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169.

Asphaltum.

Procure it possible the true Asphaltum of the East, instead of that which is usually substituted for it - the residuum of Coal Tar, after distillation.  On a Sand bath, dissolve it in Spirits of Turpentine, & add warm Drying Oil.  It may be kept in Bladders or Compressible Tubes.  It give transparency to Raw & Burnt Umber; and with Prussian Blue makes a rich dark glazing Green.  It should not be used with White nor Vermillion.  A little Magnesia ground with it lessens its viscosity.  Bouvier recommends it as "excellent for glazing, because of tis fine Bistre tint, its perfect transparency & the facility which the extreme divisibility of its particles affords of spreading it as thin as can be desired.  Besides it mixes well with all other colours, so that you can give it the precise hue you wish."  "It is adviseable to use it only in those Cases where there is little to be feared from an increase of vigour & intensity. #

Pigments

"With regard to the materials themselves, it does not appear that any Colours of importance, used by the Flemish painters, are now unknown; On the other hand, some valuable pigments have been discovered by the moderns of which those painters were ignorant."  Field.

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Transparent & opake pigments.

"With regard to transparent & opaque pigments generally, it is worthy of attention in the practice of the oil painter, that the best effects of the former are produced when they are employed with a resinous Varnish; as opaque pigments are best employed in oil."
Field

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