A paper containing such a large proportion of mineral matter intimately mixed with the fibre is naturally very weak. It easily tears, and if moistened with water goes all to pieces. At the same time it is a cheap substitute for high-class art paper, being suitable for circulars, temporary catalogues, and similar printed matter.
In an “art” paper the nature of the fibrous constituents is too often regarded as a matter of secondary importance, because in the process of printing the ink does not come into contact at all with the paper, and an impression is produced merely on a layer of clay which is bound together by the glue.
The illustrations are not absolutely permanent, and it is perfectly easy to remove the whole of the impression and the coating itself by immersing a sheet of the paper in warm water and rubbing the surface gently with the fingers, or with a camel-hair brush.
In fact the amount of coating matter which has been brushed on to a paper can be determined approximately by weighing a piece of the coated paper, removing the mineral matter and glue from both sides as indicated, allowing the paper to dry again, and then re-weighing, the loss in weight representing the amount of coating.
It is not surprising to find that the true paper is merely regarded as a convenient means of producing, so to speak, a smooth surface of clay, and an examination of the material between the two clay surfaces often reveals a paper of very low quality.
There are one or two empirical methods for testing the condition of coating on an art paper. If the coating is firm and adherent, then on pressing the moistened thumb on to the surface none of the coating matter is removed, but in a badly-made art paper some of the coating adheres to the thumb.
Another method is to crumple a sheet of paper between the fingers, and if any of the coating comes away easily the paper is considered of poor quality.
The complete examination of an art paper, apart from the practical test of printing, involves the determination of the amount of coating matter added to the paper, the proportion of glue in the coating, and the usual analysis of the paper itself.
Packing Papers.
This term may be applied to wrappings specially treated with substances which render the paper air and water proof. They are principally used for preserving food, or such articles as tobacco, which require to be kept slightly moist.