Formulation Chemistry

Formulation chemistry is the branch of manufacturing that addresses substances that do not react with each other, but have desirable properties as a mixture. These products include paints, varnishes, cosmetics, petroleum products, inks, adhesives, detergents, pesticides, and a broad range of household products.

Successful formulation requires a blend of art and science. Components are chosen for compatibility rather than reactivity. Formulation chemists think in terms of kilograms or tons rather than moles and place more emphasis on solubility than molecular weight.

Paints

Paints are suspensions of pigments, binders, drying agents, and vehicles. Vehicles are solvents in which the pigment is mixed. Pigments are powders made from insoluble chemicals, such as titanium dioxide, that give color to the preparation. Titanium and zinc oxides have largely replaced lead pigments in paint.

Binders bind pigments to surfaces. They solidify by drying, cooling, or reacting to produce polymers. Acrylic paints use polymer resins as vehicles. They can be thinned with water, but dry readily. Drying agents aid in the evaporation of a vehicle or help paint mixtures to polymerize.

Cosmetics

The cosmetic industry provides a wide assortment of formulated products. A typical bathroom contains a full range of perfumes, moisturizers, rouge, lipstick, antiaging skin products, face powder, emollients, nail polish, sunscreen, hair conditioning and coloring products, aftershave, drugs, and deodorants as well as medications.

Nail polish consists of flexible lacquers, pigments such as organic dyes, iron or chromium oxides, and ultramarine blue along with drying agents and binders or vehicles such as ethyl acetate that evaporate on drying. Nail polish remover is usually an organic solvent such as acetone or ethyl acetate.

Perfumes have been used since the days of ancient Egypt. Perfume bottles are among the most ancient glass items recovered in Asia, where perfumes were probably first extracted from plant materials such as roses, geraniums, or lemon oils. Later processes blended animal products such as ambergris and musk. The first perfumes were probably developed to mask the odors of disease or poor hygiene. Modern perfumes include components blended to produce a combination of scent that will last for several hours and provide a combination of notes (or fragrance impressions). The first note is the odor perceived when one sprays or applies perfume from a bottle. The second note or odor develops after the perfume has made contact with the skin, and the third note is the ability of fragrance to linger.

Most perfume preparations are not patented, but considered trade secrets, so manufacturers are not required to list their ingredients. High-quality perfumes are mixtures of substances that appeal to certain individuals. Typical ingredients include extracts of flowers and fragrances such as valerian, lavender, chamomile, passionflower, ylang-ylang or vanilla, geranium, mint, lemon, fixatives such as ambergris or musk, and water or alcohols. The formulation of perfumes is an art practiced by technicians who have developed abilities to perceive individual and blended fragrances. In addition to products for personal application, perfumes are used in numerous cleaning products.

Detergents

Detergents are among the most common household products and act as wetting agents . Water is a polar compound that readily dissolves most salts and polar compounds such as sugar. Nonpolar solvents such as gasoline or carbon tetrachloride (CCl₄) do not mix well with water, but dissolve nonpolar substances such as grease or oil.

Shampoo contains a mixture of ingredients, including detergents, that allow water to wet the nonpolar oils found in bodily secretions such as sebum, the oily substance which holds dirt and dead skin in hair. Common anionic detergents include sodium or ammonium lauryl sulfates. Cationic detergents, which act to condition hair as well remove dirt and oil from it, include alkyl ammonium compounds such as stearylammonium chloride or sulfate. Other components of shampoo include surfactants such as polyethylene glycol, antifoaming agents, thickeners, antistatic agents, and pH balancers. Formulation chemists add coloring or pearlizing agents and perfumes to shampoos to make them more attractive.

