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Excerpts from the book:
Theory and Practice of Goldsmithing
Niello is a deep black metal mixture that is fused onto metal for decorative effect. Though worked on three-dimensional objects, the effect might be considered graphic because it relies primarily on shape and pattern. The contrast of the lustrous black inlay against either matte or polished metal is striking and has earned niello a special place in the arsenal of decorative techniques.
The process can be thought of as having two parts - making the niello material and fusing it into position. The latter is generally a matter of filling depressions or incised lines made by engraving, etching, stamping, rolling or layering (figures 10.1 to 10.3).
This technique can be traced all the way back to ancient times. Theophilus,
a monk in the Middle Ages, has given a description of the procedure which
is so accurate that it could be used here. Unfortunately niello is rarely
used today, almost forgotten, perhaps because goldsmiths have forgotten
how to work with engraving tools and chisels. There is no reason why niello
can not be used in contemporary design and achieve again the high regard
in which it was once held.
Test the niello by striking a lump with a hammer. The piece should shatter like glass and reveal a fracture surface that is uniformly black. If the niello does not show both of these properties, remelt it, using more sulfur and stirring the mass more completely.
Applying and Firing the Niello
The niello paste is packed into the recesses with care typically sliding it off a small carrier such as a fine brush (a goosequill is specified in the ancient texts). Pack the paste as firmly as possible with a small spatula to create a dense filling. Even with this the niello will occupy less space as it fuses and should therefore be mounded up higher than the level of the piece being filled. When the piece is "loaded" it is set into a warm spot to dry, for instance under a lamp or on a heater. If the piece is fired when wet, the expanding steam tends to throw the niello particles aside.
Goldsmiths of the Middle Ages melted their niello over a charcoal fire; today an electric kiln is preferred because of its uniform, controllable heat. The dried piece can be set directly into the kiln, or laid onto a tile or piece of firebrick to make it easier to withdraw from the kiln. First the ammonium chloride spreads over the niello as a white covering layer. As the heat approaches 500°C (930°F) the black metal powder will glow red and spread to fill up the depression into which it was packed.
In flat objects gravity will pull the taffy-like molten niello into place.
When working on curved surfaces, particularly something a fully round
as a sphere, gravity of course has the opposite effect, pulling the niello
out of place. Rounded objects must be kept turning, a process made easier
if they are attached somehow to a long rod. The piece can be withdrawn
for brief times from the kiln and the niello troweled back into place
with a lightly oiled steel spatula. Prolonged heating will allow the sulfur
portion to burn away, leaving a porous brittle surface.
If there are voids or bubbles, apply additional niello paste and repeat the process, using the spatula to press the fresh melt into the first layer.
In some cases, particularly those where an engraved or stamped line is being filled with niello, the paste process just described is less successful than the following adaptation. Pour the niello into an ingot mold or similar arrangement to make rods of the raw material. Prepare the metal as described above, brush on a coat of the ammonium chloride flux and warm it with a torch. Keeping the metal warm, melt the tip of the niello rod and smear it into the prepared grooves, adding as much as necessary to fill the groove.
The niello is relatively soft and will therefore respond well to burnishing.
Be careful when using a flex shaft or polishing machine for the same reason:
the niello will be removed twice as quickly as silver or gold. Stiff polishing
equipment such as a leather stick or a felt wheel are preferred.
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