The grinding of the pigment with the mucilage is easily done on a stone slab with a palette knife. Take half a fluid ounce of the two in five gum mucilage, to which add the same quantity of water and thoroughly mix. Weigh out fifteen grains of the mixed pigment and place in a heap on the slab, add a few drops of the diluted mucilage, grind and regrind the mixture until it is completely smooth, then remove it to a cup, and clean the stone with another portion of the reduced mucilage, finally adding the whole of the ounce, intimately mix, and it is then ready for coating the paper.
For extra fine work on smooth paper, and in fact for all classes of work, the fine grinding of the colours adds materially to extend the range of gradation, and although the trituration may be carried out fairly well with a palette knife, when the finest possible grades are desired, recourse must be had to the muller and stone. Mullers are obtainable of any artist's colourman, they are made in glass, and a convenient size is about one inch in diameter.
The most convenient brush for applying the mixture of combined gum and pigment to the paper, is of the description known as bear's hair, these are usually set in tin; a flat one about two inches wide is a useful size.
In order to coat the paper evenly, pin it down to a drawing board by each corner with a double layer of blotting paper an inch or two larger than the paper to be coated. The blotting paper will absorb the excess of colour at the margins and enable you to secure an even coating up to the extreme edge.
Take a fairly full brush of the mixture after thoroughly incorporating the colour and spread it evenly over the paper, crossing and recrossing it with the brush. Allow the mixture to lie upon the paper for a second or two so that the paper may expand; now release each of the corners and pin the paper down again. Upon the next operation depends the evenness of the coating.
Take a four inch wide artist's badger's hair softener, hold it vertically and lightly by two fingers and the thumb about an inch and a half from the top of the handle, and pass it rapidly over the whole surface of the paper as quickly and evenly as possible. The motion producing the best effect is not the usual sweeping action, but a series of sudden short jerks, difficult to describe but easily acquired. Continue this softening down until the paper has an even semi-transparent surface without uneven cloudy spots. Allow it to dry spontaneously, but before it is stored for future use dry it carefully by the fire, but avoid overheating.
Uncertainty of result is a defect often brought into argument against this process; but absolute uniformity is not difficult if strictly accurate quantities only are employed. With constant strength of bichromate and gum, uniform weights and combination of pigment, similarity of repeats are obtained: but these can only be secured when each sheet of paper is coated identically with its fellow. To get this evenness the badger hair softener must be washed out and dried after coating each sheet. This is very quickly accomplished by an energetic shaking and drying upon a smooth towel. If the paper has been coated properly, it has an even semi-transparent surface slightly glassy.
Failures often occur from using an excess of pigment and allowing the gum to become too thick in consequence of evaporation. Excess of pigment gives dense heavy shadows and increases the difficulty of printing; excess of gum gives clear high-lights, tending to hardness and easy solubility endangering the half-tones.
The paper, if it has been correctly coated will work satisfactorily, if on steeping a small piece of it downwards upon cold water, the pigmented gum dissolves and drops from the surface leaving the paper nearly clean. From ten to fifteen minutes should complete this test.
The method of working without previously chromatizing the paper is as follows:—Take half a fluid ounce of four-in-ten gum mucilage and add to it an equal quantity of saturated solution of bichromate of potass; to this, with all care as to grinding and mixing, add the pigment; coat the paper as before directed. This method will be considerably slower in printing than that in which the paper had been previously saturated with the bichromate; neither are the whites as a rule quite so clear; but it will possess a peculiar grain and softness not otherwise obtainable, which is much approved by some workers of the process.