About Alcohols Used as Solvents in French Polishing

Most any kind of pure or nearly pure simple alcohol can be used as a solvent for shellac, because shellac will dissolve completely in any of them. But the choice of what kind of alcohol to use gets complicated pretty fast when issues of application, availability, price and safety are considered. The two most common choices for shellac solvents for use in French polishing are ethanol and denatured alcohol, which is nominally ethanol to which some poisonous substance(s) has been added to make it undrinkable. Both of these are discussed here in terms of all of the qualities listed above. This article originally appeared in American Lutherie.

Initial appearance: November 6, 2021
Last updated: November 06, 2021



About Alcohols Used as Solvents in French Polishing

Copyright © 2010 R.M. Mottola

[This article originally appeared in American Lutherie #105. It includes product specifications and prices that were accurate at the time of original publication.]




Photo 1 - Guitars awaiting French polishing in the author's shop.

Most any kind of pure or nearly pure simple alcohol can be used as a solvent for shellac, because shellac will dissolve completely in any of them. But the choice of what kind of alcohol to use gets complicated pretty fast when issues of application, availability, price and safety are considered. The two most common choices for shellac solvents for use in French polishing are ethanol and denatured alcohol, which is nominally ethanol to which some poisonous substance(s) has been added to make it undrinkable. Both of these are discussed here in terms of all of the qualities listed above.

The research behind this article is one of those good examples of one thing leading to another. We were discussing the general availability of ethanol around the American Lutherie cyber water cooler one day, and this led to the realization that both contributing editor Cyndy Burton and I suffered from burning eyes when we have used denatured alcohol. This seemed odd, given the generally accepted belief of what is used to denature alcohol (methanol) and how little of it (5%) we believe there is in the mix. This in turn led to a detailed look at denatured alcohols, in terms of chemistry and safety, and this turned up information which I thought fascinating enough to share.

First a bit of background and some terminology. Ethanol (also called grain alcohol, ethyl alcohol, and neutral grain spirits) is the alcohol that is in all alcoholic beverages. For this reason we generally tend to consider it to be pretty benign, although for industrial purposes it is rated as a hazardous chemical. Ethanol is made commercially by fermenting corn and then removing the alcohol by distillation. Distillation can only yield 95% pure alcohol (190 proof) and this is the form most readily available. Although it is possible to manufacture 100% pure ethanol the process is expensive (it involves stripping off the water with benzene) and so is the alcohol, and it turns out to not offer any advantage in our application.

Alcoholic beverages are heavily taxed in the U.S.A., a vestige of the repeal of prohibition. Ethanol is used industrially where, in theory at least, it would not be taxed as an alcoholic beverage. But ethanol in a bottle in a liquor store and ethanol in a drum in a factory are both ethanol, so to squelch a trend for folks to simply drink the untaxed industrial stuff, two things were done. The first was to make denatured alcohol available for industrial applications. As mentioned, this is nominally ethanol to which some denaturant has been added to render it undrinkable. Denatured alcohol is not taxed as an alcoholic beverage and so it can be used economically in industrial applications. The denaturant has to have a number of qualities. It has to be poisonous or at least noxious, to serve its function of keeping folks from drinking the stuff. It also has to be not easily separated from the ethanol by distillation, so folks don't just distill it out. And it must be close enough to ethanol in its solvent qualities so the denatured alcohol can substitute for ethanol in most industrial applications.

But denatured alcohol cannot substitute for ethanol in all industrial applications, so the second thing that was done was to create a licensing process for the purchase of untaxed ethanol. This works well enough for large scale users of ethanol, but for small scale users the permit process is generally considered to be more trouble than it is worth. And so, many small scale users simply use taxed ethanol for their purposes. This article is specifically focused on alcohol for French polishing, and French polishers by definition are small scale users. For these users then, the choice of solvent really comes down to one of taxed ethanol or denatured alcohol.

