Because of the way they are laid, tomettes are traditionally not sealed, as this would prevent them 'breathing'. More recently, it has become normal to 'traditionally' seal tomettes using a mixture of linseed oil and white spirit, but I was reluctant to do this, as the experience of the heritage conservation organisation I used to work for found that linseed oil used on wood actually attracts moisture and dirt, which then forms an almost indestructible layer of surface and impregnated gunge.
However, we did feel that it was impractical to use the tiles in a kitchen and not protect them from the likes of drops of oil and red wine spills. On the advice from the tile factory, we have impregnated the tiles with a product called Sarpasol, applied before grouting, which claims to protect against dirt and efflorescence (salts carried by moisture moving through porous material and crystalising out as a bloom on the surface).
The sole ingredient of Sarpasol is listed as 'naphta lourd hydrotraité', which I assume is the substance known in English as 'heavy naphtha'. This is a petro-chemical and an acid, which is presumably the secret of how it works. It must neutralise salts as they emerge. It is a thin colourless liquid, lightly and pleasantly scented, with a slight oiliness to the touch. I brushed it on the tiles, making sure to get the sides as well as the tops, as advised by the factory. The tiles sucked the first coat in within a couple of hours and I gave them a second coat, which they absorbed nearly as quickly. The man at the factory recommended we give them three coats if possible, but because time is short, we gave them two coats before grouting. Each coat used about a quarter litre of Sarpasol for 3m² of tiles. It changes the colour of the tiles slightly, making them darker and brighter. It was interesting to see how some tiles absorbed the substance almost instantly, whilst others must be less porous and took longer to draw it in.
Related posts: Buying Tomettes, Laying Tomettes, Tools for Tomettes