MEDIUMS AND ADHESIVES 57
a natural inference, for pitch and crude varnish were used in ship-building, and
the Egyptians used pure resins. The only historical reference, however, is made
by Pliny (John, p. 36), who, in speaking of Apelles, tells how that artist covered
his pictures with a layer of ' atramentum.' This was evidently some kind of a
varnishing process; the layer was said to be so transparent that, while it did not
affect the colors and served to protect them from dust, it was invisible, except
under close examination. According to Pliny, no one was able to imitate this
process. It is hard to understand why there were difficulties, for, from Pliny's
intimations, it can be assumed that the varnish was a semi-liquid resin, dissolved
in a little fluid bitumen. He lists resins obtained from many varieties of pine
and others from Syria and Africa, such as mastic and terebinth; but, although
resins dissolved in olive oil were used pharmaceutically, there is no mention of a
varnish being prepared in this way. Varnishes that can be applied with a brush
require the use of a thinner or of a solvent, The solvent power of turpentine was
known as early as 460 B.C. and was referred to by Pliny, but apparently spirits
of turpentine, or other volatile thinners, were not employed in the manufacture
of varnish.
Historical records about resins and their use in varnish making up to the
XVIII century are largely recipes and statements of early writers. These have
to do with oil varnishes, but it is often impossible to know the exact nature of
the resin called for. The term,c amber,' was used to cover a great variety of hard
resins, and it referred often to their color regardless of their origin. In the same
way, * mastic' and ' sandarac' are used indiscriminately, and other resins are
, called by names which are difficult to identify today. Many of the varnish recipes
given in the Lucca MS. in the VIII century and those given during the following
centuries, however, may be interpreted in the light of present knowledge about
the resins probably available at those times. Both mastic and sandarac were
used and many pine balsams were known. Of the hard resins, amber was certainly
obtainable at an early date, and African copals probably could be found in the
markets. Barry (p. 44) quotes Fr. Stuhlman (Deutsch Ost. Afrika) as believing
that the trade in copals began as early as the X century when Arabs exchanged
it with the Indians for cotton fabric. From isolated statements it appears to
have been used in the XVI century, but it is not mentioned to any extent until
the latter half of the XVIII century. The Lucca MS. (Berger, III, 15) describes
the preparation of a varnish by dissolving resin in linseed oil, and one recipe,
de lucide ad lucidas, calls for amber, mastic, three kinds of turpentine, resin,
galbanum, myrrh, two gums, a little linseed oil, and florae puppli. This would
make an extremely stiff varnish which, while still hot, would have to be rubbed
onto the picture. Here there is confusion again about the term, amber. True
amber could only have been dissolved by first fusing the turpentines and by
adding the amber before the oil. Several recipes call for a hard resin, and, although
this is called * glassa * or' karabe,' and has, therefore, been assumed to be amber,