Much of the scum that forms on the walls of showers is a soap film, actually calcium or magnesium salts of fatty acids. Most soaps are sodium or potassium salts of fatty acids that function well as surface active or wetting agents, but calcium and magnesium ions in hard water form insoluble compounds with these fatty acids that dull shower walls. Shower cleaners typically contain chelating agents such as ethylenediamine tetraacetate (EDTA) that form soluble complexes with the ions. In addition, a surfactant such as an ethylene glycol ether wets the wall so water droplets run off. Isopropyl alcohol is a solvent both for the ingredients and for substances such as oils that are not water-soluble.

Sunscreen

A well-designed sunscreen does two things: It blocks harmful ultraviolet (UV) rays and allows the skin to tan. UV rays carry high energy and are suspected to cause cancer by damaging DNA . In addition, excess UV exposure causes increased wrinkling of the skin. Zinc oxide (ZnO) and titanium dioxide (TiO₂) are the long-term ingredients of most sunblockers, considered broad-spectrum agents because they block all UV light. Many sun worshipers or outdoors enthusiasts apply a coat of reflective zinc oxide and cream to their noses and ears for extra protection.

High energy in sunlight comes in two portions: UV-A (320–400 nanometers, or 1.2 × 10⁻⁵–1.6 × 10⁻⁵ inches) and UV-B (290–320 nanometers, or 1.1 × 10⁻⁵–1.2 × 10⁻⁵ inches). Since the light of shorter wavelength radiation is more energetic, UV-B causes burning, while UV-A promotes tanning. Although not as likely to result in sunburn as UV-B, exposure to UV-A does cause eventual wrinkling and aging of the skin. p-Aminobenzoic acid (PABA) in sunscreen absorbs the energy of UV-B while allowing UV-A to pass through. Other aromatic organic compounds such as benzophenone or oxybenzone are used with or in place of PABA. In the formulation process chemists must choose emollients and sunscreens that remain dispersed without precipitating from solution and feeling gritty.

Although probably harmless at the low concentrations used, most aromatic organic compounds pose some risk of cancer or of interference with bodily hormones, but this risk is probably lower than that of skin cancer or wrinkles. New technology allows encapsulation of the active ingredients of sunblockers in tiny polymer bags that keep the chemical agents away from the skin.

Dihydroxyacetone (DHA) acts as a self-tanning agent to give sunless tans. The browning action probably involves the reaction of DHA with free amino acids to form melanoidins. Melanoidins probably offer only slight UV protection.

Ancient Romans were aware of the bleaching effects of sunlight and used this and other processes to lighten hair. Modern bleaching is done with oxidizing agents such as hydrogen peroxide that destroy melanin, the natural pigment of hair. Hair containing no pigment is light-colored or white.

Hair Coloring Products

Hair coloring products are either temporary or permanent. Temporary hair colors attach to the surface of hair and wash out after repeated shampoos. A dye is considered permanent if it penetrates into the hollow hair shaft. Coloring of hair starts with a treatment of substances such as hydrogen peroxide and ammonia. The ammonia causes hair shafts to swell and open, allowing dye intermediates and couplers to penetrate. Dyes applied during the second step of coloring react with the precursors to form pigments that remain in the hair.

Melanin compounds may appear brown, black, or red. The type of melanin determines hair color, and the density of melanin granules determines the shade. Dark shades of dyed hair contain higher concentrations of dyes. Most hair colors are combinations of organic compounds chosen to produce particular shades. Resorcinol produces a yellow color; aminohydroxytoluene produces a redder hair, and nitrophenylenediamine dye results in very red hair. Graded dyes favored by men often contain lead acetate. The lead ions penetrate into hair and form lead sulfide (PbS), a dark-colored compound.

Deodorants

Deodorants and antiperspirants are frequently compounded together. Deodorants seldom actually remove odor; they simply mask odors or inhibit the microorganisms that cause body odor. Deodorants include several strong perfumes, often with minty or musky odors. Odors can be lessened somewhat by decreasing perspiration. Most underarm perspiration comes from the apocrine or eccrine glands. Perspiration probably functions primarily to cool the skin and get rid of excess heat, but may also carry pheromones and fatty acids and excrete excess salt. The active ingredients of antiperspirants are usually aluminum salts such as aluminum chloride (AlCl₃). Aluminum ions are absorbed by cells in the epidermis that squeeze the sweat gland ducts closed. Talcum powders may be used to absorb excess perspiration.

