The following article is taken from a PDF that I was asked to do for a the guides at a Roman Villa site here in the UK. Please feel free to let me know if you have any links to other studies on this.
Vitruvius mentions the finished floors needed to be ground down to eliminate any unevenness (Vitruvius 7.1.4.). I can find no other mentions from the time on finishing floors.
There have been a number of finds of grinding stones at worksites and floors do show the signs of grinding on the surface. Amongst piles of cut tesserae, four grinding stones were found at the Fountain Court in Kenchreai, (a temple in the eastern port of Corinth, Greece) along with a ‘heap of red abrasive for polishing’ (R.Scranton. J. Shaw, L. Ibrahim, Kenchreai, Eastern Port of Corinth I, Topography and Architecture (Leiden 1978), 68, 127–8, pl.XLIX d.). From K. Dunbabin, Mosaics of the Greek and Roman World, Cambridge University Press.
Note; Red oxide is a powder that is sold now for polishing metals but I have no idea if this was a part of the ‘red abrasive’ found.
I do not know of any studies where they have analysed the surface of a Roman mosaic to see if any traces of polish have remained, though I assume someone must have considered this.
From the website concerning sculpture in the ancient world http://www.artofmaking.ac.uk/ ‘The smoothing of the surface of the carving with a series of progressively finer abrasives to achieve a range of finishes. There are different gradations of polish, ranging from a basic matt one to an extremely high gloss finish. A matt one can be achieved by rubbing emery, sandstone or pumice over the surface of the stone, usually with water to help lubricate the process. Finer levels of polish can then be achieved by using finer substances, like sand and burnt and crushed animal bones mixed with water into a paste. Matt polishes are common on high-quality Roman sculpture and high gloss finishes are also found, though these are rarer on ancient than Baroque and modern sculpture. Only certain stones can take a polish, among them marble, various hard granites and porphyries, and certain types of limestone.’ They do not seem to mention though exactly how it is applied.
Modern polishing of stone follows the same process as by hand. The stone is smoothed down using a rough grit, you then use progressively finer grit to bring up the polish.
1. The first stage is a smooth, surface which is like tumbled marble in appearance. The effect of grinding down causes the stone to have whitish marks and the colour is very dull so left at this point it does need something to bring the colours back. Wax works very well, (see below for more details). The dull appearance of ancient mosaics on display now is most probably due to some extent to this, obviously, not being reapplied.
2. If you continue to grind using finer grit pads then the surface can be made perfectly flat. This stone will still need something to bring the colours back up. I have had a piece of mosaic floor that was this flat.
3. The last stage, and I doubt too many floors were finished to this extent due to the work involved, is to have a floor surface that looks like glass. This can be done by hand, a student of mine did this with one of his mosaics using just one fine pad. This finish does not need anything else as the colours of the marble are bright and remain so.
A lot of wax polishes now use beeswax and I have no doubt the Romans used it possibly in combination with other elements. Linseed oil wax (linseed oil, beeswax and turpentine) works very well on stone. Wax needs to be well worked into the floor otherwise it will attract dirt, it does then need to be reapplied at intervals to ensure the colours remain bright.
I have read saying that the floors were kept wet as water this does make the colours more vibrant in stone. This is, obviously, totally impractical as marble will dry out in less than a minute.
Using oil would require a lot of work to get it into the stone so the floor wasn’t slippery, using wax would be the logical next step with the same effect.
There is no specific evidence for how the floors were finished but I feel that it is most likely that they were, in the main, ground down to make them smooth then wax was applied and maintained by slaves.
Wax applied to the left side of a mosaic where the tesserae had been smoothed off using a grinding stone which causes the colours to become dull.
© Lawrence Payne 2018