Enamels and enamelling; an introduction to the preparation and ...

TESTING THE RAW MATERIALS AND ENAMEL MASS. 85
when cold, dipped in water and just brought
in contact with
the powdered pigment, sufficient of which will then adhere to
suitably colour the bead.
Should, after these precautions, the colour still be too in-
tense, the bead is removed from the loop, and a fresh bead made
and dipped in the powder furnished by the old one. By this
means the colour is so diluted and lightened that an accurate
judgment will become possible. [N.B. These tests must always
be made by daylight.]
The following colour reactions are furnished by the prin-
cipal enamel pigments,
the blowpipe :
Iron ..... Bottle
with a borax or enamel bead, before
In the
Reducing flame. Oxidising flarne.
green . . Reddish yellow.
Iron and alumina . . Bottle green . . Red.
Copper . . . . Red .... Emerald green.
Cobalt .... Blue .... Blue.
Manganese . . . Colourless . . . Violet.
Chromium . . . Emerald green . . Yellow.
Antimony .... Yellow . . . Yellow.
Uranium .... Yellow . . green Yellow green.
Silver .... Grey .... Yellow.
Zinc Yellow when hot, colourless when cold.
Lead Yellow when hot, yellow when cold.
It should be noted that, .as already mentioned, strong
(oxide) pigments such as ferric oxide, manganese dioxide, cobal-
tous oxide and cupric oxide, when used in somewhat larger
amounts, produce deep black, in which event the colour must
be diluted by fusing over again with a new colourless bead.
Furthermore, that the colour of the beads while hot differs
from that exhibited at the ordinary temperature ; thus, beads
containing lead or zinc are yellow when hot, whilst, on cooling,
the zinc bead becomes colourless.
TESTING THE ENAMEL MASS.
In order to examine somewhat larger quantities of enamel
mass for colour, a thin strip of platinum foil, or, better still, a