Gasoline

Civilization runs on gasoline. Gasoline is a solution of hydrocarbons chosen by composition and boiling point as a fuel for internal combustion engines. Petroleum from oil wells is a mixture of thousands of different hydrocarbons that must be refined and separated into useful components. In general, the longer and larger molecules vaporize at higher temperatures. Gasoline is a mixture of hydrocarbon chains typically seven to eleven carbons long.

Gasolines are blended to produce mixtures that vaporize rapidly in carburetors or by passing though fuel injectors. In addition, blends are chosen to give certain octane ratings. The traditional octane rating system compares the knocking characteristics of fuels to n -heptane (C₇H₁₆), a fuel that knocks badly and is assigned an octane rating of zero, and isooctane (2,2,4-trimethylpentane, an isomer of octane, C₈H₁₈). Isooctane can be used without knocking in high-compression engines, yielding more power than low-octane fuels. The octane ratings of low-octane fuels can be raised by adding branched-chain or cyclic hydrocarbons, or by adding octane enhancers.

The first octane enhancers were lead compounds such as tetraethyl lead (TEL) or tetramethyl lead (TML). A few milligrams of either converted inexpensive, low-octane gasoline into high-test fuels. As the danger of lead to

the environment and to catalytic converters became apparent, lead compounds were phased out and lead-free gasolines introduced. Typically, lead-free gasoline contains cyclic compounds such as benzene and oxygenated additives such as ethanol, methanol, or methylcyclopentadienyl manganese tricarbonyl (MMT). Oxygenated fuels burn cleaner in cold engines.

Pesticides

Pesticides are chemical agents used to kill pests such as insects, snails, spiders, birds, or fish. Any living thing can be a pest at times; many weeds are simply plants growing in unwanted places, and prairie dogs look cute in the zoo, but their eating habits can cause widespread damage to cropland or livestock. Pesticides are, however, seriously poisonous substances.

Dichlorodiphenyltrichloroethane (DDT) is a halogenated hydrocarbon used during the 1940s to control mosquitoes and other disease vectors. Unfortunately, DDT accumulated in the environment until its high levels in the fatty tissues of birds began to cause thin eggshells and loss of life. Although its use is banned in many countries, including the United States, DDT remains a potent weapon against malarial mosquitoes in other parts of the world.

Organophosphate pesticides act by blocking cholinesterase, an enzyme that breaks down the neurotransmitter acetylcholine after a nerve impulse crosses the synapse. Most poisons require a lethal dose that depends on the weight of the animal. Because insects weigh much less than humans, the amount needed to kill an insect may be harmless to us. One early pesticide still in use is nicotine, an active ingredient of tobacco. Potent insecticide solutions can be made by soaking tobacco in water, but the formulation chemist produces safer preparations of standard toxicity.

see also Cosmetic Chemistry; Detergents; Gasoline; Pesticides; Solution Chemistry.

Dan M. Sullivan

Bibliography

Conaway, Charles F. (1999). The Petroleum Industry: A Nontechnical Guide. Tulsa, OK: PennWell.

Flick, Ernest W. (1999). Advanced Cleaning Product Formulations, Vol. 5. Westwood, NJ: Noyes.

Laden, Karl (1999). Antiperspirants and Deodorants. New York: Marcel Dekker.

Lowe, Nicholas J.; Shaath, Nadim A.; and Pathak, Madhu A., eds. (1997). Sunscreens: Development, Evaluation, and Regulatory Aspects, 2nd edition, revised and expanded. New York: Marcel Dekker.

