The term Wood Stain is used to describe literally hundreds of various formulations intended to alter the natural appearance of wood. Our desire to alter the natural color of wood, and later cloth, stems back to the earliest records of ancient history in the form of artifacts. Whether the first colorants used were natural dyes, as is generally believed, or one of the many forms of natural earthen pigments is not important. In any case we have been changing the natural color of wood for a very long time.
For the purposes of this article, a stain could simply be defined as any chemical material that changes the color of wood in a lasting way. There are three basic ways this can be accomplished:
- A pigment is bound to the surface of the wood
- A dye particle is affixed to the cell structure of the wood
- One or more chemicals are used to modify the wood such that its optical properties are altered
A pigmented stain is comprised of three elements - the pigment, the binder, and the carrier. Pigments are fine particles of inert chemical compounds. The chemical compounds used for pigments can be simple or complex, and natural or synthetic. Some of the earliest pigments in use were the iron oxides found natively in the soil of various regions of the world. You know these pigments by their old, but common names of Umber, Sienna, and Ochre. These pigments are still in use today and have provided us with a wide array of browns, reds, and yellows to choose from. To get to some of the brighter, more vibrant colors, we turned to the chemists to produce colors such as Quinacridone Violet and Phthalo Green, who else but a chemist would come up with a compound such as copper phthalocyanine for a blue or green pigment?
The binder of a pigmented stain is typically going to be a resin-based coating such as a varnish or lacquer. The binder acts as an adhesive to glue the pigment to the surface of the wood. However, in practice, the binder must be chemically treated to keep the pigment dispersed throughout the solution we call a stain. Common binders include long- oil (high oil content) linseed varnishes, alkyd varnishes, and lacquers with long-oil plasticizers. The so-called wiping stains are those that employ a long-oil binder to lengthen the open time of the stain so that it can easily be wiped on and off. As manufacturers are changing formulations to comply with VOC laws, you may find the new version less familiar or even friendly. Raising the solids content of the binder and/or using a completely different binder that relies on one of the currently accepted alternative solvents can lower the VOC content to acceptable levels. However, in one recent example that comes to mind, the manufacturer ended-up loosing some of it's loyal customer base because the re-formulated stain line performed radically different than it had for a good many years.
Spray stains typically are built around alkyd or lacquer formulations designed to set-up quicker so they can be applied to vertical surfaces and used to tone or highlight architectural details. Aerosol toners are one example of a spray stain.
The carrier used in stains is almost always just a solvent, or solvent combination, for the binder component. The carrier permits a pigmented stain to be delivered to the surface of the wood, regardless if it is to be wiped, brushed or sprayed.
Unlike pigments, dyes are absorbed into the fibrous structure of wood to effect the change in color. Dyes have been desirable mainly because they produce transparent and natural appearing results. Like pigments, dyes also have their roots in nature. Naturally occurring dyes in berries, wood & bark, and shellac are some examples of early sources for dyestuffs. One problem dyes had immediately is that they were not very fade resistant in direct sunlight like their pigment cousins. While pigments retained their color almost indefinitely, the early dyes usually faded quickly and quite badly. Dyes were really perfected in the 19th century when the customers of the great English and American textile mills demanded more vibrant and lasting colors. Consequently, and somewhat ironically, development of synthetic dyes raced in comparison to that of synthetic pigments - clothing and fashion apparently being more important than paint.
Most of us are familiar with the term aniline dye. This term originates with the discovery and use of aniline as an early chemical intermediate in the production of synthetic dyestuffs. As an interesting historical side note, the chemistry associated with dye synthesis is very similar to that of drug synthesis, and both share a common ancestry. However, aniline derived dyes are being replaced in the marketplace since greatly improved dyestuffs have been developed. One such class in common use today is the so-called Azo or "acid" dye. Azoic dyes in chemical terms are organic structures produced, for example, by the reaction of nitric acid and organic groups such as phenols and amines. Today, these azoic dyes are frequently built around synthetic metal complexes based on chromium and cobalt. These newer 'metalized' dyes are extremely lightfast and have long shelf lives (years), so they don't pose many of the problems originally associated with the use of dyes. In particular, the newer metal complex dyes are very lightfast.
