by Doug Denton
Having tried a number of various types of finishing products and experimented with different ways and means of applying them to marquetry pictures over a number of years, I now use mainly water base polyurethane for finishing. The reason for this is because it has many advantages over other finishing products:
After you have completed the gluing of your picture on the baseboard, veneering also the back and all edges, set it aside for a few weeks so that the glue may fully cure. The picture must now be levelled to a smooth flat surface with a cabinet scraper. Abrasive papers should not be used at this time as the sanding dust will clog the pores of open-grained veneers.
After scraping, use a clean white cloth dampened lightly with water or a vacuum cleaner to remove any surface dust that remains. If you have used veneers that tend to bleed, such as padauk, rosewood or ebony, you may need to spot prime these pieces separately using a small brush and the water base polyurethane. Two or three coats may be needed before these veneers are sealed. Allow about an hour to dry between coats.
To start the initial finishing coats, brush on a coat of water base polyurethane on the front and edges and after allowing an hour to dry, turn it over and brush on a coat on the back. Allow another hour to dry, then turn it over and start again. This process should be repeated until three or four coats have been applied to all surfaces. No sanding is necessary between coats. The sanding can now be started. I use silicon carbide Lubrisil paper for the initial sanding, 180 grit and then change to 240 grit, rubbing with the main direction of the grain. Be careful not to press too hard with your sanding block as you don't want to rub through and expose the bare veneer.
After cleaning up the surface dust, you may now proceed to build-up the finish adding three or four more coats on all surfaces. Set the picture aside again for another week to dry and for further shrinkage before sanding again. Repeat the build-up process until about twelve coats have been applied or until about all the grain and hollows have been filled.
For the final sanding I use Lubrisil 320 grit, and then I change to waterproof paper 400 grit then 600 grit using it wet with water. Again, be careful sanding as most of the finish will be sanded off leaving only a thin layer on the surface.
For polishing, I use pumice with a felt pad soaked in water rubbing with the grain and then change to rottenstone using it the same way.
I have used several different kinds of finishing products. The simplest involves a water based polyurethane. It has little odour, dries quickly, cleans up easily and leaves a clear, hard finish.
First, I use a heavy gauge (0.03") cabinet scraper to level the picture. All glue and tape residue must be removed from the surface. I often draw light pencil lines on the surface, 1 inch apart, across the dominant grain direction and scrape until all the lines are just removed. This minimizes the amount of material to be taken off to level the picture. Shavings are never allowed to build up on the surface. I avoid scraping oily (e.g.. African Padauk) and dyed (e.g.. black Pear) veneers towards easily stained light coloured veneers. A light gauge (0.015") scraper is used to touch up where the heavier duty scraper has left scratches.
I use compressed air to blow the dust from the surface of the picture. A stiff toothbrush sweeps out any remaining dust from the pores and the tip of a knife scrapes out any stubborn particles. The surface is now ready for the finish.
The finish I use is "Flecto Varathane Elite, Diamond Finish, 2000 Clear Gloss". (I buy it in the 1 litre size can.) The finish is stirred gently, not shaken, in its container so as to minimize the quantity of bubbles formed. It is then poured through an old nylon stocking, which acts as a strainer, into a small glass jar. The can is then resealed and stored. To the product in the jar, I slowly add 2 oz. per quart of "Hydrocote Ultraviolet Light Absorber and Stabilizer". (It is not necessary to use the stabilizer.) The room temperature should be above 70 deg. F and the relative humidity should be below 50%. A warm room temperature helps the finish to flow better and thereby fill in cracks and make bubbles easier to break. Low humidity allows the finish to dry quickly and harden properly.
If any oily or dyed woods are present, I seal these pieces first, individually, to prevent bleeding. The finish is applied with a synthetic artist's brush and requires 1 hour to dry. A light gauge cabinet scraper is used to remove any fuzz off the partially finished surface. The residue is blown and brushed away.
