The construction of this marquetry, or better said, of mosaic in metal, although a bit similar to that of mosaic in painted wood, of which I made the description, page 866, requires however to be addressed separately, due both to certain ways of working which are different and the quality of materials that one uses. These materials, brass and tortoise shell, are used as in ordinary marquetry techniques, the one forming the designs and the other the background of the work.
Since the materials that one uses for marquetry, even the most common, are of a certain value and are rather difficult to put to work, we look for ways to be sparing both hand-work and materials at the same time. That is what we accomplish in cutting out two sheets of different materials (like brass and shell) one on top of the other [simultaneously], in a way that you have created at the same time two pieces of identical marquetry. The flowers or ornaments of the one can be placed in the void of the other, of which consequently, you can remove the flowers and ornaments to place them in the void of the first one. This manner of cutting out marquetry is called “working in contra partie.”
Marquetry where the tortoise shell makes the base of the work, and brass the ornaments, is the most beautiful, and is called “la partie,” that is to say, the principal piece of work. [Where] the brass makes the base and tortoiseshell the ornament, is called “contra-partie”, [the second part] and is less valuable, although one can achieve making it very costly, about which I will speak in its turn.
When you wish to make a piece of marquetry, you begin by making the general design of the work to be executed, as, for example figure 1, where you take one or many tracings, according to what is necessary, the initial design is to be conserved for using to finish the piece.
Then you take a sheet of shell of a convenient size, and a sheet of brass [in the original French text, here the author is specifically referring to brass while elsewhere Roubo may be referring to brass or copper and we have chosen to use the term brass as it seems more sensible — DCW], which you glue one on top of the other with ordinary glue, not all over but only in a few spots, which is sufficient for fastening them together. Before gluing the sheet of brass, it is good to scrape the underside with a coarse file used in different directions, or even with a toothing plane iron, so that when it is cut out, it bonds better with the glue.
When the two sheets are thus joined together, you glue the tracing on that of the brass, and you let it all dry, after which you cut it out with the marquetry saw in the same way as for mosaic in wood.
Since the ornaments of marquetry or mosaic in metal are ordinarily very delicate and of a form often very complicated, it is necessary to use extremely fine saw blades. You often start by piercing little holes in places that are the less obvious in the work. These holes are made with the little hand drill, represented in plate 319, figures 1 & 2, or even with a pump drill, figure 12, same plate, which depends absolutely on the different occasion or taste of the worker. With much delicacy and skill he places the saw [blade] such that he always sees the line on the tracing, and that he cut out the largest quantity possible without being required to remove the saw.
[It is fascinating to read Roubo’s exhortation regarding working posture and the need for strong directional light (even without stating it explicitly), namely that the orientation of the workpiece to the artisan be such that the marqueteur can always see clearly the line he is cutting. The undeniable conclusion is that the workpiece is moving constantly and the saw and lighting orientation remain constant while the sawing is underway. Furthermore his instruction to “cut out the largest quantity possible without being required to remove the saw” may have been honored more in the telling than in the practice. There are numerous historical examples of craftsmen simply cutting “across the field” of the background to get from one element to another rather than removing and reinserting the saw blade in a new hole. Was this because the latter have been troublesome it would have placed stress on the precious saw blade, perhaps an unnecessary risk at that time? – DCW]
When the design is entirely cut out, you separate the two sheets one from the other, whether by plunging them in hot water, or simply by passing between them a very thin blade of a knife, which is sufficient, when you take care to place nothing but a few spots of thin glue, and simply that which is necessary to hold the two sheets together, as I said above. After the sheets are separated, you disassemble them, that is to say, you take out the ornaments for repositioning one piece inside the other, so each piece remains empty, like that represented in figure 2.
— Andre Roubo, with notes from Don Williams