For use on wood, we rely on dyes dissolved in water, alcohols/ketones, and vegetable oils. All of these types are useful to us. Water-soluble dyes are fairly easy to use and are available in a wide range of colors. With wood, we know that we have a material that likes water, but one that tends to swell as a result. This raising of the grain is enough to keep some folks away from using water-soluble dyes completely. Indeed, water-soluble dyes do require extra time and effort to use. With alcohol-soluble dyes, we have a solution that can be made to dry almost instantly on contact by using acetone, or more slowly using glycol ethers. Both ends of the spectrum are leveraged today to produce VOC compliant systems for spray use, and for NGR stains capable of being wiped-on without producing overlap marks. Oil-soluble dyes find their way into the products we buy and are typically labeled as pigmented wiping stains. These so-called oil soluble dyes are not completely soluble in pure oils and often require the use of a solvent such as naphtha or toluol to dissolve the dye completely before adding it to the oil.
In practice, a given brand-name line of stains will often be comprised of both pigment and dye, with the pigment varying in proportion between 0 and about 35 percent by volume. Some stains are all dye, but most are a combination of pigments and dye. The reason for this is quite simple. The consumer marketplace has determined that is prefers transparent stains better than opaque ones for interior use - the opaque stains have been relegated to exterior use where the pigments serve as the primary form of protection from the effects of the sun's ultraviolet radiation. The dye in the stain does a very effective job of coloring the latewood of many species, while the pigments are free to be trapped in the nooks and crannies of the surface. These combination stains offer the benefits of both pigments and dyes and make the product more consumer-friendly. I'll discuss the merits of using dyes and pigments together, but in separate steps, later in this article.
The use of chemicals to modify the surface of wood has been with us for at least the last 500 years. The term 'mordant', from the French word mordere, was applied to the fixatives used in dying to set or fix the dye in textile manufacturing so garments could safely be washed without the fear of losing the dye in the caustic solutions used at the time. Interestingly, these mordants would often change or shift the original color of the dye. In wood finishing applications, mordants can be used to directly modify the extractives, such as tannic acid, in wood that supply much of the natural color. One of the best-known chemical stains is a very dilute solution of lye, or sodium hydroxide, in water. Since lye is an oxidizing agent, woods such as cherry that darken upon exposure to oxygen will instantly be darkened when a lye solution is applied. The amount of darkening depends mainly on the concentration of the solution - the more lye present in the solution, the more oxygen ions there are available to fix themselves to the cherry wood. Ammonia has a similar effect on oak. With the wide array of coloring choices available to us today, chemical stains have all but become an interesting footnote in the history of wood finishing.
This area of wood finishing seems to generate more frustration and questions than any other. I'll point-out some of the areas of difficulty and how to deal with each one.
The first step in coloring wood is also one of the most important. Sanding the surface of the wood to prepare it for staining can have a significant affect on the outcome. Generally, if a wood is very hard and dense, you should sand to a coarser grit than if the wood is soft and porous. Here are two common examples. Hard maple can be difficult to stain if it has been sanded to a very fine grit like 220. The surface of the maple will be burnished using fine abrasives and it will not accept stain evenly. Completing the sanding process at 120 or 150 grit will produce a better result. Conversely, sanding pine in preparation for staining will require ending your sanding at higher grits like 220 or 280. The grit of sandpaper that you choose to 'end' with will also vary with materials. Continuing with our maple example, it might make more sense to sand to a finer level with maple if a lacquer stain or alcohol/acetone-based dye is used, especially if there is a high degree of figure in the grain. Another consideration with respect to sanding pertains to porous woods. Sanding red oak for example by starting at 80 grit and progressing to 180 grit, will produce a different result than just performing 180 grit sanding on a smoothly planed surface. If you were to examine the surface of red oak under a 20x magnifying lens, you would discover that the open pores produced by the early wood in the spring, to support high volume sap flow, are not completely open across the surface. Closer inspection revels that some of the pores are actually covered by a thin layer of the lignin that lines the cell walls of the early wood. When you use 80 grit abrasive on this surface for example, these thin layers of lignin are opened-up. So even though the wood surface is very smooth after it has been planed, you still accomplish something by using a coarse-grit abrasive. I'll also add that I have found that straight-line sanding with 80 grit garnet abrasive paper works very effectively in this regard.