The first full coat of finish is applied with a 2-inch wide foam brush. I do not scrape the brush over the jar's rim to remove excess; scraping promotes bubble formation. Instead, I merely touch the brush to the inside wall of the jar. The finish is spread parallel to the dominant grain direction. It is important to spread the finish very thinly and use as few strokes as possible to reduce the amount of bubbles that form. Bubbles will occur but most will break. Any stubborn bubbles can be burst by blowing on them before the finish films over. After the surface is dry in 1 to 2 hours, I scrape the surface with a light gauge cabinet scraper to remove the fuzz and any irregularities in the finish. The surface is blown clear and a toothbrush removes any other debris.
Another thin coat is applied and left to dry for 2 to 3 hours. The surface is lightly sanded with 600 grit wet/dry sandpaper and a sprinkle of water. The residue is wiped away with a cotton cloth and the surface washed with clean water and dried with another clean cloth. A toothbrush is used to remove debris from the pores and the tip of a knife may be required to get stubborn particles. A visual inspection is made to determine whether a third coat is required. A third coat, if necessary, is applied in the same manner as the second coat.
The next step is to very lightly spray the surface of the picture with the aerosol version of the "Flecto" finish. The thinner the coat, the faster it dries, and the less air borne particles it picks up. A extremely light sanding is given with 1200 grit wet/dry sandpaper when the surface is dry. The surface is wiped clean and washed if necessary. A second extremely light spray coat is applied. The picture is left to cure and harden for a few days.
This process leaves a slightly pebbly gloss finish. It is less reflective than a smooth mirror-like finish and reveals fewer faults. The veneers' texture can also be seen and felt, which to me is very important for a wood picture.
Pumice powder is an abrasive material used in the application of varnish, shellac, or lacquer to produce a fine hand-rubbed finish. It has obtained by grinding volcanic ash into powders of different degrees of coarseness. Pumice is used for coarse rubbing between coats when building the body of the finish. Use rottenstone for fine polishing, usually only on the final coat.
Pumice is worked with a pad. Different type of pad can be used (cotton, wool, or felt). Felt rubbing pads are available from ¼" to 1" thick in various weaves. The finer the weave the smoother the finish. Coarse rubbing of the undercoat can be done with medium pumice and medium pads. Sprinkle just enough pumice on the work to do the piece. As you run it in, the powder will ground smaller, so if you must add more pumice, remember it will be coarser and scratch up what you have accomplished.
Rottenstone is a very fine powder ground from slate or limestone. It can be worked with a pad for the final finish.
A lubricant must be used with pumice and rottenstone because dry powder will cake up and could cause heat or friction damage. Note that water will turn shellac white, so must be avoided in favour of oil when shellac is your finishing material. Through the years several kinds of oil have been in and out of favour. We suggest paraffin oil as a good quality, light oil. If using oil, make a thin mix of pumice and oil to be applied with the pad, or you can soak the pad with oil dip it into the pumice to transfer enough powder to do the piece. If using water, sprinkle some pumice on the surface of the piece then add enough water to make a paste.
With the pad, rub and try to use the same pressure and number of strokes over the whole surface. Avoid using circular motion as this will leave visible scratches. Keep the abrasive wet, keep the pad free from build up, and check the surface often to make sure that you do not rub through. Do not rub too long in one place or the finish will burn from friction or from lack of oil. Be very careful at the corners and edges as it is easier to go through at these spots. Work until a very flat and dull surface is achieved, then clean off the pumice. Oil leaves a film which must be removed with a very soft cloth and dewaxer. For the last application of pumice, using a fine pumice powder with oil, rub only enough to dull and smooth the finish, then clean it. Wait 48 hours before fine rubbing with rottenstone.
Oil finishes can use veneer to separate from the substrate if contact cement was used as the adhesive. Avoid this problem by treating the veneer with glue size. Prepare glue size by using the mixture of 25% PVA white glue to 75% water. Apply a coat to each side of the veneer and then sandwich it between sheets of paper and plywood, placing wax paper between veneer and paper. Put weight of veneer to keep it flat. Let it dry thoroughly over a few days, but change paper every day. Glue size does not affect finishing, and it strengthens veneer.