Comparing ethanol with denatured alcohol in terms of application, availability, price and safety would be a fairly straight forward process if only denatured alcohol was manufactured using a set formula. But it is not, and contents vary widely from manufacturer to manufacturer. And some manufacturers don't even have a set formula for their denatured alcohol, further confusing things. In any case, the denaturant most often used is methanol, and we should discuss this substance a bit before proceeding.

Methanol (also called methyl alcohol, wood alcohol and methylated spirit, although the latter term is also used to refer to denatured alcohol) is another simple alcohol. It is also generally made from corn, but from the stalks, not the kernels. It is poisonous to drink in any quantity and it can't be separated from ethanol by simple distillation. Its solvent properties are very similar to those of ethanol. All these qualities combine to make it a good choice as a denaturant in the manufacture of denatured alcohol. There are a bunch of other substances used in some recipes for denatured alcohol, but I won't discuss them much here except in terms of how they affect safety of the product.

Because the recipes for denatured alcohol vary so greatly we really should treat each brand separately when comparing to ethanol. It turns out that this is really only necessary when discussing price and safety, as will be seen later. All of the discussions reference table 1, which compares ethanol from two sources, and seven brands of denatured alcohol. The discussion of safety also references table 2, which compares the recipes of all of the products in table 1.

Product Manufacturer Approximate price per quart,
Sept. 2010
Availability Product PEL (derived,
worst case mix, ppm)
Spectrum ACS 190 proof
Ethyl Alcohol (tax paid)
Spectrum Chemicals
14422 S San Pedro St.
Gardena, CA
$40 mail order from lab supply 1000
Acros Organics Ethyl alcohol
denatured w/ 5% wood spirit
Acros Organics
One Reagent Lane
Fair Lawn, NJ
$18 mail order from lab supply 971
Everclear 190 proof Luxco
5050 Kemper Avenue
St. Louis, MO 63139
$17 retail liquor stores in some states,
Internet liquor stores
1000
Behlen Behkol RPM Wood Finishes Group, Inc.
P.O. Box 22000
Hickory, NC
$9 woodworking supply and stores 848
Sunnyside denatured alcohol
(note: 93% active ingredients)
Sunnyside Corp.
225 Carpenter Ave
Wheeling, IL
$4 paint and hardware stores 870
Crown denatured alcohol Packaging Service Co., Inc.
1904 Mykawa Road
P O Box 875
Pearland, TX 77581
$4 paint and hardware stores 286
Startex denatured alcohol CSD/Startex
PO Box 3087
Conroe, TX
$4 paint and hardware stores 900
Klean Strip XLS denatured alcohol W. M. Barr
2105 Channel Avenue
Memphis, TN 38113
$3 paint and hardware stores 400
Klean Strip Green denatured alcohol W. M. Barr
2105 Channel Avenue
Memphis, TN 38113
$4 paint and hardware stores 915

Table 1 - Two ethanol and seven denatured alcohol products are compared by price, availability, and derived product PEL (see text), a single measure of safety. Note that not all products are packaged in quarts. For those that are not the quart price is normalized from that of the nearest package size.



Application. The first thing I researched was the suitability of the products in the tables as solvents for use in French polishing, without considering any of the other factors. Although I could find no hard research data on the subject, opinions abound and often differ, with some folks opining that there is no functional difference and others maintaining that ethanol was superior. Pressing for details on the latter end of the opinion spectrum, the most common complaint was that the methanol in denatured alcohol made it evaporate faster, and that this adversely affected the finish, sometimes causing it to fracture due to internal stresses. I did not find anyone of this opinion that had first hand experience with this problem, and I consider it to be unlikely. Methanol does evaporate faster than ethanol, all conditions being equal. The boiling point of methanol is 148.4 deg. F, lower than that of ethanol, at 172.6 deg. F. But these values are pretty close, so close in fact that you can't remove methanol from ethanol by simple distillation. The reason they may not look that close when simply compared to typical ambient temperature is that the relationship between boiling point and rate of evaporation is nonlinear. The percentage of methanol varies considerably from one denatured alcohol to another in the tables, with a high of 75% possible for the Crown denatured alcohol. For most of the denatured alcohols though, the percentage of methanol in the recipe was small enough to put any effect due to differences in evaporation rate in the noise.

