Page:Paper and Its Uses.djvu/77

Rh papers are made entirely of esparto, finished to produce as bulky a paper as possible, consequently the fibres are not well rolled together, and the books printed on such paper are anything but durable. Imitation art papers give a mineral residue of 25 to 35 per cent. on ashing, and have very little strength, owing to the large proportion of china clay present.

Art papers do not fold or stitch well, as the mineral coating, although firmly fixed to the paper, behaves as a non-fibrous material might be expected to do, breaking down, and the paper beneath tends to give way too. If kept in a damp place art papers absorb moisture at the edges, and in the presence of a large amount of moisture the sheets will stick together. Rag art papers are procurable (the body being a rag paper), and possibly it will be found that such papers, kept from air and moisture, will be very durable.

Papers containing a large proportion of mechanical wood, whether coated or otherwise, are certain to deteriorate rapidly. A newspaper exposed to sunlight for a day or two becomes discoloured and brittle, the same result following in a longer time if exposed to light and air without the sun. For this reason papers containing mechanical wood should never be employed for work which is to last. Cheap reprints of standard works are sometimes printed on such paper, but it is a very doubtful economy on the part of the publisher.