Applying a glycerine solution to brittle or wrinkled veneer makes it much easier to bond to a substrate. Mix two ounces of glycerine into one litre of water. Apply the solution to both sides of the veneer with a sponge or spray bottle, and then sandwich the veneer between brown paper and plywood. Clamp or apply weight to this set-up as soon as possible. The paper may have to be changed occasionally if it becomes saturated with water. The veneer should be ready for use or bonding after a few days under pressure. Since writing the above paragraph, Paul Armstrong gave a discussion at one of our meetings on the method he uses to straighten veneers. His method is to dampen the veneer with a fine spray mist of water only, then place them in a plastic bag for a couple of days. He then remove the veneers from the plastic bag and repeats the spraying of water only mist and returns them to the plastic bag for a couple more days. Paul advises us he has not had any problem with the black spot using this method.
Before sandpaper was developed in 1860, wood was often smoothed by fine sand under a piece of damp, soft leather.
by John Sedgwick
from Canadian Marquetry May 1995
Throughout modern history one finish, French polish has gained an almost mystic reputation. "Oh yes this is a charming Regency table, Circa 1812, the original finish has been cleaned and rejuvenated, at great expense". Conversations like this go on at antique dealers everywhere, but very few dealers are likely to pay to restore old French polish. There are two reasons: first, few people know how, and second, those that do have difficulty getting paid for their time at market rates.
For the hobby and experienced woodworker alike, French polishing is an exercise that will challenge. However, "to those that persevere, go the rewards". Apparently, like apple pie everyone has a slightly different recipe on how to French polish. With at least 2 dozen variables which are in play at any time, only practice and many failures will eventually teach you how. A competition would be interesting, like a cake bake-off perhaps, to see who has the best formula with the winner's method finally recorded on video. Ah! but then the mystery and the reverence would be gone. What now follows is my way.
Where to begin? Well perhaps some biology or chemistry is in order. The principle ingredient in the process is lac, a resin-like substance excreted by the parasitic beetle, Laciffer lacoa, which lives in certain trees in India and other eastern countries. The resin is hand-picked and washed. At this stage it is known as seed lac. The lac is placed into canvas tubes and heated over a fire. The lac melts and is squeezed onto a cold surface to form approximately 3 inch discs that are left to harden. Purchased in this form it is referred to as button shellac. Further refinements of the lac are achieved by diluting and straining. The lac is then spread out very thinly and allowed to dry. In this form it is sold as orange shellac. In the past these flakes were sun bleached, but now they are chemically treated; the resulting flakes are sold as lemon shellac. With each of these treatments the shellac gets lighter, the lightest now being blond shellac. It is sold as either super fine or blond shellac flakes. With each process from button shellac to blond shellac, the flakes and subsequent finish become more transparent. The bleached shellac is mixed with alcohol and sold as white shellac.
If the surface has not been stained and you wish to show the natural colour, super fine or blond shellac flakes are the only logical choice. However you should know such flakes did not exist until recently and would not be appropriate for restoring that Circa 1812 table. Personally I prefer to mix my own shellac primarily because of its limited shelf life (about 6 months). Commercially prepared white shellac is mixed (cut) in about a 5:1 ratio (5 parts alcohol to 1 part flakes) and it is not dated. As mixed shellac ages, it takes longer to dry and hence longer to body-up the surface (i.e. build up the finish's thickness). It is however suitable for resurfacing a previously polished surface.
There are various methods of measuring the mixture of alcohol and lac flakes. Commercially it is done by weight. I have found that mixing by volume works providing I crush the flakes in the same manner every time. I often use lemon shellac on old marquetry as it imparts an antique yellowish patina, but the flakes are bigger and thicker and so measurement by volume will give a different proportion than the same volume of blond flakes. Crushing the flakes to a granular consistency will enable the lac to dissolve quicker and will give a consistent measurement.