A note on water content. Most ethanol and denatured alcohol has a water content of about 5%. This is an effect of the fact that the last 5% of water cannot be removed from alcohol by simple distillation. Empirically it is obvious that alcohol with 5% water content is usable in French polishing. What is not so clear is what percentage water content is detrimental. Alcohol is hygroscopic and absorbs water from the air. So any alcohol exposed to the air will be absorbing water and thus increasing its water content. The absorption rate is nonlinear - the purer the alcohol, the more readily it will absorb water. All of this explains why alcohol and shellac should be kept in sealed containers. It also explains why there is nothing to be gained by the use of 100% pure alcohol, which can be obtained from chemical supply houses. Exposed to the air, it absorbs water fast enough to make it equal to 95% alcohol in no time. It may not be too useful to obsess over water content, at least in this range. Although the chemistry of shellac is not completely understood there is really no evidence that removing all the water from the solvent makes for a better coating.

Availability. There are a number of sources for alcohol. Denatured alcohol can be purchased retail from hardware stores and home centers, specialty woodworking stores, and mail order from specialty woodworking suppliers. But as you can see from table 2 there is a big difference in the recipes for various brands, and if you are looking for a particular brand then your choices for suppliers will be limited. Laboratory and chemical suppliers sell both ethanol and denatured alcohol mail order. You can generally get denatured alcohol that is minimally denatured from lab supply houses. For example, the Acros Organics Ethyl alcohol denatured w/ 5% wood spirit listed in the tables has less than 5% denaturants. Chemical supply places are also a source of ethanol. They will stock both 95% and 100% (also called reagent grade) ethanol. And they will almost always carry tax paid alcohol, which has been taxed as drinking alcohol and can be bought without special permit and in any quantity. With the exception of purchases by large industrial users, most ethanol is purchased this way. To sum this issue up as briefly as I can, your choice here is to buy it as liquor and pay the tax on it as liquor, or to buy it as an industrial chemical and have to deal with some regular paperwork. Personally, I opt for the former.

190 proof (95%) ethanol can also be purchased from liquor stores in a small number of states. The two brands that I know of are Everclear and Clear Spring. Please note that most states do not allow the retail sale of 190 proof ethanol, but some do allow the sale of lower (151) proof versions of these products. The lower proof ethanol has too much water to be used as a solvent for French polishing, so if you are looking for one of these products be sure to specify that you want 190 proof. You can also purchase 190 proof Everclear in any state from Internet based wine stores. The regulation of Internet-based alcohol sales is something of a gray area, but at least at the time of this writing you can usually find one that will ship to any state. Note that not all Internet wine stores list 190 proof Everclear, but most can get it if you contact them and ask for it.

Price. The prices that appear in table 1 are typical and do not include shipping. Where shipping is a requirement (ordering from a lab supply house or Internet wine store for example) the charges are fairly high since the material is liquid, flammable, and the shipment must be signed for by an adult. I generally buy Everclear from an Internet wine store that is about 200 miles away. Although the price is about $17 a quart for four quarts, it costs an extra $4 per quart to get it here. Chemical suppliers generally charge the highest prices to begin with and shipping charges must be added to those, so buying from them can mean some expensive shellac thinner. Contrast this to the $3 - $4 per quart that you pay for denatured alcohol from the hardware store, and it is pretty clear that price can be a big factor when choosing alcohol.

Safety. For most folks safety is a serious concern when choosing solvent. French polishing is time-consuming and when you do it you spend most of that time in close proximity to the source of alcohol fumes. The major hazard in typical use of alcohol solvents is from the vapors. Objective safety data for each of these products is available in the product Material Safety Data Sheet (MSDS), a government mandated collection of safety information on any hazardous product used commercially. All manufacturers and sellers of hazardous materials are required to make MSDSs available, and these days most companies find it easiest to provide these online. So if you are looking for the MSDS for, say, ethanol, a search engine search for "ethanol MSDS" will find one for you quickly and easily. All of the recipe data and all of the safety info in the tables here comes from MSDSs found online.

