It must be understood that colour printing can be done well only upon a perfectly even and smooth surface, therefore it is wise with first-impression blocks, as well as larger colour plates, to repeat the impression, only slightly warm after having given a very hot impression the first time.
Surface papers are an exception as they generally stand very little heat.
It helps here either to let the newly made covers get half dry or, better, to let them lie longer in a damp room.
When different colours are to be superimposed, the order in which they are printed is a matter of some importance. As a rule, the so-called warm tones (yellow, brown, red, or colours containing much of these) are printed first; the cold tones (blue or others containing much blue) being afterwards laid on. White is also classed as a cold tone, but for technical reasons it must nearly always be printed first. If these rules are observed, serious mistakes will seldom be made.
Moreover, proof impressions of the different colours must be taken upon white cardboard, which should be marked for reference, in the same way noting the proportion of the colours used in mixed colours. If the outline block is to be printed in colour, it comes last; gold stamping being done first.
The process of printing in bronze is similar to colour printing, as the bronze is dusted over a preliminary impression in colour (gold upon red or yellow ochre, aluminium upon white, coloured bronze upon a suitable similar colour). Bronze is not printed upon colour, because the bronze sticks too easily to such surfaces in places where it is not wanted; it is then much better to use gold-leaf.
Should it be necessary on the score of economy to make a double working in bronze, the cover must be previously washed with glair.
Leaf metals, pure or alloyed, are printed upon colours after they have been slightly dusted with gilding powder, and with moderate heat. The colours, however, must previously be allowed to dry thoroughly. Larger surfaces are wholly prepared with glair to prevent the gold from adhering where not required.
Most of the colours must cover well where it is not intended that they should have a translucent effect. A good covering is not secured by laying the colour on thickly but by properly mixing the colours, and often also by giving a second coat after the first surface has dried. Above all, one must avoid too thick and greasy colours. Black must always be printed with a strong good covering colour, as must also the outline block.
For colour printing, the press must be of the same temperature as the room. In winter it is slightly warmed an hour before use, and when necessary this must be repeated later.