There are two basic applications of the shellac: first, the bodying-up, or the filling of the pores and smoothing of the surface, and second, finishing or polishing. It is quicker to use a thicker mixture at first. My mix is 4:1 crushed flakes and alcohol at the start and then 5:1 for finishing.
To crush the flakes, first put them in a ziploc-type bag or double a sandwich bag, and wrap the bag in a cotton rag. Whack the daylights out of it, with a flat piece of wood. After crushing the lac, measure by volume 1 part flakes and pour them into an old clean narrow-necked plastic 2 litre juice container. Using the same measuring device, pour in 4 parts methyl hydrate (wood alcohol). Put the cap on the container and shake well. It will take an hour or so to fully dissolve the lac flakes. Periodically inspect the solution and shake it vigorously. Screen out the lac that will not dissolve with a kitchen sieve, or better, buy a new one (the cook will not be amused). Remember to rinse the sieve with methyl hydrate after you use it.
Most hardware stores carry clear plastic containers for mustard and ketchup. Buy 2 of these, perhaps when you buy the sieve. Pour the shellac mixture into container #1, and mark it 4:1 mix. Into container #2 pour 1 portion of alcohol, measuring it in the original measuring container, then pour in 4 portions of the 4:1 mix. This is then your 5:1 mix. Mark this container 5:1 mix.
By mixing your own flakes, you will have removed one of the many variables, the age of the shellac. The older the mixture the longer it takes to dry so unless you intend to use any leftover shellac within few months, throw it away. Shellac is environment friendly and will kill weeds by stopping photosynthesis in foliage.
If you have struggled with me so far, you are becoming aware of what I said initially, this is not a finish for the faint of heart.
You are now about half way to completing your supplies: next is the shellac rubber. You will need some heavy cotton rags, the thickness of cotton jeans. If you cannot find any, visit a material store and buy a yard of plain white cotton. Next you will need a package of cheesecloth; this is available at most automotive stores as polishing supplies. Cut two pieces of white cotton about 10 inches square. Place a folded pad of cheesecloth about the size of an egg in the centre of one side of one piece of cotton. Fold two sides of the cotton over the cheesecloth then roll everything into a tight pad. This is a shellac rubber. Make another rubber and place them both flat side down in plastic food savers. Charge them with shellac, (see below step lA Bodying-up) one with the 4:1 mix, the other with the 5:1 mix. Put the lids on and seal them. The rubbers, should be conditioned after a couple of days soaking in small quantities of shellac. Make a third rubber using a 10 inch square piece of linen instead of the cotton. Use an old white pillow case, shirt or bed sheet. Into this third pad pour 6 caps full of methyl hydrate, assuming your caps are about 1 inch in diameter.
A ponce bag is a bag or pouch filled with pumice. The bag is made in the following manner. Cut an 8 inch square of linen. Pile about 6 tablespoons full of pumice powder in the centre. Fold the four corners to the centre and twist into a ball. Use a garbage bag tie to prevent the bag from unravelling. Put the ponce bag in a fourth food saver.
The final ingredient is olive oil. This needs to be in a container that will allow you to dispense small quantities.
In the preceding paragraphs I have noted a series of supplies required. The following is a summary:
You are now ready to begin. Do not start without all the ingredients as I have described. Here are a couple of useful tips:
There are usually only three basic steps attributed to French polishing:
STEP 1A (Bodying-up) This part of the process will use the 4:1shellac mix and the rubber or brush to quickly coat the surface.
STEP lB (Rubbing) This is done exclusively with a rubber, the 4:1mix, olive oil, and the ponce bag.
STEP 2 (Polishing)
STEP 3 (Spiriting-off)
Sand the surface with 320 grit paper. Wipe the surface using a piece of clean linen and methyl hydrate. What you will notice being lifted from the surface is the sanding dust that had been trapped in the pores. Now leave to dry for half an hour. The previously smooth surface now feels rough: the wood fibre ends are now standing up. Stroke the surface lightly with fresh 320 paper in the general grain direction and wipe clean with a dry cloth.
