SUPPORTS 235
Glass Fabric. Molten glass can be blown or drawn into fine fibres which can
be woven into fabrics. Although such fabric is used chiefly for electrical insulation,
it is also made into finer stuffs. Its use as a support for paintings has been sug-
gested and small quantities, glass fabric with a prepared ground, have been
marketed by artists' supply houses. Because of its inorganic nature and, hence,
its resistance to mildew and other agents of organic decay, it may be good for
special purposes. The fabric has high gloss and sheen, is harsh to the touch, is
not elastic, and is brittle when sized with glue. Glass fibre is marketed under
the trade name, 'Fiberglas/ by the Owens-Corning Fiberglas Corporation.
Gum Wood, red gum, and satin walnut are names given to the Liquidambar
styraciflud) a tree indigenous to North America. Species from other genera, how-
ever, may be called * gum wood/ The wood from L. styraciflua is medium hard, is
easily worked, and has no important variations in structure. The heart-wood is
reddish brown and the sap-wood, pinkish white. The pronounced figure in tan-
gential and radial sections is caused by dark streaks that are distinct from the
annual rings. The pores are small and rings are not easily noticeable in sections,
either cross or longitudinal. Medullary rays are quite clearly marked in strictly
radial sections. The wood is strongly inclined to shrink and warp.
Gypsum Cement. Plaster of Paris is not the only cement or plastering material
made from gypsum. When gypsum is burned at a higher temperature than that
used in making plaster of Paris, it loses all, or nearly all, its water of crystallization
and the product has poor setting qualities. If, however, this product is dipped in
certain salt solutions, such as alum, borax, cream of tartar, etc., and is again
calcined, a very useful cement, slower to set and more easily worked than plaster
of Paris, is obtained. These highly calcined gypsum products are known under
various proprietary and traditional names, such as Keene's cement, Martin's
cement, and Parian cement.
Hemp is derived from the bast fibres of the stem of Cannabis sativa. It has
been used for centuries in the making of ropes and sailcloth, and the finer varieties
have been spun into yarns and woven into textiles. It probably originated in
India but will grow almost anywhere, and commercial varieties include Russian,
Italian, Chinese, and Arabian hemp fibres. In a few cases, chiefly during the XIX
and XX centuries, hemp cloth has been used as a support for paintings.
Hydraulic Cement is any cement that hardens by chemical combination with
water, but the term is usually restricted to that kind which is obtained by
burning a mixture of lime and clay and pulverizing the resulting clinkers, and
which, ordinarily, goes by the name, Portland cement (see Portland Cement).
Pozzolanic cements also come within this classification (see Pozzolana),
Iron. Since mediaeval times iron has had first place among the metals. Its
properties and method of preparation are familiar. Although for purposes of
armor-making it was beaten into thin sheets quite early, it probably was not rolled
into sheets to any great extent before the middle of the XVI century. Its tendency