Internet Resources

Chemical and Engineering News web site. Available from <http://pubs.acs.org/cen>.

The Cosmetic Industry Resource Center web site. Available from <http://www.gmp1st.com/csind.htm>.

"The Extraordinary Chemistry of Ordinary Things." Available from <http://www.howstuffworks.com>.

Correction Fluid

Correction fluid is a liquid product designed to cover mistakes made while typing, hand writing, or photocopying markings on paper. Typically, it is applied to paper using a brush. When it dries, it forms a solid film that effectively covers the error and allows the correct mark to be written over it. Correction fluids are composed of pigments, polymeric binders, and solvents that are mixed together in large batch tanks. First developed during the late 1950s, correction fluid formulations have steadily improved over the years.

Background

The need for correcting mistakes made during writing has been around for as long as writing itself. While erasers worked well for pencil marks, they did little to remove mistakes made with a fountain pen, type-writer, or ball point pen. At some point, it was realized that a mistake could be covered using an ink that was the same color as the paper. This led to the development of the first correction fluids. These fluids were typically white inks. These products were inferior because they did not match the paper color very well, took a long time to dry, and were difficult to write over. Correction fluids were greatly improved during the 1950s when polymer technology was utilized. This allowed production of a product which would adhere better to paper, spread easier, and remain flexible when dry. Over the next 40 years, a variety of patents have been granted which show how steady improvements have been made in correction fluid technology.

Correction fluid is a liquid product designed to cover mistakes marked on paper. It is typically sold in a small jar with a brush applicator and works in much the same way as paint. First, the fluid is applied to the paper over the errant mark. Then it forms a film, which bonds to the paper fibers. This film is an elastic polymer that is both strong and flexible. Fixed in this film are pigments, which are supposed to match the color of the paper and cover the incorrect ink mark. When the film is dry it can be written over.

A variety of correction fluid products have been developed for different applications The most common types are those designed to be used on standard, white typing paper. These formulas are typically white and designed to dry relatively quickly. Other fluids are available for special types of paper. For bonded paper, correction fluid formulas are made which give a different texture when they dry. This makes the correction less noticeable. For corrections on paper that is not white, various colored correction fluid are available. Products are also available for photocopying applications. These formulas are made with special additives that reduce the reflection of light off the film.

While the standard product is sold in a plastic jar with an applicator brush built in the cap, this is not the only kind. Some fluids are sold in a pen, which uses a roller ball applicator. These products give better control over the application and the amount of fluid used. Other correction fluid type products are sold as solid films. These products are designed to be placed in front of the typing hammer of a typewriter. When the type-writer hammer hits the film it transfers the correction formula onto the paper in the exact shape as the letter providing a perfect coverup. As computers gradually replace conventional typewriters, this product will be used less frequently.

Design

Before a correction fluid can be made for the first time, a formula must be developed. This is done by trained chemists who are knowledgeable of a variety of raw materials. These scientists begin by choosing what characteristics are required for the fluid. They decide on functional features such as how long the product will take to dry, how strong the film has to be, and how stable it will be during storage. They also consider aesthetic features such as how thick it should be, what color it will be and how it will be delivered from the package. Often consumer testing is employed to help with these determinations.

Preliminary formulae are first prepared in small beakers in the lab so the performance aspects of the formula can be evaluated. Tests for the correction fluid's effectiveness are done on these initial samples. Other tests may be run, including stability tests, safety tests, and consumer acceptance testing. Stability testing is used to detect physical changes in characteristics such as color, odor and thickness over time. It helps ensure that the product on the store shelves will work just like the formula created in the laboratory. Using the information obtained during this testing phase, the formula can be adjusted to produce the best product.

Raw Materials

There are many different types of ingredients that can be used to make a correction fluid formula. In general, the formulas are composed of an opacifying agent, a polymeric film former, a solvent, and other miscellaneous ingredients.