STEP 1A (BODYING-UP)
Put on a pair of disposable latex gloves (put some powder inside the gloves to reduce perspiration from your hands). Remove the conditioned rubber from the 4:1 food saver. Open the rubber partially from the back. To charge the rubber, slowly feed it with 4:1 mix, allowing the enclosed cheese cloth to soak up the shellac mix. Do not put shellac directly on the rubber face. Do not over charge the rubber, too little is better than too much.
Pass the rubber over the surface in a left to right motion like a pendulum moving about 10 inches each time. While doing this, squeeze the shellac from the rubber. Do not be concerned with what appears to be very uneven spreading of the shellac. This will disappear as the shellac builds, cover the desired area once, then wait about 15 minutes between each additional rubbing. Start with initial applications of shellac as though you were brushing it on. The main objective is to fill most of the wood pores and to build an even coat. You can also at this stage use a brush. Whichever method you choose, alternate the direction of the application with each coat, this will insure complete coverage. Mistakes such as smudging occur when the shellac becomes tacky and you stop, leaving the rubber still on the surface. Such mistakes at this stage are easily covered with the next application.
After about 4 coats, leave it to dry for 24 hours, then sand the surface smooth with a power sander or sanding block using 180 grit and finish with 320 grit. Wipe the shellac dust clear with a dry cotton rag or use a vacuum and brush. The surface is now ready for rubbing.
STEP lB (RUBBING)
The most commonly described motion for French polishing is moving the rubber in a figure 8 covering an area 8" to 10" long by 3" to 4" wide. Personally I find this motion cumbersome and more prone to smudging. I prefer a circular or oval motion that enables me to quickly change the diameter of the circle, or move from area to area. This is where you finally start the process that has eluded and challenged wood finishers since its use on fine furniture. I have read many descriptions of this technique and in almost every case the description is inadequate. Perhaps it is impossible to adequately describe and interpret this process that has so many variables. I will say from the beginning that a degree of application flexibility with pressure, shellac, and oil is required.
ARE YOU READY ?
Holding a charged rubber in your right-hand squeeze 6 drops of olive oil onto the face with your right hand and spread it over the surface (reverse for lefties). Then do the following:
For obvious reasons your first attempt should be on a small piece not a board room. It might help you to visualize what is happening on the surface, when you are doing it right of course. When you French polish, you are trying to create one seamless layer of shellac not layered levels like other finishes. You do this by keeping the surface in a semi-liquid state and adding more shellac. The tacky state is isolated from the rubber with olive oil which is used continually as required. The rubber will slowly level and smooth each new charge of shellac. You can see from this that excess pressure, at the wrong time can tear out previously laid shellac.
Continue the coverage with the first charge in the rubber repeatedly on the same area. You will notice that at first the rubber makes a harsh swishing sound and it is difficult to move over the surface even when additional oil is applied. Work without stopping through this stage, applying oil as required. Slowly the surface tension will change as the shellac dries. If a problem area develops, move away from it and allow it to dry. Do not stop. Do not pause with the rubber on the surface.
1B.1 Levelling a smudge.
Smudging, at this stage, is easily repaired by one of the following methods:
The traditional method of levelling a smudge is method #3, however I only use this during the bodying-up stage, as it is helpful in quickly filling large open pores. Note these techniques as you will need all of them before you finish.
You will notice as the surface begins to smooth out and the shine slowly appears that the rubber leaves a cloudy trail behind. Ignore it at this time; you will take care of it later. The cloud trail is the oil rising to the surface: some oil mixes with the shellac but most stays on the surface. When the cloud trails appear, extend the diameter of the circles, changing to an oval stroke, and eventually cover the widest part being worked. Finally make long wide strokes lifting the rubber at the end of the stroke, placing it back on the return.
1B.3 Filling grain, pores, and defects.
There are three methods used in French polishing to fill grain, pores, and defects. (Although having said that there have probably been many new filling products on the market since the invention of French polish.)
These sticks have been used for centuries to seal official documents often with the seal of the sender pressed into it. Various dyes are added to match the wood colour. These sticks are available in various colours including clear, from various mail order and quality finishing suppliers.
