which can now be bought at a reasonable figure, are well worth a trial as being lighter and less liable to break than earthenware pans.

PREPARING PIGMENTS OR COLOURS BY
PRECIPITATION.

Many colours-the chrome-yellows, Prussian blues, Brunswick greens, lakes, &c.—are prepared by a process of precipitation, the principle of which is that when two or more substances in the state of solution are mixed together a reaction sets in-what the chemist calls double decomposition occurs-and new products are formed; one of these being insoluble in the liquid used is thrown down or precipitated out of the solution, usually in the form of a fine powder. Thus when to a solution of nitrate of lead, one of chromate of potash is added, a yellow powder falls down; this on examination is found to be chromate of lead, while the liquor contains nitrate of potash in solution; thus there has been an cxchange of constituents, the chromic acid has left the potash to form lead chromate, while the potash has combined with the nitric acid of the lead nitrate to form nitrate of potash; the

chromate of lead forms a precipitate because it is insoluble in water.

Put into the form of a chemical equation this reaction is expressed as:

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Another example of precipitation met with in colour making is that of zinc sulphide, from solutions of zinc chloride and sodium sulphide, which is represented in the following equation:

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Here, again, there has been an interchange of constituents, and the zinc sulphide being insoluble is thrown down as a precipitate.

As precipitation is a chemical reaction it always takes place in fixed and definite proportions; thus in the preparation of chromeyellow it is found that 331 parts of lead nitrate interact with 194 parts of potassium chromate, the result being that 323 parts of lead chromate are precipitated while 202 parts of potassium nitrate are left in solution; should the salts be mixed in any other proportion, then one or the other must be in excess, and this excess will be wasted; thus, suppose 150 lbs. of lead nitrate and 95 lbs. of potassium chromate are used; the latter quantity is not sufficient to precipitate all the lead from solution; consequently, the excess, which is 7 lbs., remains, and is practically wasted. The necessity of using equivalent proportions of the materials is a matter of importance as regards economy in making colours by precipitation.

Every case of precipitation is a case of double decomposition, so that the main product is always associated with bye-products, which are sometimes worth recovering, or which may be utilised in other ways. Thus, in making chrome-yellow by the process mentioned above, potassium nitrate in solution is a bye-product; in places where fuel is cheap it might pay to boil down this solution and recover the salt. Then again, by paying attention to the bye-products, and their probable use in other ways, it is possible to effect economies in the production of colours. Thus, supposing lead sulphate is to be made, this can be done by precipitating a solution of lead acetate, with either sodium sul. phate or sulphuric acid, as shown in the following equations:

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In the first case, sodium acetate is formed, and although this has cost money, yet it must be thrown away, because it cannot be economically recovered by the colour-maker. In the second case, the acetic acid formed may be utilised in producing a fresh stock of acetate of lead from metallic lead; in this case there are no waste products, and the manufacture of the lead sulphate is conducted most economically.

The character of the precipitate formed is modified in regard to its tint, its consistence, and in other ways by the conditions under which it is obtained. Thus, if barium sulphate be precipitated from cold solutions, it falls down as a very fine, rather light powder, which is difficult to filter; but if thrown down from hot solutions, it is not so fine, and can be filtered more readily. Again, in making chromes, the conditions under which the operation is performed have a wonderful influence on the result; a difference in the lead salt influences the tint, the nitrate giving a finer product than the acetate. The temperature also modifies the result considerably. Thus, the chrome which falls down from cold solutions is much paler and more voluminous than that obtained from hot solutions. The acid or neutral state of the solution also has some influence, while the presence of such bodies as alum or sulphate of soda has a material influence.

In making colours by precipitation, the following conditions affect the character of the resulting pigment:-(1) Strength of solution. Generally, weak solutions yield finer and more voluminous precipitates than strong ones. (2) Temperature. From cold solutions the product is usually finer and more voluminous than from hot solutions. (3) Proportion between the interchanging bodies. This also has some influence on the result. Thus, in making chromes it is preferable to keep the lead in excess; if a soluble Prussian blue is required, then the potassium ferrocyanide must be in excess. Other examples might be given illustrative of this point. (4) The order of mixing is important. If the lead salt, in making chromes, were added to the bichromate of potash, the pigment obtained would not be so fine as in adding the bichromate of potash to the lead salt. In making soluble Prussian blue it is important to add the iron salt to the potassium salt, not vice versa. In making emerald green, the

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