Shaw, Henry: The decorative arts, ecclesiastical and civil, of the Middle Ages (London, 1851)

This being done, the crucible is to be soon withdrawn, and the whole to
be slirred well together with a cinder held in the tongs, because the lead
always naturally making a little scum, it must be removed as far as
possible with the cinder until tlie three metals are thoroughly incorpo-
rated and quite pure. Then have ready an earthen vessel, as large as the
first, with a narrow neck, sufficiently large to admit the finger. This
must be half filled with well pounded sulphur, and the above mentioned
metals being well melted, are to be poured hot into this vessel, which
must be immediately stopped up with a little fresh earth, keeping the
hand over it, and finally closing it up with a large piece of old linen.
While the composition cools, it must be kept continually agitated by
the hand, and when cool it must be taken out of the vessel and broken,
when it will be seen that by virtue of the sulphur the fusion (wh ih is
called niello) will have taken its black colour. The niello will have
many grains, although the objedt of shaking it was to mix it well toge-
ther. However, in the state in which it is, it must be put again into a
crucible, as in the first instance, and melted over a slow fire, putting on
it a small piece of live coal. This re-melting must be repeated two or
three times, and every time the niello must be broken, and the grain
examined, until it is found to be very close, when the niello is perfeit.