stirring rods is made to revolve by suitable gearing so as to mix the materials in the vessel. Arrangements are provided by means of which the stirrers can be lifted in or out of the vessel as may be required. This mill is an effective one, and capable of getting through a large amount of work.

It is a good plan in mixing oil with the dry colour to pour the oil over the colour at least 24 hours before mixing and grinding, as the gradual absorption of the oil by the colour promotes and quickens their intimate mixture in the pug mill.

The quantity of oil required to grind colours into the stiff paste in which they are now so largely sold varies very considerably with different pigments; some only require a comparatively small quantity of oil, others a relatively large quantity. Even with different samples of the same colour the proportion will vary a little. Different colour makers, too, use different proportions of oil and dry colour in grinding. The following table will give some idea of the proportions usually adopted, which are essentially the same both for raw and for boiled linseed oil :

White lead,
Zinc white,

7 per cent. of oil.

22

7

These figures are based on practical working, but, as mentioned above, are liable to vary a little from time to time.

Finally, the mixing and grinding of paints and colours cannot be too well done, as these operations materially influence their brilliance and covering power.

MIXING AND SIFTING MACHINES.-Of late years there has come into use in colour works, for the purpose of mixing and sifting dry colours, a class of mechanical sifters and mixers. A good type of these is shown in Fig. 63, which represents Messrs. Wm. Gardner & Son's "Rapid" sifters. The engraving shows the construction of the machine, which consists of two parts, the upper portion being the sifter, the lower portion the mixer. The material is fed through a hopper

which is fitted with a vibratory feed board and falls upon a brush of a peculiar construction; this brush rubs it against fine wire gauze, the fine particles being forced through, while, owing to the spiral form of the brush, the material is carried forward; it next comes in contact with hard brushes which break the coarse particles up into fine ones and send them through the sieve into the mixer; while, if there be any particles which are too hard for the brushes and too large for the sieve, these are sent forward through a spout out of the machine.

The mixing portion consists of a set of revolving curved blades which act very rapidly and efficiently upon the materials contained in the mixing chamber. The machine is made in various sizes to take charges of from 15 lbs. to 1 ton, and to work either by hand or power.

359

CHAPTER XII.

PAINT VEHICLES.

Each

THE most convenient way of applying pigments to the surfaces of bodies is by mixing them with certain fluid bodies called "vehicles," which act both as carriers and as fixers. Vehicles may consist either of a single fluid, of a mixture of liquids, or even of a liquid containing a solid body in solution, which may act as the real fixing agent, the fluid simply acting as a solvent for this and a convenient medium wherewith to mix the pigment with it for use as a paint. The usual vehicles are certain oils, turpentine, shale spirits, benzoline or petroleum spirit, benzol, coal-tar naphtha, methylated spirit or alcohol and water. of these is used in certain classes of paints and varnishes. PAINT OILS.-The oils are a numerous group of bodies derived from both animal and vegetable sources. The characteristic features of oils are that they are lighter than and insoluble in water, are rather viscid fluids, are greasy to the feel, and impart a permanent greasy stain to paper; they are only partially soluble in alcohol, but are freely soluble in ether, petroleum spirit, and turpentine, and some other solvents of a similar character. Boiled with caustic soda (sodium hydroxide), they are decomposed, yielding soap and glycerine. All the oils are not suitable for use as paint oils; they may be divided into two principal groups with some sub-groups:-1st, Those oils which on exposure to the air do not change, or, at most, become slightly more viscid. 2nd, Those oils which on exposure to air gradually become hard and dry; these oils are called the drying oils, and are used as paint oils on account of their possessing this important property. The first group of oils are known as the non-drying oils; they are quite useless as paint oils, and are never used for that purpose.

The oils belong, chemically, to that group of bodies known as salts, which may be defined as compounds containing two radicles, one of which is of acid origin, the other of basic origin.

In the case of oils the latter is always the body known as

5

glyceryl (C, Hg), which when combined with hydroxyl (H 0) forms the well-known compound, glycerine, C, H, (HO); hence oils are frequently known as glycerides because on saponification they yield glycerine. In the oils this glyceryl is united with various acids; for, while there is only one base present in any oil, there are rarely fewer than two acids present, and often there are many more. These acids are known as the fatty acids, and form a rather numerous group, or rather several groups of bodies. Some of these, such as oleic, stearic, palmitic, linolic, are found present to a greater or less extent in all oils; others, such as arachidic, ricinoleic, valeric, &c., are only found in small quantities, and often only in certain oils of which they are the characteristic constituent, such as ricinoleic acid in castor oil, arachidic acid in ground nut oil, valeric acid in fish oils, rapic acid in rape oil, linolic in linseed oil.

The fatty acids may be divided into five groups, which, from the most prominent acid they contain, may be named-1st, the stearic acids; 2nd, the oleic acids; 3rd, the linolic acids; 4th, the linolenic acids; and 5th, the ricinoleic acids.

The first group is a very numerous and important series of acids, and is often called, from the most important member of the series, the acetic acid group. Many of these bodies, such as acetic acid and stearic acid, are used on a large scale in various industrial operations; others are of importance as occurring in products which are of great industrial value. The following lists comprise all the known members of these series:

Formic and acetie acids are liquids having a powerful acid odour, are soluble in water, and can be distilled without change. The next few members of the series are liquids more or less soluble in water, and can be distilled without change; they have a slight odour of rancid fat, and are known as the soluble fat acids, being present in such fats as butter, cocoa nut oil, and palm nut oil, and are occasionally found present in small quantities in fish oils. The higher members of the series, or from capric acid upwards, are solids; they are insoluble in water, and cannot, as a rule, be distilled without being decomposed.

The fat acids are soluble in alcohol, ether, turpentine and similar solvents; they are monobasic acids combining with one equivalent of potassium hydroxide (caustic potash) or sodium hydroxide (caustic soda) to form soaps, which are more or less soluble in water, the salts of the lower fatty acids being freely soluble, while those of the higher acids are rather difficultly soluble, the solubility decreasing with the complexity of the fatty acids.

These acids are very characteristic of fats and oils; oleic is by far the commonest of all fat acids, as when combined with glyceryl it forms olein, the fluid constituent of almost all oils. The lower members are more or less soluble in water and volatile by heat without decomposition; the higher members are insoluble, and are decomposed by heat.