In the finishing world, we are concerned with such attributes of appearance as color, texture, shape and sheen, as well as attributes of feel or tactile sensation, such as smoothness, slipperiness, sharpness and hardness.
Sheen, gloss and luster are often used interchangeably, although they have somewhat different technical meanings and can be measured with different instruments as well as appearing differently to the eye. In wood finishing we are likely to see the sheen listed on a product data sheet in this form:
Gloss 90+, Gloss 90 +/- 5 or Gloss 80≥
Those numbers refer to a scale of gloss units. The testing protocol and instrumentation for such testing are specified in ASTM D523, the most common standard for wood finishing coatings. The scale ranges from 1 to 100. The most common instrument used for such measurement for many years has been the Gardner 60 Gloss meter, but more advanced and versatile instruments are now available, some with the ability to import data wirelessly to computers.
In simple terms, these instruments measure the light emitted from the instrument at a specified incident angle, reflected from the surface of the dried coating being measured at the same angle, to a receiver. The instrument compares the emitted light to the received light and computes the difference to determine the amount directly reflected versus the light scattered or diffused by that surface.
It compares the results to results of the same test performed on a calibrating surface, such as a highly polished black glass plate. The calibrating surface should register 100 on this scale. For readings above 80 GU or below 20 GU, a different instrument is recommended for greater accuracy. A different setting on some instruments could be all that is needed to accommodate these higher or lower sheens.
The geometry of the instrument is given as the angular deviation from vertical, when the surface being measured is horizontal. For sheens between 80 and 20 GU, a 60-degree angle of both incidence and reflectance is used. For sheens above 80, a 20-degree angle is used, while an 85-degree angle is used for sheens below 20 GU.
The American Society for Testing and Materials is an internationally recognized source for rigid standards to ensure uniformity of testing and description of a tremendous range of materials and applications. It has standards for appearance and performance of all measurable aspects of coatings and provides a reference that enhances communication between companies and individuals.
For example, while each coating company might have its own words to indicate the sheen level of a product, there is no standard recognized in the wood coatings industry that tells you, for example, the sheen level for satin. An online search of several leading manufacturers’ product data sheets shows satin lacquer listed as 40 sheen, 35-40, 25+/-5 and 12-18. One company calls its 20 sheen lacquer “dull-rub” while another calls its “matte.”
Here’s an interesting quote from Barry Law, president of the Master Painters Institute: “We found 40 different names between flat and semigloss. One company creates a new name for gloss level between flat and semigloss every year. There’s nothing wrong with that from a marketing perspective, but that creates a great deal of confusion from an industry perspective.”
It is important to understand that the laboratory methods of measuring sheen, as precise as they are, do not represent the real-world results that you will get. First, let’s look at what the standard requires.
The coating is applied as a “draw down” at a specified thickness, much more precisely than you could spray or brush it, to a standard Leneta 5C card at 77 degrees and 50 percent relative humidity, and allowed to dry for seven days at that constant temperature and humidity. When testing, the instrument should be calibrated to the supplied standard. The card to be tested should be placed on a flat and level surface. Take readings at several points on the surface within a 3” x 6” rectangle. If the range exceeds two GU, take additional readings and calculate the average. Factors that can affect the reading include flatness of the specimen, brush marks or similar texture, care of the specimen (fingerprints, dust, etc.), care of the standard and care of the instrument.
When you use a coating product, it will more than likely be applied to a variety of substrates, prepared in many different ways and under varying environmental conditions. You will experience different results than in the laboratory. However, it is incumbent that the manufacturer of the coating product provides consistent materials. It could be helpful for you to test incoming materials, especially if you are producing larger batches of goods that have to match in sheen. The test equipment is still expensive, but I expect that, like all electronics, the cost will continue to come down. For a less accurate standard, you can make up cards or boards coated with a representative sample of the coating you use. They can be used to visually compare similar cards coated with each new incoming batch.
Getting the right sheen (Martin, not Charlie)
Now, how do we achieve different sheens in a coating? Simply, the basic coating chemistry provides a smooth flow of liquid that dries to a smooth film. The better it accomplishes that, the higher the reflectivity of that surface. If we add particles to the film that affect the reflection of light by scattering it in all directions rather than reflecting light primarily in one direction, we change the appearance of sheen. These particles can be pigment (colored particles, such as in paint) or translucent, constituting a flattening powder or paste, as we would use in a clear finish. The flattening agent can be talc (magnesium silicate) or amorphous silica (silicon dioxide) or similar translucent particles, thoroughly mixed into a paste with solvents compatible to the coating.
More of the flattening agent gives a lower sheen, but at the expense of some hazing at low viewing angles, as much of the light striking the surface penetrates the film and passes through several of the flattening pigments. Some of the light reflects from each particle encountered and some passes through, perhaps to be reflected from another particle deeper in the film.
The flattening pigments have a tendency to settle in the liquid coating, so thorough agitation before and during use is needed. When decanting the material into a cup or pot, you should filter the coating to avoid the possibility of including particle clumps or agglomerations in the material to be sprayed.
Failure to adequately agitate the coating may result in varying sheens coming from the same can. Usually this means higher sheen from the upper portion of the liquid, to very low sheen from the bottom, where the flattening pigments settle.
Sheen control can be also accomplished by abrading or polishing the surface. A very smooth coating without flattening agents in it will typically be quite glossy. When the surface is sanded, or otherwise abraded by rubbing with steel wool, abrasive pads, pumice or polishing compounds, a fine scratch pattern is produced. The scratches reflect light in all directions, creating the same type of scattering effect that the flattening pigments do. Coarser scratches result in a lower sheen. Extremely fine scratches, invisible to the eye, result in a higher sheen. If the scratches are too coarse, customers will find them objectionable (4/0 steel wool is about as coarse as you can commonly go). In today’s market, very good and very low sheen off-the-gun (not rubbed) finishes are quite popular because they don’t show the scratch pattern that a very low sheen rubbed finish does. However, they are a bit of a maintenance problem as normal rubbing of the surface with clothing, dishes and even hands will tend to burnish the surface, smoothing the flattening pigments and leaving a shiny area.
The sheen of the surface coating will affect how you see the color. A lower sheen will typically look lighter and the higher sheen will look darker. When making repairs, matching the sheen is as important as matching the color.
This article originally appeared in the June 2012 issue.