There are specific tools available for this process: an electric burn-in knife, or a manual curve-bladed knife, and an alcohol lamp are the latest toys. Some of you will be familiar with the tablespoon and candle method. The handle is held over the candle flame, and the hot spoon handle is then used to melt the shellac stick.
This method is only suitable for filling small wood defects and is not practical for large areas.
Pumice and 4:1 mix.
Originally pumice was grated over the area to be filled and the shellac worked over it. This method most probably is still in use today on large pieces of dark furniture. However I always use the ponce bag: the linen will only allow the finest pumice powder to screen through.
Tap the ponce bag on the area to be filled to lightly cover it with pumice powder. Now with a freshly charged rubber begin lightly rubbing, in a circular motion, the area. You will soon notice that the shellac and pumice combine to fill the pores, although you have not yet achieved a very deep coating. Be careful to avoid drag-out. The added abrasive capability of the pumice increases this possibility. Wait at least 30 minutes before repeating the procedure on the same area.
4:1 shellac mix only.
This is my choice for most of the bodying-up process. If you wish, you can dab obvious spots using the 4:1 mix and a brush. Leave these areas to dry over night. Sand the filled areas smooth. Afterwards you can recharge the rubber and continue bodying up. All remaining flaws should now be filled.
1B.4 Drag out.
You will know it when it happens, and it will happen. This is when too much pressure is used or too much time is spent on one place: the shellac below the working surface is softened and all the polish is dragged off one spot.
There is only one fix: sand the area and feather it off over a large area.
1B.5 Preparing the surface for final finish.
How do you know you are finished bodying up? When the following has happened:
Assuming the above is so, wait a minimum of 24 hours, preferably longer, before moving onto the step 2, polishing. Put away the 4:1 rubber after charging it with a little methyl hydrate. Make sure you seal the top tightly. This will leave it ready for next time. By now you may have had a glimpse of the shine that is to come while bodying up. But before you can continue you must sand away any polishing swirls or surface blemishes. Sand with 240 grit wet/dry sandpaper and oil, or dry 240 grit. Sand in one direction with a flat hard sanding block.
STEP 2 (POLISHING)
The freshly sanded surface will now be dull, flat, and blemish-free. Wipe any residue from the surface. Now open the 5:1 rubber and charge it with 5:1 mix. The procedure is precisely the same as bodying up. Apply an initial covering of oil to the rubber and begin circling. Be warned however that this mix is thinner and softens the surface quicker; moreover you have a rather thick layer of shellac to tear out if you mess up. Continue with this step until you have covered the entire surface many times. The mirror-like shine should now be showing itself through the transparent clouds. The more shellac you apply, the deeper it will look. However before you apply each new charge, allow at least 30 minutes to dry.
Mistakes at this stage are very costly to repair. You must wait for the surface to dry and hope you can wet sand it out. If not, you should dry sand, using the 4:1 mix to body-up if necessary and sanding again. The rules here are if it looks like a problem is developing, leave the problem area alone and work on another area allowing it to dry, increase the circle diameter, add more oil, and lighten up on the surface pressure. All or any of the above will be used throughout the process and by now you should be beginning to understand when and what is needed during these French polishing steps, including a new arm!!!
When you are satisfied with the surface, you can begin the process of removing the oil. Recharge the 5:1 mix rubber with small quantities of methyl hydrate. Using about 8" overlapping circles move the rubber lightly over the surface until the rubber becomes dry. Finish with wide sweeping strokes. Repeat this until the oil clouds have thinned and are barely visible. Put the 5:1 rubber away in its sealed container with a small charge of methyl hydrate. Do not touch it for at least 24 hours.
STEP 3 (SPIRITING-OFF)
For those of you who cannot handle stress this may be a stage you want to miss. One week from now the remaining oil can be removed with a good quality furniture polish, but the shine will dull 25% or more.
To begin spiriting-off, remove the clean linen rubber from its sealed container and check to feel that it is still damp from your original charge. If the rubber is not charged, recharge it and put it back in the sealed container for a few hours. When you remove the rubber it should be damp.
