Thus if we want to produce bright shades in dyeing, we must avoid the introduction of complementary colours. A bluish red mixed with a reddish blue will produce a bright shade of violet, but if we mix an orange-red with a greenish-blue, we introduce yellow into the mixture, and obtain a dull maroon or puce according to the proportion of the other colours. In a similar way, the introduction of a blue dye will dull a bright orange to a brown, and a little of a yellow dye will dull a bright purple to a maroon. This fact is frequently used in producing the quiet shades of colour often required from the most brilliant dyes. If to a bright orange we add black, or a blue dye which as its complementary produces black, we convert it into a brown. If instead of blue we use green for dulling, we give the brown a yellower shade, since the green produces black at the expense of the red of the orange. Violet similarly used gives a redder brown, since it produces black by combination with the yellow. This shading, if small in amount, is frequently done by direct mixture of a suitable dye, but if considerable, it is generally better to top one colour with another. Thus a blue, topped with a powerful orange, will produce a Havanna brown. For dark colours, it is frequently convenient to produce a dark ground with some cheap dye, such as logwood and iron or chrome, and to top it with a bright shade of the colour required. In this way cheap dark blues and greens can be easily produced. For reds and browns, mixtures of logwood and Brazil-wood, or Brazil-wood and fustic may be used, topped with coal-tar colours. Tanning materials, such as quebracho and mangrove extracts, which give browns with bichromate, are also employed on cheap goods. It is also frequently wise to dye with a basic colour and top with an acid one, or vice versa; as in many cases the one fixes and combines with the other, and an increase of fastness is obtained.
Morocco and many other coloured leathers are finished by damping the surface of the dried leather with a very dilute “seasoning” of water, milk, and blood or albumen, allowing the leather to become quite or nearly dry, and polishing by friction under a cylinder of agate, glass, or wood in the glazing machine. Many leathers are also grained by printing from engraved or electrotype rollers, or by “boarding,” or a combination of the two. “Boarding” consists in pushing forward a fold in the leather on a table with a flat board roughed underneath, or lined with cork, in a way which is difficult to describe, but which in skilful hands wrinkles or “grains” the skin in a regular pattern.
The colour of a dyed skin is much altered by finishing and especially by glazing, which always darkens and enriches the colour. In dyeing to pattern, it is useful to glaze a little bit of the rapidly dried skin by friction with a smooth piece of hard wood for comparison, and a portion of the pattern may also be wetted for comparison with the wet skin. Colours which look full and even in the dye-bath, often go down in a most disappointing manner on drying, though to some extent they regain intensity on finishing.
In comparing the dyeing value of colours, the most practical way is to make actual dyeing trials with equal or known quantities of the colours and of water. Such trials may be made, either by “turning” the samples in photographic porcelain trays, kept warm in a water-bath (a “dripping tin” may be used for the purpose, the trays being supported a little above the bottom on tin supports soldered to the tin), or the leather may be hung from glass rods, by hooks of copper wire, in glass vessels (square battery jars), also placed in a water-bath. The leather samples should be of equal surface in every case; for suspension, pieces of “skiver” (sheep-grain) of 8 by 4 in. or 20 by 10 cm. are very convenient. These may either be “pleated” or suspended by the two ends grain side out, with a short glass rod to weight the fold, and keep them flat. The weight of colour used for a sample 8 in. by 4 in. multiplied by 54 times the area of a single skin in feet, will give approximately the weight of colour needed per dozen; which is, however, a good deal influenced by the mode of dyeing, and the quantity of water used.
In dyeing on the large scale, iron, zinc and even copper are to be avoided, the latter acting very injuriously on many colours, and on the whole wooden vessels are to be preferred. Though these become deeply dyed, they become very hard, and if well washed with hot water, and occasionally with dilute acid, they may be cleansed so as to give up no colour in subsequent dyeing operations, though of course it is not desirable, if it can be avoided, to use the same vessel for very different colours. Zinc rapidly bleaches many colours, especially while wet and slightly acid, and discharge-patterns may often be produced by pressing the wet leather on perforated zinc plates.
CHAPTER XXVI.
EVAPORATION, HEATING AND DRYING.
Questions of evaporation, whether for raising steam, or for the concentration of tanning extracts and other solutions are of considerable importance in the tanning industry, and as the same natural laws which apply to these equally govern the drying of leather, it is convenient to study the theory of the whole subject in one chapter, rather than to divide it, and place each part in a different portion of the book.