An MSDS contains a lot of information, but the key information as far as safety goes for French polishing solvent is the Permissible Exposure Level (PEL). This is a single number, generally presented in units of parts per million (ppm) or milligrams per liter (mg/L) or milligrams per cubic meter (mg/m3). It specifies a mix of the hazardous material and air, so the smaller the number the more hazardous the exposure is. The PEL is calculated on a time weighted average basis, which assumes the actual exposure varies over an eight hour day. It is important to note when considering PELs that these state actual permissible exposure levels for eight hour days, every day, for a typical person's working life. They are therefore generally very conservative. The PELs I use in this article come from the U.S. Occupational Safety and Health Administration (OSHA). Other countries have their own PELs, and you will sometimes see these in product MSDSs as well. They usually agree with each other, but when they don't I personally just assume the one with the lowest value is the one to follow. This brings up a general rule of thumb about safety related exposure level data, which is to always assume the worst case. It's what most people in industry do.

If you just want a simple measure of relative exposure safety you can also often find the National Fire Protection Association's health, flammability, and reactivity ratings (NFPA HFR) in the MSDSs. These appear as a symbol, generally referred to as the "NFPA 704 diamond" and shown in figure 1. Each box in the diamond contains a number from 0 to 4. The blue box contains the health rating, and a value of 4 means the substance is most dangerous and a value of 0 means the substance is least dangerous (but it does not mean the substance is benign). You can find a nice table of typical solvents that includes NFPA HFRs, OSHA PELs and also odor thresholds at http://www.chem.purdue.edu/chemsafety/chem/solvents.htm.



Figure 1 - The "NFPA 704 diamond", a simple graphic for rating hazardous substances.

Ethanol is a hazardous material and it has an OSHA PEL of 1000 ppm. But denatured alcohol is a mixture of any number of hazardous materials (see the recipes for denatured alcohol in table 2) and so in the MSDSs for these products you'll find a separate PEL value for each hazardous ingredient. What you'd like of course is a single PEL for the whole product, so you could easily compare brands of denatured alcohol and ethanol. Unfortunately product PELs are not available and there aren't any simple guidelines that are available for calculating them either. The reason for this lack is that it is not clear whether or not exposure to each hazardous component of a product should be considered separately or added together. OSHA has indicated that some mixtures should be considered as the sums of their component parts (see OSHA Technical Manual, section II, chapter 1, Personal Sampling for Air Contaminants, heading XI.k., at http://www.osha.gov/dts/osta/otm/otm_ii/otm_ii_1.html) but I could find no clear indication that this applied to alcohol mixtures. My personal approach to this problem is to assume that each component is to be considered separately when doing so yields a worst case product PEL, and to derive a PEL for the product as a sum of component PELs otherwise, as described below. This is how I calculated the derived product PELs listed in table 1. Please note that this is my personal approach and not a general recommendation for safety and compliance.

The first step in this approach involves weighting the PEL of each ingredient by multiplying that PEL by the quotient 100 over its percentage of the product, like this:

weighted ingredient PEL = (100 / ingredient percentage of product) * ingredient PEL

and then identifying the ingredient with the lowest weighted PEL. If the weighted PEL for that ingredient is less than the PEL for ethanol (1000) then I use that weighted ingredient PEL as the PEL for the product. Using this value as the PEL value for the product means that, as long as exposure to the product is under that value, exposure to all of the components of the product will be under their respective PELs.

