Sodium silicate can be diluted in water and will have some sealing properties when absorbed into porous clay surfaces. But, of course, it will shrink as it dries, thus not completely sealing. However the product "Liquid Quartz" is claimed to be very effective for ceramics because it is doped with nano-sized particles of quartz. It is claimed to be a non-toxic, food-safe, water-based sealer. It is also much more expensive than other options.
Liquid Quartz from Walker Ceramics
Liquid Quartz manufacturer website: What is it, how to use it, what is can do, what it cannot do.
Many ceramics are either porous by nature or by necessity. For example, stonewares need to be non-vitreous enough that they do not warp or blister on firing. Red earthenwares must be porous in order to have the red color (they go brown when fired higher). White talc or dolomite low-fire clay bodies always have high porosity. Bricks must have minimal firing shrinkage, which guarantees substantial porosity. Even porcelains can blister and it is common to cut back on feldspar to give them more margin for overfiring - that brings porosity. If water penetration must be prevented all of these need to be sealed, these are some of the methods.