Wooden objects change color when they are cut, sanded or scraped because of chemical changes occurring when the surface is exposed to air, light, water, chemicals or minerals. One of the reasons for finishing wood is to protect it from undesirable changes in color.
We also finish wood to emphasize certain characteristics such as depth, figure and color to enhance the appearance of the piece or, in some cases, to play down some of the characteristics, such as excessively prominent pores, mineral deposits, discolorations and defects. Proponents of the “natural” look often forget that the natural look of wood is bark. Once we remove the bark we are selecting characteristics that we want to emphasize, minimize or eliminate.
Most of the color we’ll apply to wood will be in the form of a stain. The most commonly used stains are pigment stains — composed primarily of pigment, solvent and binder — and dye stains, which are composed of a dye dissolved in a solvent.
Pigment is a finely ground colored matter of organic or inorganic origin, insoluble in the chosen solvent. The pigment is suspended in the solvent/binder mix and must be agitated to keep the particle distribution even. Pigments can be opaque or transparent, but have some hiding power as they are distributed on the surface of the object. Because the pigments are very small, they can lodge in minute cavities in the wood’s surface, such as open pores, sanding scratches and swirl marks. Consequently, any inconsistency in the topography of the surface will cause an inconsistency in the way the stain takes.
The binder is a resin or film-former dissolved in the solvent, which binds the pigment to the surface of the wood when the solvent evaporates. Reducers or retarders are added to control the rate of evaporation. Stains that dry too rapidly could be difficult to apply on larger pieces or during hot and dry weather. Stains that dry too slowly can slow down production and give off lingering fumes.
Because many pigment stains contain a binder that can be dissolved by the solvents used in the coating, brushing or ragging a sealer or topcoat over those stains can result in streaking or removing the stain. Stains with a stronger binder are available to fix that issue. Streaking is generally not a problem when the sealer or topcoat is sprayed.
Pigment stains with little pigment in them are more forgiving, but getting sufficient color on the wood with these stains can be a problem. If the stain is applied too heavily, the effect is likely to be muddy or streaky. It might be possible to apply a second coat, but with some stains you will either rewet and move the first stain or get a streaky effect. Pigments are generally quite stable and able to resist fading because of sunlight and other environmental influences and are not highly reactive.
Dye stains consist of a dye dissolved in a solvent. Since there are no particles larger than the molecule of the dye, a dye stain will penetrate into the upper layers of the wood and is transparent. When the solvent evaporates, the color is in the wood.
Dye stains are generally more difficult to control than pigment stains. Since they are applied wet and not wiped off, they can absorb more in some places and less in others, leaving a streaky appearance. They can be very difficult or impossible to remove if applied too heavily. They won’t hide defects, but will enhance the appearance of highly figured woods such as maple, mahogany, walnut or satinwood.
Dyes stains can be reapplied after previous coats have dried. The color becomes more intense without appearing muddy. Dyes typically do not need a binder as they penetrate the wood and attach to the wood fibers. They are not easily removed without extensive sanding or bleaching.
Dyes can be purchased as a premixed stain, concentrate or in undissolved powder form. Retarders and reducers can be added to control the drying process. Dye stains are commonly labeled as vegetable, mineral, animal, aniline, coal tar, metallized and acid dyes.
Aniline dyes are seldom used professionally, having largely been supplanted by more modern, light-fast and bleed-resistant metallized dyes, available from a number of suppliers serving the hobbyist market that can be mixed to a powerful intensity. They are available in oil, lacquer, water and alcohol solutions, and sometimes in oil/lacquer and water/alcohol solutions.
Both pigments and dyes can be added to coatings, provided that the solvents are compatible, creating a toner or shading stain.
What are toners?
If we add small amounts of pigment to a lacquer, we get a semitransparent toner. If we add small amounts of dye to lacquer, we get a transparent toner. We can put either of these into aerosol cans for easy application. They are also sometimes called spray stains and can be used to apply the first coat of color. This process is often termed “spray to color” and produces a very even coat of color on woods that tend to blotch easily, although at the expense of depth and grain definition.
Toners can also be used to add another layer of color and depth by applying subtle coats that contrast or complement the wood and primary stain color and the sealer has been applied.
Another toner technique, called uniforming, is used to match different species or cuts of wood in a single piece.
How to tell the difference
I’ve run across a fair number of finishers who aren’t sure if they’re using a dye or pigment stain. Here’s how to tell.
If the container has not been agitated for several days, the pigment will tend to settle in a “mud” in the bottom of the can. A dye won’t.
Pigment stain, when applied liberally, will hide the wood’s figure and remain on the surface. Dye will remain transparent and eventually absorb into the wood.
To add to the confusion, stains are often labeled as oil, alcohol, water or lacquer. An oil stain usually means a pigmented stain that uses a drying oil as a binder, but there are dye-type oil stains dissolved in a variety of petroleum distillate solvents. A lacquer stain could be dye or pigment in a lacquer thinner. The pigment type contains a binder — usually a lacquer or other resin — and the dye type does not. A water stain could be a water-soluble aniline dye or any of the more recently available waterborne pigment or dye stains.
The only way to know for sure is to read the product label and Material Safety Data Sheet or find a vendor who is candid and truthful about what their products will and will not do.
This article originally appeared in the April 2014 issue.