Polymetaal

These inks were especially developed for intaglio techniques. Intaglio is different from relief printing, because cut lines contain the ink during printing. The image is actually below the surface (intaglio = insicion), as with relief printing the image is on top of the plate.

MAKING THE INK

Etching inks are made of ground pigments mixed with a binder and a polymerised linseed oil.

Preparation

The pigment powder is mixed with linseed oil in a mixing vat which resembles the machine to make dough in a bakery. This paste is made to be able to mix the pigment with the binder.

The binder is a colourless substance that gives back the full colour to the pigment and determines largely the properties of the final product. It gives the paste its viscosity, it improves and standardises the drying time and gives a harder and longer lasting top layer. Making the pigment paste takes about three hours.

GRINDING

Now the paste will be ground: the paste is pressed against a hard surface (granite or steel) which makes the pigment particles being crushed. This grinding makes the product smooth, shiny and creamy and gives the pigment the full colour power. For this job a three roller paint mill is used (developed in the first half of the 19th century). The longer the grinding time, the finer the particles and the better the quality of the ink later on. The grinding time is also related to the hardness of the pigment.

The colours of Charbonnel are ground 3 to 5 times; for a 100 liter ready made product, the grinding takes about 6 hours and five times about 12 hours..

CONDITIONS DURING THE GRINDING

While making the pigment paste it may not become too warm (friction) because the consistency of the oil may change making it too thick. Mixing must be done slowly and should be interrupted regularly.

After mixing the paste, it stays over normally a whole night, allowing the pigment to absorb the oil completely, before grinding starts.

During the grinding the space between the rollers is changed as the grinding progresses. The distance between the rollers is made smaller each grinding run, except during ther last run. During the last run the space must be slightly more, allowing the paste to be mixed thorougly.

QUALITY CONTROL

After grinding, three quality criteria apply:

COLOUR:

The colour will be compared with the standard colour as well as the transparency.

CONSISTENCY:

The consistency of the paste is checked; adhesion properties (tack), viscosity, and wiping properties.

The adhesion properties are checked by applying some ink on to a glass plate with a spatula. The ink should not stick to the spatula and form a layer on the glass surface.

Wiping properties are checked by applying ink to a plate and wiping it with tarlatan of paper.

SIZE OF THE PIGMENT PARTICLES

This is cheched with a gauge. Some of the ink is applied to the gauge with a scraper. Depending on the thickness of the ink layer in the gauge, the ink should be uniform, without visible particles.