With regard to the proportion of the addition, the
following circumstances may serve by way of guide:
When the colour of a decoction is darkened by the addition, without any
precipitate being produced, no detriment can easily arise from using a
redundancy of it, because the colour will not be further darkened by it.
But if the colour be required to be brighter, the trial must first be
made, which is the proportion by which the colour is darkened the most,
and then less of it must be employed.
When the colour of a decoction is brightened by an addition without a
precipitation ensuing, this addition can never be used in a larger
quantity without hurting the colouring particles; because the colouring
particles would be made too light, and almost entirely destroyed.--Such
is the consequence of too large an addition of the solution of tin or of
a pure acid.
When the addition produces a brighter colour, and part only of the
colouring substance is separated without a further addition occasioning
a fresh separation, somewhat more of it than what is wanted may be added
to produce the requisite shading; because experience shows that, by this
means, a greater quantity of tingeing particles is united with the
woolly fibres of the cloth, and is capable of being, as it were,
concentrated in them: for which purpose, however, these barks must be
boiled down.
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