Here's a hypothetical example for a denatured alcohol containing 50% ethanol (PEL = 1000 ppm) and 50% methanol (PEL = 200 ppm):

weighted ethanol PEL = (100 / 50) * 1000 = 2,000

weighted methanol PEL = (100 / 50) * 200 = 400

So the weighted ingredient PEL for the methanol (in this case 400) is taken as the product PEL since it is lower and also less than the PEL for ethanol. If the lowest weighted ingredient PEL value is greater than or equal to the PEL for ethanol (1000) then I calculate a PEL for the product by multiplying each ingredient's PEL by its percentage of the mix and summing these together. Here's a hypothetical example for a denatured alcohol containing 90% ethanol and 10% methanol:

0.9 * 1000 = 900

0.1 * 200 = 20

product PEL = 900 + 20 = 920

Using this two step process to derive a PEL for a denatured alcohol product yields worse case but reasonable results.

Some recipes for denatured alcohol contain fixed percentages of each component, but some provide ranges of percentages instead. In these cases I use the rule of thumb mentioned above - assume the worst case, which in this instance means to assume the most hazardous mix indicated by the recipe.

I'll discuss how to make use of the PELs in a bit, but first I want to point out that some general observations can be made from the PEL data found in the tables. As mentioned, the OSHA PEL for ethanol is 1000 ppm, and the OHSA PEL for methanol, the denaturant used most often and in the highest concentrations, is 200 ppm. That permissible exposure to methanol is five times less than for ethanol did not correlate at all with the straw polling I did for this article, asking folks to assess the risks in working with these two chemicals. Most folks considered ethanol to be pretty benign, and methanol to be a deadly chemical. I didn't ask the question in this manner, but I suspect that the general consensus of those polled would be that methanol is way more than 5 times more toxic than ethanol. There are a lot of potential explanations for this, but one I want to consider here is that the odor threshold for methanol is way lower than that for ethanol. Methanol smells stronger and therefore we may consider it to be way more toxic. My point in mentioning all of this is to suggest that when assessing the hazards of exposure, the objective data in the PELs are much more reliable indicators than smell, gut feeling or culturally received wisdom.

How to make use of the PEL data is simple in theory, but in practice not so much. In theory, you simply make sure that the air changes in your work space at a rate that is high enough, and that the evaporation rate of the alcohol is low enough, to keep concentrations below the PEL. Let's look at ethanol, with a PEL of 1000 ppm, used in a room that's an eight foot cube. When considering hazardous vapor mixed with air, it is more convenient to use a PEL in the form of mass/volume. Converting (see http://www.industrialventilation.net/ClassDownloads/IHS_725/Calculations%20for%20PPM_dilution_gasLaws/OSHA-PELs%20and%20TLVs%20and%20density.pdf) or looking up the PEL for ethanol in this form gives us 1900 mg/m3. The volume of the 8'x8'x8' room is 14.5 cubic meters, so the PEL permits constant exposure to the air in this room mixed with 27.6 grams of ethanol, which is about 35 ml or 1.2 fluid ounces. Interestingly enough, this is a little less than the shot glass of solvent a lot of French polishers use to wet the fad.

Now consider that all the solvent you use while French polishing is going to end up in the air. Also consider that for most residential and commercial buildings the air completely turns over about every 0.5 to 4 hours just by natural convention and leakage. Considering the worst case (always consider the worst case) you could use about 1.3 ounces of alcohol and shellac (which is mostly alcohol) in a closed room of this size every four hours and be under the OHSA PEL.

That's in theory, but in practice things get complicated real fast. Consider you enter a sealed room with a capped jar containing a cotton ball soaked in alcohol. You take the cotton ball out of the jar and put it on a table in the center of the room. If you were to measure the concentration of alcohol in the air near a wall right away, you'd find there was no alcohol there. A measurement of the concentration of alcohol in the air right on the surface of the cotton ball would be very high. And the concentration in the rest of the room would vary by location and would change over time. Until all the alcohol was evenly dispersed throughout the room though, the concentration would always be higher the closer you got to the source of the alcohol.

