A technical dictionary of printmaking, André Béguin.

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Drying
[1]. DRYING OF INKS.
The drying of a substance implies that a liquid or a semi-liquid substance hardens. In the case of ink, which is made up of pigments and of a vehicle, the disappearence of part of the vehicle leaves the dry colour behind. Essentially this disappearence concerns the solvent* (water, alcohol, etc.) since the rest of the vehicle (oils, synthetic or natural resins, and others) ensure the cohesion, the keying, and the look of the ink.
The drying in itself does not imply that the substance having undergone this process will not dissociate itself, will not be indelible, or will not be altered. In fact a dry ink can usually be dissolved (see * diluting agent, washing, restoring , and solvent), will often run when it comes into contact with other inks ( see run and bleed), with a solvent, with a vehicle*, with a dusting agent, with a medium, and remains more or less sensitive to the effects of light [* conservation, restoring].
The drying process of ink depends on the nature of the vehicle used. The drying process may come about in one of the following manners:
-BY EVAPORATION. This is the case for water base inks, cellulose base inks, and (to a certain extent) oil base inks, especially if the latter are used on a non-absorbant surface. This type of drying process requires airing in function of the amount of ink that has been applied and in function of the atmospheric humidity. Such drying can he accellerated by blowing hot or cold air over the surface to be dried.
-BY PENETRATION OF THE SURFACE. This is the case with water and oil base inks applied to absorbent surfaces [dusting agent , paper].
-BY OXYDATION IN CONTACT WITH AIR. This is the case with oil paint and glycerophtalic inks. The oxydation process, as opposed to penetration which is a physical process, is nothing less than a chemical reaction of the ink with oxygen. The oxygen coming into contact with these inks brings about polymerization and therefore a progressive thickening of the ink until it reaches its final, hard stage. The drying agent used accellerates the oxydation process. Heat helps to activate drying processes depending on oxydation.
Whatever the drying process may be, all of them will be slowed down by humidity, poor air circulation (piling up of impressions), and the acidity of the water used to dampen the paper in lithography and offset printing.
Typographical inks dry by penetration (newspaper ink) and by means of oxydation. Photogravure inks dry by evaporation and by penetration. Silkscreen inks dry by means of oxydation and by penetration (in the case of oil base inks). With cellulose and ethylcellulose base inks, fluorescent inks as well as some synthetic inks and some water base inks, drying is achieved by evaporation and penetration. Clycerophtalic inks dry by oxydation after the evaporation of the solvent they contain.
Different inks have different drying times. Typographical inks are fast drying for machine printing and medium fast drying for manual impressions. This is also true for lithographic and offset inks which are fast drying for machine printing. Photogravure inks (which are perforce liquid inks) are fairly quick drying due to the volatile nature of the diluting agents used (acetone, alcohol, esters ). The drying speed of these inks is calculated in function of the speed of the machines used. The inks used for manual intaglio printing are fairly quick drying.
The inks used in silkscreening are more delicate and somewhat problematical because their drying process must be slowed down so that they do not dry in the meshes of the screen. Furthermore, in printing silkscreen images one uses brilliant finish inks (often glycerophtalic inks) which take more than eight days to dry completely on an absorbent surface. In order to avoid the clogging of the silkscreen meshes a retarder is often mixed into the ink which further slows the drying of the printed image.
Once the printing has been done it is essential that these various steps be kept in mind: place a sheet of tissue paper between each impression using intaglio ink, between each impression of a manually executed relief print, and between each lithographic print. The sheets of tissue paper may be changed after a while if the ink stains too much. The clean sheet can then be left in place for several months, or even longer if there is danger of rubbing motion An impression will loose more ink if the relief of the ink is high.
In silkscreening each sheet must be dried separately on a rack or hanging from a clothes line [* dryer].
To accellerate the drying process one can provoke a draught or pulse hot air onto the impression (unless the ink be a glycerophtalic one). Infra-red tunnels are also used in industrial silkscreening but such drying requires much caution.
The drying time of inks is relative since it depends on the ink used as well as on the atmospheric conditions. A printer should be able to evaluate these variables. Actually drying time can be anywhere from less than a second to more than a month. The drying time can be divided into two distinct phases:
the first is the dust-free drying phase, which is the amount of time it takes for the ink to dry encugh not to catch any dust, the second phase is the setting, which is the time it takes for the ink to dry enough not to stain when the impressions are piled up. As long as the dust-free phase is not complete one must be very careful, especially with very sticky inks, such as glycerophtalic ones. The setting phase, on the other hand, is not always easy to determine, the reason being that the size ni the pile influences the outcome of this phase in two different ways. First of all the weight of a big pile may be such that staining continues to appear. The second reason is that a big pile and the consequent pressure reduce the air circulation thus slowing up the drying process. In silkscreening and when using glycerophtalic inks in particular, the setting process might be very slow and might still not be complete even when a finger no longer sticks to the ink. In fact, the ink may remain pasty below a dry crust which forms very quickly and acts as an air barrier. In such cases accidents may happen such as sticking when the impressions are piled up. To avoid such damage impressions whose drying point is not certain should be piled up on their sides.
The drying of inks before use is another problem that should not be forgotten; first of all because the inks should be kept properly and secondly to reduce expenses. Oily inks form a layer on the surface which is often called the skin. The skin protects the ink below it from being oxydized but, in the long run, the ink below becomes thicker and thicker until it is unusable. Synthetic inks also dry up and form a skin which does not protect the rest of the ink from thickening. Cellulose and ethyl cellulose inks do not dry up if they are kept in a hermetically closed container. Glycerophtalic inks form a skin on the surface which may protect the ink below it for quite a long time.
Varnish reacts more or less in the same way as ink. Alcohol base varnish dries by evaporation. Oily varnish dries due to oxydation and penetration [* ink, paper , dryer].

[2] DRYING LITHOGRAPHIC PREPARATIONS. The drying of preparations is of great importance in lithography. Depending on the work, drying must be done immediately after the applications or then might be done after a varying amount of time. Traditionally drying was done with a tourniquet or a fan but today a pulsed air dryer is used [* lithography].

[3] DRYING LITHOGRAPHIC STONES. The stones used in lithography must be perfectly dry before being worked on. If they are a bit damp they can be dried out in a stove or in front of a heater. The heating of the stone must be progressive (not brutal) and the stone must be kept standing on its side. At times this drying process is called "making the stone perspire".

[4] DRYING PAPER . Paper used for intaglio printing is often dampened and so is the paper used for printing relief engravings and lithographies. In the past even paper for delicate typographical work was slightly dampened. Because of this dampening one must then dry the paper after impression. This drying is done with the sheets lying flat and by applying pressure [ * pressing].
The sheets that have already been printed on are put between two very clean, white sheets of blotting paper making sure that the side carrying the image is protected by a sheet of tissue paper. All of these sheets are then put between two pieces of strong but absorbent pieces of cardboard which are, in turn, weighed down. It may, however, be better to use boards made of wood shavings which stay perfectly flat and are particularly absorbent. The drying done with such boards is very rapid and it will not be necessary to use blotting paper. All that is necessary is to cover the image with a sheet of tissue paper making sure that the "felt" side of the board, which is softer, lies against the paper. In fact, such boards will be found to have a "felt" and a "cloth" side. The boards should be chosen for their perfectly level surfaces. When drying paper one can pile up many such boards and impressions and thus it will not be necessary to weigh them down [* dampening , paper].

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