Cerusing Oak, and Why It Works on Oak
Cerusing is the technique of packing a contrasting paste into the open pores of a coarse-grained timber so that the grain reads as a drawn line across the surface. It is an old treatment that periodically returns to fashion, and it is worth understanding because it only works on certain timbers, for a reason that is structural rather than aesthetic.
Oak, ash and elm are ring-porous. Each year's growth begins with a band of large vessels, visible to the eye as a row of open pores, followed by denser tissue. When such a board is sanded flat, those vessels are cut open and left as a network of fine channels lying just below the surface. Maple, cherry and most conifers are diffuse-porous or have no vessels at all, so there is nothing for a paste to sit in, and the same treatment simply leaves a milky smear that has to be removed.
The sequence matters more than the products. The pores must first be opened, traditionally with a brass or bronze brush worked along the grain, which scrubs out the soft early wood a little deeper than the surrounding surface. A steel brush is a poor choice on oak because oak is high in tannin and any steel left behind will react with it and leave grey-black staining.
The surface is then usually darkened or at least sealed, because the contrast depends on the paste being lighter than the ground around it. If the paste goes onto bare pale oak, the effect is close to invisible. A sealing coat also stops the paste staining the flat surface between the pores, which is the difference between a crisp grain pattern and a cloudy one.
The paste is worked across the grain so it is pushed into the channels rather than dragged along them, left to firm up, and then the excess is taken off with a cloth pulled along the grain. The timing here is the only real skill involved. Too soon and the paste lifts out of the pores; too late and removing the excess turns into scrubbing, which takes the pore filling with it.
Finally the whole surface needs a protective film over the top, because the filled pores are not durable on their own and will be worn out of a tabletop within a season. A clear film also deepens the surrounding timber and pushes the contrast further, which is generally the point of having done any of it.