The modern conception of evaporation and vapour pressures has been described on [page 75], but it will be necessary to recapitulate a little. It is a well-known fact that most liquids, if left exposed in an open vessel, gradually disappear by evaporation into the air, even at ordinary temperatures. If the vessel is heated sufficiently, the liquid “boils”; that is, bubbles of vapour are formed in it, and escape, and the evaporation is therefore much more rapid. To avoid complication, let us first imagine a liquid sealed in a glass flask, which contains no air, but which is only partially filled by the liquid. It has been pointed out that the motion of heat by which the molecules of the liquid are agitated, enables some of them to break away from the attraction by which liquid particles are held together, and pass into the form of gas or vapour, which will fill the empty part of the flask. This evaporation will, however, soon reach a limit, since the vapour cannot escape from the flask. The flying molecules of vapour produce pressure by striking the walls of the flask, while a proportion of them will strike the surface of the liquid, and again be caught and retained by its attraction; and as the pressure rises, the number of these necessarily increases till a point is reached when as many fall back and are retained (or “condensed”), as those which evaporate, and the pressure will then remain constant. The amount of the pressure will vary with the nature of the liquid, and will be the greater the more volatile it is, or, in other words, the less the power of its internal attraction. It will also increase with rising temperature, which, by increasing the velocity of motion of the molecules, renders their escape from the liquid easier, and their recapture more difficult. It will not be at all affected by the volume of vapour or the size of the flask, but so long as any liquid is present, it will depend merely upon the nature of the liquid, and the temperature. If the flask is large, more of the liquid will evaporate till the same pressure is reached. If at the outset the flask is not empty, but filled with air, it will make no difference to the pressure or quantity of the vapour in it, which will be added to that of the air, whatever that may be. If the sealing of the flask is broken so that it is open to the atmosphere, air and vapour will escape, or air will pass in, till the total pressure is equal to the atmospheric pressure outside, (about 15 lb. per square inch). As, however, the vapour in the flask is always renewed by evaporation, so that the full vapour-pressure of the liquid is maintained, the “partial” pressure (as it is called) of the air in the flask will be less than that of the outer atmosphere by the amount of the vapour-pressure, which makes up the difference. Once this balance is attained, evaporation will go on very slowly in the flask, as it can only replace the small quantity of vapour which escapes. If, however, the vapour is removed by blowing fresh air into the flask, it will rapidly be replaced in the old proportion by fresh evaporation. Thus goods in a close room will dry only very slowly, even if the temperature is high, unless the moistened air is replaced by dryer air from the outside by some effective system of ventilation. In absence of this, evaporation only becomes rapid when the temperature of the liquid is raised to its “boiling point,” that is, when the vapour-pressure becomes slightly in excess of that of the atmosphere, so that the freshly formed vapour can push out that already in the flask or chamber into the outer air, and at the same time, bubbles can be formed in the interior of the liquid by the escaping vapour. As the vapour-pressure of a liquid rises continuously with increasing temperature, and its boiling point is defined as that temperature at which it is equal in pressure to the air (or vapour) in contact with it, it is evident that the boiling point must entirely depend on the pressure. Thus the boiling point of water in a boiler at a pressure of 55 lb. per square inch above the atmosphere is 150°C., and in a partial vacuum equal to 5·8 inches of barometric pressure, is only 60° C., a fact which is made use of in the concentration of extracts and other liquids at a low temperature in the vacuum-pan. (Atmospheric pressure is taken at 30 inches or 760 millimeters of the barometer or 14·7 lb. per inch, or 1·033 kilos per square centimeter.)
If a piece of iron is placed over a powerful gas-burner, it will go on getting hotter till its temperature is nearly or quite equal to that of the gas-flame. On the other hand, a pan of water, in the same condition, once it has reached its boiling point, becomes no hotter till all the water is evaporated. It is evident that the whole available heat or energy of the gas-flame is consumed in converting the water into steam. We might convert a proportion of this energy into mechanical work, by using the steam in a steam engine; but even without this, work is actually being done by the escaping steam in raising the weight of the atmosphere, and in overcoming the attractive force which holds the particles of water together in the liquid form. It is of course known to everyone, that energy may change its form, as from heat to work, but that it cannot be destroyed, diminished or increased; and therefore the whole of the work performed in converting the water into steam is again recovered as heat when the steam is condensed. In this connection a clear distinction must be made between quantity of heat, and temperature, which in popular language are often confused. It is for instance obvious that if we mix a pound of water at boiling temperature with another pound at freezing point, the temperature is altered to 50° C., but the total quantity of heat is unchanged. It is equally clear that no change in quantity of heat takes place when 1 lb. of mercury at 100° is mixed with 1 lb. of water at 0°, though in this case, owing to the small capacity of mercury for heat, the common temperature would only be raised to about 3°. We must therefore have some measure of heat apart from the mere direct indications of the thermometer, and that most generally used is the quantity of heat required to raise 1 kilo of water 1° C. (kilogram-calorie).[180] In England the heat required to raise 1 lb. of water 1° F. is also in use as a unit. The k.-calorie is equal to 3·97 (very approximately 4) lb. × F. units. For our purpose it may be taken that 100 k.-calories of heat are required to raise 1 kilo or liter of water from freezing to boiling temperature. If, however, the water is actually frozen, we require 80 k-calories merely to melt the kilogram of ice without perceptibly raising its temperature, and when the water is raised to 100°, 536 calories of heat are still necessary merely to convert it into steam at the same temperature. To melt 1 lb. of ice requires 144 lb. × F. units, to raise it to boiling point 180 more, and to evaporate it 965 additional. The quantity of heat required for actual evaporation varies a little at different temperatures, being somewhat larger at lower temperatures, but the total heat required to raise water from the freezing point, and convert it into steam at any pressure is nearly constant, being 635 calories at atmospheric pressure, and only about 650 calories, or 1180 lb. × F. units at 50 lb. per sq. inch. The quantity of heat evolved by the combustion of 1 lb. of good coal is 13,000 to 15,000 lb. × F. units; or of 1 kilo, 7200 to 8300 k-calories, but in raising steam in a good boiler coal will only evaporate 10 times its weight of water at 100° (5360 calories or 9650 lb. × F. units), the remaining heat being lost. 1 horse-power (33,000 foot-pounds per minute)[181] in the best engines requires about 11⁄2 lb. of coal or 15 lb. of steam per hour, but in those of worse construction may run up to many times that amount. As, even theoretically, not 20 per cent. of the total heat can be converted into mechanical work in a “perfect” engine working at 75 lb. pressure, it is often economical to use waste steam for heating or evaporation, and where this can be done profitably, the additional cost of the mechanical power is very small.