Raw linseed oil is matured and oxidised until its consistency is considerably reduced. It is still further reduced by being boiled at a high temperature, and is known commercially as “boiled linseed oil.” As this boiling is continued the fumes which quickly rise can be ignited, and the liquid soon assumes a syrupy or stringy character, according to the length of time during which it is subjected to the action of fire. By extending or discontinuing the burning the varnish is produced in three grades—thin, medium, or strong.
Lithographic varnish is a good servant but a bad master, and it is a generally recognised fact that, beyond a certain point, lithographic varnishes as a reducing medium will depreciate the value of colour.
The chief requisite in colour printing is the production of a solid flat impression, and for this purpose almost all printing inks must be reduced to a suitable working consistency. To accomplish this, and at the same time retain the full colour strength of printing ink, a soft, free-working composition will be useful and desirable.
There are several commercial varieties of solid oil from which the excess of grease has been extracted. These form excellent reducing mediums. They break down the tack of stiff pigments and enable them to work freely during the printing operations. The drying of inks thus prepared is not seriously retarded; they lift readily and usually produce brilliant impressions. There is a reasonable and logical explanation of these peculiarities which is both interesting and suggestive. Whatever the character of a reducing medium may be, its effect on the strength of colour will of course be in proportion to the quantity used. In all probability 1 oz. of a solid oil composition, otherwise known as lithographic reducing medium, would soften down a quantity of printing ink for which at least three times its bulk of varnish would be required. Consequently, the depth of colour and covering power of an ink reduced with “litho medium” would be proportionately greater than that reduced with varnish. Vaseline in some of its commercial forms is frequently used by American printers, and even in this country its use is being tardily, though none the less surely, recognised.
A few remarks anent the intelligent application of a softening medium may not be inopportune. Considerable care and judgment must always be exercised or there will be a loss of cohesion in the colour pigments which cannot fail to prove disastrous. The tendency of lithographic varnish is to bind the colour pigments together, and this should not be entirely counteracted by the addition of fatty compositions, lest the printing inks run “scummy” during printing operations, and in drying leave the colouring matter, from which they have been detached, on the surface of the paper in the form of a dry powder.
Such lack of cohesion may, however, be an inherent feature of the ink itself, and not be produced in the manner just indicated. Bronze blue affords a striking example of a printing ink of this character. It is, in fact, a “constitutional weakness” which cannot apparently be prevented, but which is fortunately not incurable. The addition of a little Canada balsam to bronze-blue ink will add considerably to its working qualities. The loose particles of the pigment appear to be held together without becoming harsh or stringy, as might easily happen if varnish of sufficient strength was added to produce the same effect.
Other colours, again, such as vermilion and yellow, owing to their weight and texture, will always require a fair percentage of varnish in their composition. At the same time, a little reducing medium might also prove beneficial. Referring once more to the fact that vermilion, as distinguished from its imitation, is unusually heavy, etc., it may be useful to know that for “blocking out” work it has no equal in all the range of printing inks. It possesses unrivalled opacity, and as a “blocking-out” agent frequently plays an important part in colour printing.
Of the other printing inks, few possess characteristics of a sufficiently striking character to require special mention. Their working qualities present no exceptional difficulties, and their employment either under primary or secondary conditions is almost invariably determined either by the character of the work or some such conditions as have been already indicated.
When the strength of a colour is problematical, or its effect more or less a question of experiment, it is a safe plan to mix it a little lighter than will be required. For obvious reasons it is much easier to alter the line or tone of a light colour than that of a darker one.
The arrangements for extensive and economical ink mixing need not be of a very elaborate character. Standard colours might with advantage be mixed in large quantities and kept as stock shades. Fleshes, pink, blues, greys, etc., are all useful colours which are in constant use. A warm or cold tone could be imparted to a stock grey as required, and a similar method adopted with regard to the other colours. Other peculiar conditions could be met in a similar manner, and many economies thereby effected.