Richard Jones Furniture

An Unusual Polishing Technique for Open Grained Woods. Richard Jones © 2012

The polishing of the table top to achieve the effect seen here used a chemical that reacted with natural extractives found in the wood and other chemicals found in water. The later finishing stages used tinted Plaster of Paris with the panel finally sealed with lacquer, see photograph beside this text. The blue grey background colour comes from the chemical reaction of the tannin in the white oak wood with water and green copperas, AKA ferrous sulphate which is the chemical dye. Ferrous sulphate reacts in a similar way with maples and European sycamore to produce what is known as harewood or greywood, with more images of harewood at the Tweed Table page, as well as an example of the result near the bottom of this page. In the case of the maples, not known for their high tannin content, I am not sure what the ferrous sulphate reacts with, but it certainly works.

To achieve a similar result you start by raising the grain of the wood by wiping the surface over with hot water applied with a clean lint free cotton cloth. Let the wood dry thoroughly and sand back the raised grain lightly with 220 or 240 grit abrasive paper, then brush off or blow off the surface dust.

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good base proportions around which to adjust the strength later is one heaped teaspoonful of ferrous sulphate per half a litre of water (or one pint if you prefer to work in either Imperial or American measure); some people say you should use distilled water, but I have never used it in my concoctions. If you look closely at the sample to the right you can see the solution hasn't penetrated entirely successfully into the bottom of the open pores because the brown of the wood shows somewhat. A way to help penetration of water based dyes and colourants is to add two or three drops of dish-washing soap to the mix. This reduces the surface tension of the water, discourages 'bridging' over pores, and helps it flow better into those pores and crevices. The sample shown at right had a little shellac applied to the right hand side, and remained unpolished on the left. In use the solution needs regular stirring, easily done with the brush used to apply it to the wood. Apply it quickly with the grain, preferably with a brush for large surfaces, but a cloth also works well. Keep the solution wet and flooded on the wood for three to five minutes and wipe off the excess liquid, again with the grain, using dry cotton rags or paper towels. The colour gradually appears as the wood dries.
 
Apply a thin barrier coat of a film forming polish over the dyed wood, eg, shellac, varnish or lacquer. I used a thinned out sprayed pre-catalysed lacquer on this table top. This seals the background colour off from the subsequent grain filling. This is important for the look required here because without the barrier the grain filling colourants alter the background colour as well as filling the open grain of the oak. 

Filling the grain using Plaster of Paris is the next task. Put some Plaster of Paris in a bucket and add a little dry powder colourant. I used green powder paint and mixed it in-- about one heaped teaspoon of paint per four or six heaped tablespoons of Plaster of Paris is a good place to start for experimenting with colour intensity.
 
Use a cotton cloth, dip it in some water, pick up a little of the powder mix and scrub the plaster of paris across the grain in a roughly elliptical motion to force the damp powder into the pores of the wood. Working in sections about one foot square keep dipping the cloth in water and picking up more dry powder mix. With an approximate one foot square section done grab a piece of rough hessian -- burlap to North Americans, and vigorously scrub off the Plaster of Paris sitting on the surface of the wood, mostly by working across the grain. I finish off this bit of the job by more gently working the hessian with the grain. This removes the cross grain striations that develop in the earlier part of removing excess filler.
 
Go over the whole panel in small sections as just described and leave the filler dry, preferably overnight. Quite often a second application is needed, and that was the case with this job because I'd missed patches here and there. After the second application the panel was left to dry and sanded very lightly with the grain using 220 grit paper to remove the last of any cross grain marks. Two coats of pre-catalysed lacquer completed the job.

Grain filling with a contrasting colour as seen here is a little unusual. Most grain filling is done with the intention of blending with the background colour, and of course to create a level surface in an open grained or 'coarse' wood; contemporary wood finishers mostly use convenient ready mixed grain or pore fillers out of a can. Using Plaster of Paris is an old technique, and it is not commonly seen today, in part I think because many workers in the past probably didn't use some sort of pigment or other colorant in their mix, with the result that the Plaster of Paris tended to reveal its whiteness after time, often years or decades later. But it remains an effective and valid method for achieving either a fully filled or ‘semi-filled’, AKA a ‘half-filled’ surface if used appropriately.