The opacifying agent is a key ingredient in the correction fluid formula. It is the material responsible for covering the errant marking. The most common opacifying agent is titanium dioxide. This is an inorganic material derived from various titanium ores. It is an opaque material, which does not significantly absorb visual light. Since it has a high refractive index, it produces a predominantly white color. By changing the processing method and mixing the titanium dioxide with different materials, a variety of other colors may be produced. These are used for the different colored correction fluids. In general, the opacifying agent makes up from about 40-60% of the formula.

Although the opacifying agent actually covers the error, a polymeric material is used to affix it to the paper. This polymer creates the film that strongly bonds to the paper fibers when it dries, or cures. The film is designed to be strong so it will stay in place, but also flexible so that it will not crack, flake, and fall off under normal conditions. A variety of polymeric resins can be used such as acrylic resins, petroleum resins, chlorinated polyolefin resins and even synthetic rubber. To make the optimal film, often a copolymer is used. One type copolymer system is a latex emulsion. This is made by polymerizing methacrylate with a nitrogen containing monomer in the presence of ethylene vinyl acetate. In a typical correction fluid formula, the polymer resin comprises 5-15% of the formula.

To control the viscosity and dry time of the correction fluid, a solvent is necessary. In general, the correction fluid is made thin so it can be applied evenly and smoothly. The solvent works by diluting the formula and quickly evaporating to leave a dried film. Additionally, the solvent improves stability and helps to make the other materials in the formula more compatible with each other. In developing a correction fluid formula, the solvent must be chosen carefully. On the one hand it must evaporate quickly, so it can be quickly written over. On the other hand, it can not evaporate too fast or the polymer may solidify in the bottle.

Two types of solvents are used including aqueous based and organic based. The aqueous based solvents are used for correction fluids that will cover oil based inks. They are typically a mixture of water and alcohol. Organic based solvents use volatile organic compounds (VOC) and generally dry more quickly than aqueous solvents. They are better for covering water based inks. A variety of organic compounds can be used including acetone, toluene, xylene, ethyl acetate, and methyl ethyl ketone. Some newer formulae include both types of solvents. These "amphibious" type formulae are useful for all types of inks. Recently, environmental concerns have led to the development of formulae, which use little or no volatile organic solvents. The formula can be composed of anywhere from 25-50% solvent.

A variety of other ingredients are added to the correction fluid formula to optimize stability and performance. Since titanium dioxide is not generally soluble in the solvent it has a tendency to settle out over time. For this reason suspending agents and dispersing agents are added. Examples of the former include hydroxyethylcellulose, xan-than gum or guar gum. Examples of the latter include phosphate esters, ethoxylated alcohol, and polysorbitans. Sometimes glass or metal mixing beads are included in the container to help re-disperse the titanium dioxide. In this case, the user has to shake before using. Other ingredients added include chelating agents that help protect metal parts in the applicator, defoamers that prevent excessive bubbling, and preservatives that prevent biological contamination.

The Manufacturing
Process

The manufacturing process can be broken down into two steps. First, the batch of correction fluid is made and then it is filled into its packaging. The following description details the production of an aqueous based correction fluid. Other types are made in a similar manner.

Compounding the batch

• 1 The batches of correction fluid are made in large, stainless steel tanks which can hold 3,000 gallons or more. These tanks are equipped with mixers and a temperature control system. Workers, known as compounders, follow the formula instructions and add the correct types and amounts of raw materials at specified times and temperatures. Using computer controls, they can regulate the mixing speed and temperature of the batch. The correction fluid batch is made in three phases.
• 2 In the first phase, the main batch tank is filled with some of the water. The suspending agents and other miscellaneous ingredients are added at this time. Mixing is done at a low shear rate to get adequate dispersion without incorporating air into the mixture. As the suspending agent is hydrated, it thickens, and the mixing speed is increased.
• 3 A pigment dispersion is made next. This is done by adding the pigment to an amount of water and dispersing it at a very high shear rate. When the size of the particles is sufficiently small, it is slowly added to the main batch. In the final phase, the resin is slowly added. Additional ingredients such as colorants and preservatives may also be added at this point.