The problem is that in a real life scenario there are so many variables that affect alcohol concentration in any given place in the room that it is pretty much impossible to accurately model this. Someone that is French polishing spends a lot of time in close contact with the source of alcohol, where the concentration in the air is high. All this points to the need for added ventilation in the workroom. Ideally that ventilation would move the air in a direction away from the person doing the polishing. If adequate ventilation can be achieved this way, great. If not, you may want to consider using a mask rated for alcohol. The bigger the room the better. The great outdoors is the biggest room of all.

How can you tell if you have adequate ventilation? Industrial users make use of chemical concentration testing services and devices to be sure they are compliant with the PELs. But this route is prohibitively expensive for the small shop. If you are using ethanol as a solvent, the sniff test works pretty good. If you can smell the ethanol, you need more ventilation. This works because the odor threshold for ethanol is 1000 ppm - the same concentration as the OHSA PEL for it. Unfortunately this test doesn't work out for all components of denatured alcohol. For example, the odor threshold for methanol is around 4 ppm, way lower than its PEL value of 200 ppm.

The PEL data are very useful as indicators of relative toxicity due to long term exposure, but they aren't of much use in determining individual sensitivities and allergies. People can be allergic to just about anything (peanuts, cocobolo, etc.), including ethanol and all of the components of denatured alcohol. If you are allergic to any of these things then your exposure threshold may be way lower than the PEL values. In addition to frank allergies, folks have individual levels of sensitivity to different substances, so if you are sensitive to ethanol or any of the components of denatured alcohol you may need to keep exposure to a lower level than is indicated by the PEL.

Product Components Ethanol (Ethyl Alcohol,
grain alcohol)
Methanol (Methyl Alcohol,
Methylated spirit,
wood alcohol)
Methyl isobutyl
ketone
Ethyl Acetate Isopropanol
(Isopropyl
alcohol)
Naphtha Heptane Isobutanol Acetaldehyde Acetone
OHSA PEL
(TWA, ppm)
1000 200 100 400 400 Not est. 500 100 200 1000
Boiling Point
(deg. F)
172.6 148.4 243.5 170.6 226.4 226.4
Spectrum ACS 190
proof Ethyl Alcohol
(tax paid)
95
Acros Organics Ethyl alcohol
denatured w/ 5% wood spirit
95.5 2.9 1
Everclear 190 proof 95
Behlen Behkol 70-100 0-0.1 1-10 1-10 0-0.1
Sunnyside denatured alcohol
(note: 93% active ingredients)
85.7 3.6 1.9 1 0.8
Crown denatured alcohol 20-30 65-75 0-10 0-10
Startex denatured alcohol 80-90 0-5 0-10 0-1
Klean Strip XLS denatured alcohol 45-50 45-50 1-4
Klean Strip Green denatured alcohol 90-100 0-10 0-10 0-5

Table 2 - The recipes of two ethanol and seven denatured alcohol products are compared. OSHA PEL values (see text) are provided for each ingredient.



Summary PEL data and concentration data from MSDSs is the only objective data that is available for use in considering safe exposure levels to hazardous substances. Although this data can serve as useful guidelines for the general population, individual sensitivities and allergies may be present and may require significantly lower exposure levels for comfort and possibly also for safety. The difficulty in estimating concentration levels of hazardous substances means that a general rule of thumb for ventilation is this: more is better. Ethanol probably offers no application advantage to denatured alcohol for French polishing and is a lot more expensive. The relative safety of exposure to ethanol and denatured alcohol depends on the recipe for the denatured alcohol, which varies widely from brand to brand. For example, with a worst case derived product PEL for Crown denatured alcohol of 267 ppm, the product offers roughly 1/4 of the permissible exposure level of ethanol. On the other hand, the Klean Strip Green denatured alcohol, with a best case derived product PEL of 1000 is the same as the actual PEL of 1000 for ethanol. All of the substances used as shellac solvents including ethanol are hazardous and all should be treated as such.

Acknowledgments. I am most grateful for the thorough review of a draft of this article by polymer physicist Sjaak Elmendorp, and for concentration calculation information provided by industrial hygienist Ed Hribar. Technical accuracy here can be credited to them; any errors are all mine.

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