The word "denatured" is not very specific. I believe it means that
it refers to the fact that the alcohol is, or has been rendered,
poisonous, therefore not subject to tax as an alcoholic beverage.
(I don't remember the origin of my belief, so treat it as a factoid)
At one time, the shellac thinner I purchased at the hardware store
was methanol (wood alcohol, very poisonous) In recent times, the
shellac thinner is a mixture of methanol, ethanol, and acetone. The
hardware guy tells me this is because methanol is used as a
precursor to methamphetamine. While methanol is not illegal to
possess, they have been asked to inform the police if anyone is
attempting to purchase quantities of methanol.
Based on the properties I saw listed in my Merck Index, Tenth
Edition, I believe that boric acid would be more soluble in methanol
than the current mixtures of alcohol available: "One gram dissolves
in...4 ml boiling water...6 ml boiling alcohol...soly increased by
HCL, citric or tartaric acids." I base my belief on the listed
solubilities increasing with the polarity of the solvent. Since
methanol is more polar that ethanol, the solubility should be
greater in methanol. With regard to the original question, I would
expect a methanol solution to give a slightly better boric acid
coating than ethanol. However, how do you know that "denatured" at
the pharmacy means that the alcohol is really methanol? Like many
other respondants to your question, I use the hardware store stuff.
One of these days, I plan to try out the methanol that Rio Grande
Jeweler's supply sells. I think 5 gallons costs about $50.00
For people using alcohol as a flame coolant for their water torches,
there is a difference between the various alcohols: methanol,
isopropanol, and ethanol. Each produces a flame at a different
temperature. Source of Advertising material from the
Spirig company. (http://www.spirig.com/) Their chart is not on
their web site but in a hardcopy document entitled OPERCOSTS04.NTE
and lists the following for their brand of torch:
Hydrogen-Oxygen: 3300 C. or 5972 F.
H-O plus methanol 2700 C. or 4892 F. isopropanol 2500 C or 4532 F
H-O plus isopropanol 2500 C. or 4532 F.
H-O plus methyl ethyl ketone (methyl-ethanone or MEK) 1700 C. or 3092 F
H-O plus acetone 1500 C or 2732 F.
Someday, when I can afford to buy one of their torches, I plan to
abandon the hardware store brew. As you can see, the mixture makes
a real difference in flame properties.