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Desiccation Or Drying The Well-known Additions Of The Acetate Or

sugar of lead, litharge, and sulphate of zinc, either mechanically
ground, or in solution, for light colours; and japanner's gold size, or
oils boiled upon litharge, for lakes; or, in some cases, manganese and
verdigris for dark colours, are resorted to when the pigments or
vehicles are not sufficiently good dryers alone. It would be well if
lead and copper could be banished from the list of siccatives
altogether: assuredly, no artist with any regard for the permanent
texture of his work should employ them except in extreme cases, and in
the smallest possible quantity. The best of pigments may be ruined by
their injudicious use, and obtain a character for fugacity which they in
no way deserve. It requires attention that an excess of dryer renders
oil saponaceous, is inimical to drying, and is otherwise injurious. Some
colours dry badly from not being sufficiently edulcorated or washed.
Sulphate of zinc, as a siccative, is less powerful than acetate of lead,
but is far preferable in a chemical sense. It is supposed erroneously to
set the colours running; which is not positively the case, though it
will not retain those disposed to move, because it wants the property
the acetate of lead possesses, of gelatinizing the mixture of oil and
varnish. These two dryers should not be employed together, since they
counteract and decompose each other, forming two new substances--acetate
of zinc, which is a bad siccative, and sulphate of lead, which is
insoluble and opaque. The inexperienced ought here to be guarded against
the highly improper practice of some artists, who strew their pictures
while wet with acetate of lead, or use that substance in some other
mode, without grinding or solution; which, though it may promote present
drying, will ultimately effloresce on the surface of the work, throw off
the colour in sandy spots, and expose the paintings to peculiar risk
from the damaging influence of impure air.

It is not always that ill drying is to be attributed to the pigments or
vehicles, the states of the weather and atmosphere have great influence
thereon. The direct rays of the sun are powerfully active in rendering
oils and colours siccative, and were probably resorted to before dryers
were--not always wisely--added to oils, particularly in the warm climate
of Italy. The ground may also advance or retard drying, because some
pigments united by mixing or glazing, become either more or less
siccative by their conjunction. Many other accidental circumstances may
likewise affect drying; and among these none is to be more guarded
against by the artist than the presence of soap and alkali, too often
left in the washing of his brushes, and which, besides other bad
results, decompose and are decomposed by acetate of lead and most
siccatives. In such cases desiccation is retarded, streaks and patches
are formed on the painting, and the odium of ill drying falls upon some
unlucky pigment. To free brushes from this disadvantage, they should be
cleansed with linseed oil and turpentine. Dryers should be added to
colours only at the time of using them, because they exercise their
drying property while chemically combining with the oils employed,
during which the latter become thick or fatten. Too much of the
siccative will, as before noticed, often retard drying.

The various affinities of pigments occasion each to have its more or
less appropriate dryer; and it would be a matter of useful experience if
the habits of every colour in this respect were ascertained. It is
probable that siccatives of less power generally than the compounds of
lead and copper might come into use in particular cases, such as the
oxides of manganese, to which umber and the Cappagh browns owe their
drying quality.

To other good attributes of pigments, it would be well if we could in

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