Quality control check

• 4 After all of the ingredients are added, a sample of the batch is taken to the quality control lab for approval. Physical and chemical characteristics are checked to make sure the batch falls within specifications outlined in the formula instructions. The quality control scientists run tests such as pH determination, viscosity checks, and appearance and odor evaluations. If the batch does not meet all of the specifications, an adjustment may be made. For example, if the color of the batch is off it can be adjusted by adding more pigment. After the batch is approved, it is pumped to a holding tank where it is stored prior to filling.

Filling and packing

• 5 The filling operation is dependant on the type of packaging in which the product will be sold. For the typical bottle of correction fluid the process begins with empty containers at the start of the filling line. These bottles are held in a large bin and physically manipulated until they are standing upright. They are then moved along a conveyor belt to the filling heads holding the correction fluid.
• 6 As the bottles pass the filling heads, they are injected with the exact amount of product needed. The bottles are then moved to a capping machine, which sorts the caps, places them on the bottles, and tightens them. At this point, the bottles may be passed through a labeling machine if necessary. The bottles are then put into boxes and stacked onto pallets for shipping to whole-salers and retailers.

Quality Control

Beyond the quality control tests made during the batching process, other checks are performed during filling. Line inspectors are stationed at various points on the filling line, and they watch the bottles to make sure every thing looks right. They check for things such as label placement or filling weights. They would also see that enough finished bottles are packed into cases. Occasionally, product performance tests are run. For example, the opacity may be checked using a colorimeter. The flexibility and adhesion of the film may also be examined using a fold test. In this test, the fluid is applied to paper and allowed to dry. The paper is then folded numerous times and the film is checked for cracking and flaking. These types of tests are crucial to the production of a quality product.

The Future

There are a variety of challenges facing developers of correction fluids. Many of the correction fluid formulas continue to have certain drawbacks. For example water based correction fluids are still prone to a problem called bleeding when used with water based inks. When this happens, the inks often show through the coating. The new amphibious formulas, which contain both a water based and organic based solvent, help alleviate some of these problems. However, these formulas will be more difficult to produce as governmental regulations require a reduction in the amount of volatile organic solvents used. Other formulation challenges include producing new colors, reducing drying time, reducing the incidence of product dry out in the container, and making the products less poisonous. New and improved forms of product delivery are also expected.

Where to Learn More

Books

Carraher, C. and R. Seymour. Polymer Chemistry. New York: Marcel Dekker, 1992.

Kirk Othmer Encyclopedia of Chemical Technology. New York: John Wiley & Sons, 1992.

—Perry Romanowski

Wood Stain

Background

Wood pieces are often decorated to add color and appeal. Wood products are often imparted with a wood-tone stain to enhance the natural grain or add depth or tone to the wood. Stain may alter the color and appearance of the wood or hide unattractive grain. Stains are available in a variety of wood tones, including very light, semi-transparent stains to dark, nearly opaque stains.

Stain is a combination of dyes and pigments suspended in a solvent. Soluble dyes dissolve in compatible solvents and provide greater grain clarity, meaning the grain shows through the stain. Insoluble pigments are finely ground coloring materials that disperse but do not dissolve in the solvent. These insoluble pigments tend to cloud the grain. Stains need to be mixed frequently so that the pigments remain evenly dispersed and neither completely reveal or obscure the grain. Stains are generally characterized by the type of solvent that is used in their production. Thus, the most frequently used stains include alcohol (sometimes called non-grain raising stain), water, and oil stains. Each solvent affects the way the stain looks and handles. Today, oil stain is manufactured in the greatest quantity and the most familiar to the amateur woodworker. There are two types of oil stains. These include penetrating oil stain, which sometimes bleeds and fades, and wiping oil stain (sometimes called pigmented stain), which is more consistent and does not streak.

