Summary of the experience attempting to clean the hearth tiles of the principal fireplace on the first floor.
The hearth is paved with Victorian encaustic tiles. There was partial, conditional success in the effort to clean the black stains from the tiles. Surprises and problems emerged to such an extent that I would not have embarked on this had I known at the start what I know now.
Over the years I have cleaned black carbon deposits from around many vintage fireplaces. I never had much success removing the black staining on the section of hearth closest to the middle of the fireplace opening. The black stains in this location seem to be literally burned onto the surface of the tiles. rather than simply built up on top of the tile surfaces.
In this case I followed some advice for cleaning by means of abrasion. In retrospect I believe this would be good advice for removing dirt and stain from most encaustic tile floors. I surmise that situation of hearths presents a special problem due to the factor of high heat. The cause is presumed to result from episodes in which burning embers occasionally manage to find their way outside the floor of the firebox and onto the hearth. I can’t remember ever seeing a hearth which , after many decades of use, did not have a pattern of similar extreme burn marks in the location closest to the center of the fireplace opening.
There is no other type of tile for which I would have attempted to use abrasives to try to remove stains. (This method could be appropriate in certain situations involving natural stone.) The solid color tiles used in such an installation are called “geometrics”, and they are made of a single color of clay throughout their bodies. (In other words, there is not a thin glaze on the surface of a tile that is different from the color of the clay body beneath it). The decorated encaustic tiles use a contrasting color of clay which is inlayed 1/8” deep into what would otherwise be a geometric tile. This kind of tile originated in Europe in the Middle Ages, mainly for church floors. The idea of the thick inlay was that over the centuries the abrasion from foot traffic would gradually wear down the tile surfaces but that the decorative patterns would remain unchanged.
My plan was to use diamond hand pads to remove the stained surface, exposing the unstained clay body underneath. I planned to grind wet, by hand, starting with 50 grit, moving through 100 grit, 220 grit, and 400 grit. There proved to be two factors which prevented this method from succeeding according to plan.
(1) Some of the black stains permeated the tiles to depths even beyond 1/8”
(2) some of the inlays proved to be less deep than the normal 1/8”
The upshot is that I had to grind down far enough on the solid color geometric tiles to remove most of the stains. I hesitated grind away any deep stains on the encaustic (decorated) tiles for fear of erasing their decorative designs.
The finished work showed success in removing some of the worst stains from the light-colored geometric tiles. However, their previously flat surfaces were left with shallow depressions in spots where the grinding had gone deep, chasing after the black stains. A great deal of staining was left untouched on many encaustic tiles. The visual effect of these stains was not as noticeable against darked or patterned backgrounds as it was on the larger light-colored geometrics.
My overall message to any DIYers who might face this challenge is this: DON’T do what I did! I do not have an answer for what you should do.
What was accomplished in this case ended up as a trade-off between good results and bad. I hope there was a net gain on the side of good results. But it violated a fundamental rule that should be followed in historic restoration: namely that the changes should be reversible and should not permanently alter the historic material.