A quick foreward to this post – I originally did some “lets see what happens when I try some methods of finishing castings off that I think they might have used” last year, about 5 minutes worth. That convinced me that using files and stones was possible. Then of course I realised it would be a good idea to do some research as well, using my well stocked library. Hence this post, which follows on nicely from the previous one.
So, now you have a nice cast object. It is still discoloured and a bit rough from the casting process. What do you do?
To answer this I turn to the usual two printed sources that bracket my period of interest, Theophilus and Biringuccio.
The 12th century monk Theophilus describes the use of files to finish a cast silver chalice off before engraving it.
More interestingly, he writes, page 175 (Of the Dover paperback edition), after casting a bell and bracing it on the lathe, “…the bell can be turned and smoothed all over with a sandstone.” Which is surely good evidence for the use of a natural and often freely available precursor to sandpaper! You can get different types of sandstone too, some fine, some coarse, which would give you great ability to smooth it out properly.
Earlier, he writes of the making of burnishers, which are for finishing the work, they are like scrapoers but more rounded. They are still in use today for smoothing out the surface of a metal object by plastic deformation.
You should also, (page 93) have files made out of pure steel, hardened with a form of carburisation. Large, medium sized, four cornered, three cornered and round. The descriptions are not so clear, but he does refer to making finer files “.. work whic h has previously been filed with other coarser files should be smoothed.”
Obviously I need to expand my toolkit, get some more files and burnishers as well as different grades of sandstone.
Biringuccio writes on page 377, regarding the finishing of flasks, saltcellars etc, “With rasps and scrapers and other cutting tools they are smoothed, polished and made beautiful.” I get the impression that the finishing of metal items is so well known about and widespread that he doesn’t see any need to go into details, whereas the art of casting in bronze and such is tricky and difficult and he wants you to know all about it (And that he knows all about it).
I have an interesting book on English pewter, called “A history of British Pewter” by John Hatcher and T. C Barker, published in 1974. It has an outline of the manufacturing process, and it is clear that you used the lathe for it as told by the period sources, with several types of iron tools similar to those used by wood carvers for burnishing the surface. Interestingly though you also hammered some types of wares, like plates.
But it obviously only covers pewter.
Another source is the treatises of Benvenuto Cellini, of the mid or later 16th century. Page 57 of the edition I have talks about finishing a work of art, some sort of cast and soldered together object, and he wrote “I took some four or five hard pointed stones, which are sharp at the ends and thicken upwards in the manner and I used them with some well powedered pumice stone. The object of using these stones is to take out the markes of the steel tools, the punches, chisels, files and suchlike and to give it a fine uniform surface; and last, but not least, a brilliancy of colour which would not be so easy if the marks of the steel tools (And the skin they make) were not obliterated.”
Page 94 tells us about the cleaning up of a large silver statue. He used pumice to clean and smooth it, then heated it to red hot and put it in a blanching solution of water with tartar and salt. Then it was scrubbed and when the silver started going white it was put in another vessel with clean water, to wash the blanching solution off. After that it was ready and clean for gilding.
Between them all that gives me enough information to improve my displays and production processes. But as usual, practise is required.
For contrast, here’s a photo of a later medieval bronze cooking cauldron from Dublin. Note that it isn’t really well finished off and clearly shows the flashing line where the outer mould was split in half to enable it to be taken off the mould it was shaped on.