Regardless of solvent, stains generally penetrate only the top layers of the wood. Thus, the stain can be stripped and sanded away, revealing the original color of the wood. Stain must be topcoated or finished, meaning that once it is dry some kind of surface finish is applied to protect the wood surface and stain from moisture, scratches, unwanted stains, dirt, and chemicals. Wood stains are compatible with natural finishes such as varnish or shellac, and synthetic finishes such as polyurethane or acrylic.

History

Woodworkers have stained wood for centuries using natural pigments and dyes from plants and minerals. Iron nails soaked in vinegar render a dark gray or ebony stain, brown stain may be devised by soaking tobacco in ammonia and water, and so forth. Many of the earliest stains were essentially thinned paints that rendered opaque color and tone. It is estimated that over 100 years ago stains were first mass-produced, and around 1920 American companies such as Pratt & Lambert not only made a wide variety of oil stains, but were actively advertising and marketing their products.

More recent developments in stains include a wider variety of those with solvent-bases. Water and alcohol stains are considered less environmentally unfriendly. (Mineral spirits essential to oil stains have restricted disposal policies as it may contaminate water supplies.) An interesting array of semi-transparent colors has recently been developed by stain manufacturers to render colorful, non-natural colors sought by some woodworkers. Synthetic pigments have been developed as well, resulting in more consistent coloration than some of the pigments found in the natural world. Gel stains are pigmented stains in a thickened form resembling jelly. Pigments stay mixed evenly and the stain does not drip or splatter as much as a liquid stain.

Raw Materials

The raw materials essential to the production of wood stain vary by type. Water stains use water as the solvent and include water-soluble aniline (chemically derived) dyes to impart color. Non-grain-raising stains, sometimes referred to as alcohol stains, are manufactured using alcohol or glycol as the solvent with alcohol-soluble aniline dyes used in their production. Because alcohol dries almost instantly, this dye is not able to be manipulated much and essentially the stain is set as it is applied.

Oil stains utilize mineral spirits for the solvent. Mineral spirits help the product's viscosity and ease of application and are the volatile ingredient in stains (rags soaked with stains have been known to instantaneously combust and must be carefully disposed). Oil stains also generally use linseed oil as the resin or binder that has been treated with special acids so that it will not penetrate too deeply into the surface of the wood. Pigments come in 50-lb (23-kg) bags and are generally iron oxide pigments (although this may vary). Metallic salts are important ingredients as they help the product oxidize and permit the oil stain to dry. Finally, a thickening agent that also helps control penetration into the wood is needed. These thickeners are often proprietary and may not be discussed by the manufacturer.

The Manufacturing Process

There are many different solvent-based stains. Oil stain is one of the most produced and sold in greatest quantity.