Starting at the top of the piece and using long sweeping strokes, lower the rubber onto the surface, lifting and raising it at the end of each stroke. When you begin, the surface will go dull under the rubber; this is because the shellac surface dried with the oil still on it. Slowly the oil will be released as the surface softens. As you continue to stroke back and forth the shine will return. Continue until the oil streaks disappear. Recharge the rubber if there are still major streaks, but wait for the methyl hydrate to soak in before reuse.
There may be some small streaks that appear at the beginning and end of each stroke. Remove what you can. Do not change directions. Small smudges will vanish overnight or fade in a few days. Many projects have been sent back to square one by the over zealous chasing of smudges. You are finished for now!
In reality you are never finished. Unlike other finishes, you can always add more shellac. Even 5, 10, 15 years from now, you can dust the surface, spirit off any furniture polish residue with methyl hydrate and add a new finished surface. There is no need to strip the surface to make a repair or restore the shine. Because this finishing technique is complex and time consuming it becomes obvious why this finish is not in common use and has been replaced by other methods. We do know however that furniture which has been French polished 200 years ago is able to be re-polished to its original splendour. No other finish can make that claim.
FRENCH POLISHING EDGES
Polishing edges, table legs, moulding and contoured areas cannot be done with the same circular motion. The same process however can be duplicated, by changing the stroke from circular to lateral. Depending on the size of the piece it may be necessary to make a small rubber. The motion is changed to left to right short strokes at first, increasing the length as it dries. The same bodying-up and polishing procedures are required and logically should be done at the same time as all flat surfaces.
by Frank Harris
Canadian Marquetry March 1997
In case anyone is ever tempted to purchase and use what is
sometimes called "white spirits" or "odourless mineral
spirits", you might want to consider the following reply which I received
from "Fine Woodworking" magazine.
I wrote to them asking "what exactly is mineral spirits?" and
mentioned the fact that white spirits and odourless mineral spirits was
generally three or four times the price of mineral spirits or paint thinner and
asking, was there any advantage to using it?
Mineral spirits is not one simple chemical or substance but is a complex
mixture of many chemical compounds and does not have a definite chemical
composition. Chemically speaking,
mineral spirits is a mixture of aromatic hydrocarbons, naphthenic hydrocarbons
and paraffinic hydrocarbons. Regular
mineral spirits is distilled from petroleum and the exact composition depends on
the grade and type of crude oil processed by the refinery.
For instance, mineral spirits processed from Southwestern crude is high
in aromatic content (usually about 25%) while Mid-Continent crude produces
mineral spirits with about 12% aromatic content.
Even though the chemical composition varies from refinery to refinery,
the distillation range or "cut" of the mineral spirits fraction
remains constant. To be labelled
and sold as mineral spirits the solvent mixture must distil or boil between 310oF
On the other hand, odourless mineral spirits is a chemically synthesized
product. That is to say, odourless
mineral spirits does not occur naturally in the crude petroleum feedstock but
must be chemically manufactured at the refinery.
The manufacturing process is quite demanding (hence the higher price) and
involves catalytic cracking of large molecules into smaller ones; then
chemically recombining these smaller molecules in a specific way to produce the
desired product. Odourless mineral
spirits formed in this process is almost 100% isoparaffinic in character and
contains no aromatic and few naphthenic hydrocarbons.
Regular mineral spirits and odourless mineral spirits have little in
common except the same distillation range and unfortunately similar names.
Odourless mineral spirits is primarily used in products where solvent
"power" is not a big concern, such as in the cosmetics industry and in
other specialty household products. Odourless
mineral spirits is definitely the poorer solvent of the two and should not be
used for any finishing applications.
Regular mineral spirits, often simply labelled paint thinner or varnish
thinner, is a much better choice as a finishing solvent.
The aromatic hydrocarbon portion of regular mineral spirits adds a little
odour to the product but greatly enhances the solvent "power" and
allows it to be used as a degreaser or paint thinner.