Some woods, especially highly figured examples of these woods, do not take color evenly. Wood fibers do not run in perfectly straight lines, so they are not like a box of drinking straws where each one is parallel to the next. Twists and turns in the growth of a tree fortunately make most woods visually interesting. However, as you know, applying a stain to end grain does not produce the same result as applying it to the sides of the wood fibers, i.e. the surface of the wood. The density of the wood at any given part of the lumber will also vary and this can dramatically affect the how absorbent the surface of the wood is.
This is where sealing comes in. When the fibers comprising the wood suddenly shift, as they do near a branch for example, more of the open-end of the fiber will be exposed when the surface is planed smooth. Using a sealer helps to even-out the surface of the wood in terms of how readily it will absorb a liquid medium.
Some years ago I was building a project in white birch, one that would ultimately require matching it to existing cherry furniture. I planned to use a dye stain to at least get the color close. As I expected, the dye did not produce quite as much unevenness as a pigmented stain would have, but the result was still unsatisfactory. I had recently read about the merits of using lacquer-based sanding sealers. After sealing a test board and re- sanding its surface, I once again applied the dye (an NGR stain) with terrific results. When the finishing was completed, it was difficult to tell that the wood had been stained at all - that it was not naturally the color it appeared to be.
Sealers come in various types - vinyl, varnish-based, lacquer-based, glue-based, catalyzed, etc. One of the best sealers for all-purpose use is de-waxed shellac. In addition to drying and hardening quickly, shellac provides an excellent barrier between incompatible finishing materials. With respect to their use in staining, most sealers are applied, allowed to dry, and then sanded with the last 1 or 2 abrasive grits that you plan on using before staining takes place.
If there were only one point that I could get across to you, it would be that you don't have to always use stains full-strength. This notion applies to all stain types. You can dilute that stain in a can, that dye you mixed, and that lye you wanted to use on cherry to produce a more subtle effect that might be part of a multi-step finishing schedule that builds the color in steps rather than all at once. Simply using more of the main solvent or adding some of the clear stain base for that product line will reduce the concentration of the stain. Using a very dilute stain of burnt sienna on cherry produces a more vibrant appearance that using no stain at all. This technique adds contrast without imposing very much color of its own to the wood.
Stains can also be used to produce more than a simple monochromatic effect. If for example you apply a burnt sienna dye, a wash coat of shellac, followed by the application of a van dyke brown pigmented wiping stain, you will have produced a color effect that is virtually impossible to get with a single color. A similar effect would be obtained with an application of dilute lye solution over cherry wood followed by the sealer coat and the van dyke brown pigmented stain. Again, compared to unstained wood, the result is superior to the eye.
Earlier I mentioned that the stain manufacturers often use both dyes and pigments together for their stain formulations in an effort to produce a better overall effect in one application. However, this is not to suggest that a yellow dye and a dark brown pigment can be combined to mimic the result of using these products by themselves in separate steps - in fact you would find that mixing such a concoction would yield little more than a mess.
Some stain lines are built to be intermixable such that mixing 2-3 stock stain colors can generate a wide array of tones. A truly intermixable stain line has the following characteristics:
- A single binder system is used
- Monochromatic pigments are used for each stock color
- The stock colors are kept simple
- The color strength is kept consistent across each stock color
Each of these characteristics is necessary to produce both predictable and consistent results, and all of them together help ensure that you will quickly learn to perform color mixing. I will also add that it is vital to maintain consistency in the color of the pigments over the years such that the contents of a 3-year old can of raw sienna matches what was purchased yesterday.
Copyright 2000 Russ Ramirez