1. First, components must be mixed together in order to begin the process. The main component—linseed oil used as a binder within the stain—is pumped into a tank. Only about half of linseed oil that is needed to make stain is added to the tank in this stage. Next, the solvent, generally mineral spirits, is pumped in. Finally, the pigments are added. The pigments are mixed in powdered form, pre-measured carefully elsewhere and dropped in by hand. This amount is carefully monitored in order to acquire the depth and tone of stain the consumer is expecting. These powdered pigments also have some oil absorption qualities and help thicken the mixture. Finally, a dedicated thickening agent (varied, and in some cases, a proprietary ingredient)is pumped into the tank.
2. The ingredients must be thoroughly mixed in a process referred to as "the grind." A high-speed dispenser, essentially a saw-tooth blade that rotates at very high speeds, is lowered into the large vat of chemicals and pigment. This blade agitates the slurry for approximately 20 minutes, ensuring that the powdered pigment is evenly distributed throughout the liquids. As this high-speed dispenser rotates within the chemicals for several minutes, the temperature of the mixture rises.
3. The batch must be cooled down. In order to cool down this thick concoction, the rest of the linseed oil is pumped in along with additional solvents (more mineral spirits) and various metallic salts. The nascent stain is quickly cooled and thinned to nearly the viscosity required for a high-quality wiping stain. This single batch of stain is approximately 250 gal (946 L) in volume.
4. Presuming that the batch of stain under production requires no further adjustments for quality standards, it is hooked up to a filtration system that essentially removes all sediment from the oil stain so that the liquid is without grain or lumps. Some companies put their stain batches through two filtration systems to ensure the undesirable solids are eliminated. From the point at which the tank was initially filled with ingredients to the completed decanting may take as long as 2.5 hours.
5. The decanted stain, currently held in a large vat or tank on the second floor of a factory, is now dropped into a filling machine on a lower level of the factory. Here, the liquid is ready to be individually dispersed into cans and packaged. This filling machine automatically fills each can by shining a beam of light into the can with a label already affixed. If the beam remains unbroken, it indicates that the can needs to be filled, and fills it in a quick stream. When the can is filled to the desired level, the beam is broken and the filling stops. Another can moves into its spot, a beam of light is shone in, and the filling of another can commences until all 250 gal (946 L) are gone from the machine. The cans are moved away from the filling machine and they are packed in cartons and readied for shipment.

Quality Control

The creation of oil wood stain is a carefully controlled cooking process. The ingredients are very carefully measured as they are pumped into the mixing tank, and pigments are hand weighed according to proscribed recipes that render the tone desired. The quality of the raw materials—particularly the proprietary thickener, the linseed oil, and the pigments—are essential to producing a quality product. Machinery must be working properly in dispersing the pigments and filtering out undesirable particles. Finished batches are checked for proper viscosity, weight, and color.

Byproducts/Waste

Oil stains generally utilize mineral spirits, a combustible material with a high flash point. Most unused solvents are easily reused and re-mixed into the stain manufacturing process so that the solvent is generally not a hazard. If for some reason the solvent is contaminated and may not be reused in the product, then the mineral spirits are considered hazardous waste and must be disposed according to federal regulations that pertain to such wastes. These contaminated solvents are shipped to a hazardous waste facility.

The Future

Because oil stains are made with solvents considered hazardous, many woodworkers are turning to the water stains because they are environmentally friendly. Water stains move deeper into the wood than oil. But they don't always have the depth of tone or color that oil stain imparts on the first coat. It may require a few coats to get the desired color. Also, water stains tend to raise the grain, considered undesirable if one wants a smooth, even surface when the piece is topcoated. The future of all paints and oil stains made with linseed oil and mineral spirits is in question as the disposition of the used and contaminated products are becoming an issue.

Where to Learn More

Books

Umstattd, William. Modern Cabinetmaking. South Holland, IL: The Goodheart-Wilcox Company, Inc., 1990.

Other

American Furniture Design Company. http://www.americanfurnituredsgn.com (January 2001).

Antiques Resources.Com. http://www.antiqueresources.com (January 2001).

Lowe's Home Improvement Warehouse. http://www.lowes.com (January 2001).

—NancyE.V.Bryk

sol·vent / ˈsälvənt/ • adj. 1. having assets in excess of liabilities; able to pay one's debts: interest rate rises have very severe effects on normally solvent companies.2. able to dissolve other substances: osmotic, chemical, or solvent action.• n. the liquid in which a solute is dissolved to form a solution. ∎  a liquid, typically one other than water, used for dissolving other substances. ∎ fig. something that acts to weaken or dispel a particular attitude or situation: an unrivaled solvent of social prejudices.DERIVATIVES: sol·ven·cy n. (in sense 1 of the adjective ).

solvent Liquid that dissolves a substance (the solute) without changing its composition. Water is the most universal solvent, and many inorganic compounds dissolve in it. Ethanol, ether, acetone, and carbon tetrachloride are common solvents for organic substances. See also solution