There are many techniques in which contrasting metals are placed side by side. Those that can properly be called inlay require a mechanical attachment to secure a soft material into a harder one.
|10.12 Gunlock, Steel with silver inlay, some looses. Middle 18th century. (State museum, Schweirn)|
Proper inlay requires materials of contrasting hardness to hold well, and contrasting color to look good. A combination that does well on both counts is the use of pure silver or gold into blackened steel. Another popular combination has fine silver pressed into copper or brass. Examples can be found in museums, particularly in weapons and decorative objects from Asia.
The principal behind metal inlays is that wire or pieces of sheet are forced into cavities or hollowed out areas. These are cut so their walls slope inward, or are undercut. This insures that a piece of metal will get stuck there once it is forcibly rammed in. It is a simple process and judging from the fact that we can find inlays that are many hundreds of years old, an effective one. While the pressing in requires sensitive hammering, the most important part of inlay work lies in cutting the cavity in the first place. There are several methods available to a metalsmith.
The process is worked as described in Chapter 10, section 7, with nitric acid being the preferred mordant in this case. As the acid works its way deeper into the metal it has a natural tendency to also work outward, creating exactly the undercut that is needed. Unfortunately this is not the natural tendency of ferric chloride, which has the usually agreeable tendency to leave a vertical wall.
In a miniature milling operation, a bur can be used in the flexible shaft machine to remove metal. Once the cavity has been excavated with a ball, barrel, or similar bur, the walls are given the necessary undercut with an inverted cone bur or with a narrow graver.
Each piece will present unique approaches, so it is best to let your instincts guide you. The point is to create a cavity with sharp edges, a uniform depth, and undercut walls. A tube or other curved surface can sometimes be decorated with a sawblade, a chisel or a bur, or a combination of tools.
An ideal approach for delicate linear designs is to make a pair of cuts with a narrow onglette graver. This tool is driven along a drawn line at a steep angle, as shown in figure 10.18b. The same tool is then passed along the same line at the opposite slant, creating a cavity with a swallowtailed cross section.
|10.18 Inlay with engraved recesses
a – raising a bead, side view
b – First cut
c – Second cut to inset wire
e- Inserted wire hammered into place
In cases where the desired inlay is larger than a wire, sheet material is used. In these cases, a square graver is used to excavate the area, then the onglette is used around the edges to create the undercut. This is shown in figure 10.10c.
Using chisels to cut away sections of metal sheet is an effective technique that has the advantage of being relatively easy to learn. The metal is secured on pitch as for repoussé, or in some similar way clamped onto a sturdy work surface. Chisels of several sizes will probably be needed, each taking the shape shown in figure 10.19b. The tool is held between the thumb and the two first fingers and struck lightly with a small hammer. Particularly when first learning, avoid the mistake of gouging out too much at a time. Think of shaving off layers of metal until the proper depth is reached. The cavity should have smooth walls and a flat bottom and attain a uniform depth throughout.
|10.19 Inlay with chiseled groove
a – chisel cut, side view
b – chisel cut
c – chiseled groove
d- tapping down the ridge
e- hammered inlay completed
It is usually best to first make the cavity walls straight, focusing on the correct shape for the cut. When this is achieved, use another chisel and go around the inside of the form to undercut the walls slightly. Where small wires are to be inlaid, an alternate method is to use a chisel to pull up a bur along the edge as shown in figure 10.20d.
|10.20 Inlay with punched grooves
a – punch side view
b – incised groove
c – flat punches side view
d- enlarging the channel
e – hitting down the edge
f – finished inlaid wire
g – rounding off the inlaid wire for relief inlay
This method is similar to chiseling except that no metal is removed. Because of this it is not recommended for large inlays, though it can be very efficient for small wires. Select a sharp liner that will cast up a bur on both sides of its mark. Use magnification to check the results of the too (figure 10.20b).
Follow the first cut with another punch, this one a flat-ended punch that will widen the groove to accept the inlay (figure 10.20d). Only a small amount of metal is needed to secure the wire but it must stand up above the surface as shown. It is important to select tools of the proper size and strike each blow cleanly so the tiny bur is not accidentally pressed down.
Use an annealed wire that is a proper fit for the groove. A wire that is too small will not be securely held while an oversize wire risks distorting the inlay (as well as requiring tedious extra finishing). Start at one end of the groove, tapping the wire lightly into place for a short distance, than proceed to the next area. The idea, seen in figure 10.18d and e, is to press the wire outward into the swallowtail or undercut area, locking it into place. The action has the added advantage of work hardening the inlay.
Once the wire is firmly pressed into place, use a smooth-faced planishing punch to even the surface and refine the inlay. Make a series of light passes over the wire and surrounding area, feathering the surface until it is flush.
In the case of a groove cut with a chisel or punch, in which the edge of the cavity has been thrown up in a delicate bur, the process is a little different. The wire is set into position and a slightly roughened matting punch is used to gather the material immediately adjacent to the groove and press it onto the wire, figure 10.20e. A variation on this is to create a roughened punch with a slight arc in its center, shown at figure 10.20g. This tool spans the wire and presses down on the inlay to lock it in place. Once it is secure, planishing hammers, files and sandpapers are used to make the surface flush.
A cavity is made as described above, using either chisels, gravers or through acid etching. The walls of the cavity are undercut, causing them to slope inward. A piece of sheet metal a little thicker than the depth of the cavity is cut out and filed so its edges are beveled to resemble the slope of the walls. The piece is domed slightly, which will allow it to drop into the cavity. Measurement and close tolerances are clearly very important.
Once it is in place the sheet is flattened with a mallet or punches, which causes it to press against the undercut walls, locking it into place. The surface is planished then refined with sandpaper.
While most inlays are flush, it is also possible to create an inlay that stands above the surface of the base. In the case of sheet inlays, the piece might be formed through repoussé or casting. The inlay process is the same as described above but of course the inlay panel must not be planished. Instead it is pressed flat with carefully located punches that will not damage the shape on the panel.
In the case of wires, the round wire mentioned above is replaced with a flattened wire, the smaller dimension of which fits snugly into a groove that has had a bur thrown up along both sides. A matting punch is used along the side of the wire to press the bur against it, alternating from one side to the other. To cinch the inlay use a flat-faced punch that has a groove the width and height of the projecting element cut into it. This punch, called a veining tool, travels along the metal, tapping the base piece against the sides of the wire and trapping it into position.