Lithography

Lithography is a printing technique in which the printing mould is made up of a plate which is quite thick and smooth made from a special calcium stone.

The principle fundamental of lithography is based on the property that the calcium carbonates have of changing their chemical composition superficially in contact with acids, and of easily accepting greasy substances.

The printing principle is very simple, seeing as the calcium has the property of holding the grease, also the picture executed with similar substances sticks perfectly to the surface of the plate.

The stone, deliberately smoothed and therefore designed for a grease pencil, maintains the parts not drawn, contrografismi, a subtle veil of water, with the grease stain, called grafismo, on the contrary is refused. Passing the ink on the stone treated thus, it is refused by the dampened parts and maintained by the greasy parts. In the printing phase, therefore, the sheet of paper receives only the ink that is deposited on the parts drawn and not on the others.

The printing plate, made of limestone, which is granulose and made up of calcium carbonate, must have a thickness that varies between 6 and 12 cm. The surface of the stone is ground down with a pumice, sand or, carborundum, to remove any marks.

A line is drawn with a lithographic pencil or with lithographic ink made up of greasy substances, so that the calcium carbonate maintains the greasy substances with ease, but the drawing can also be executed with wax and soap and blackened with carbon black (nerofumo).

On the printing plate, the images must be drawn mirrored.

Once the drawing is finished, the stone is brushed with a liquid, a preparation with a base of nitric acid, acidic gum arabic and water. The reaction of the nitric acid which transforms the non protected by the ink parts, changes the calcium carbonate into calcium nitrate, an absorbent substance, which tends to absorb water.

It is dried, so as to render the drawing more stable.

The printing occurs 24 hours after the preparation, using a lithographic press, the drawn printing plate is dampened and then inked using an India rubber roller.

Seeing as there is incompatibility between grease and water, the greasy parts drawn reject the water and maintain the ink. On the contrary, the wet pieces of stone reject the ink because it is greasy.

The ink sticks where the drawing is and is rejected by the wet stone.

The sheet of paper to print is positioned, other sheets and a piece of card are added and at the end it is all compressed.

At the end of the operation, the sheet is extracted and dried.

You can obtain many copies from the same printing plate, repeating the procedure each time.

Today stone is substituted frequently with plates of porous metal.

The procedure of lithography on zinc is called zincography and on aluminium it is called algrafia. Both metals are grained. The grains serve the double objective of maintaining more water and improving the adhesion of the greasy substances and the preparation of the lithograph. Senefelder also invented the auto lithography method using which you do not have to do the drawing in a flipped way.

In auto-lithography you draw with greasy ink diluted upon special pelure paper, which has a light coat of Tragacanth, which is viscous and insoluble in water. The drawn card is pressed on a well smoothed and pumiced stone, it is slightly dampened, gradually the pressure is increased, and then it is separated from the stone so that no drops of water fall on the drawing. You pass over with a thin layer of rubber solution, and then, with a sponge in the very diluted lithographic ink, this is applied sweetly until a shiny and clear image appears in all its particulars. It is dried rapidly, it is dusted with impalpable colophony powder and then with talcum powder, it is acidified with water, nitric acid and rubber, and it is washed. Finally you pass over it and you leave an even layer of very diluted gum arabic to dry.


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