The C2 wiki presents only a very abbreviated version, and the chapter in question actually tells a whole bunch of stories. Here’s a reasonably full excerpt:
Old man Cometto added
that life is full of customs whose roots can no longer be traced:
the color of sugar paper, the buttoning from different sides for
men and women, the shape of a gondola’s prow, and the
innumerable alimentary compatibilities and incompatibilities, of
which in fact the one in question was a
particular case: but in any event, why were pig’s feet obligatory
with lentils, and cheese on macaroni.
I made a rapid mental review to be sure that none of those
present had as yet heard it, then I started to tell the story of the
onion in the boiled linseed oil. This, in fact, was a dining room
for a company of varnish manufacturers, and it is well known
that boiled linseed oil has for many centuries constituted the
fundamental raw material of our art. It is an ancient art and
therefore noble: its most remote testimony is in Genesis 6:14,
where it is told how, in conformity with a precise specification
of the Almighty, Noah coated (probably with a brush) the Ark’s
interior and exterior with melted pitch. But it is also a subtly
fraudulent art, like that which aims at concealing the substratum
by conferring on it the color and appearance of what it is not:
from this point of view it is related to cosmetics and adornment,
which are equally ambiguous and almost equally ancient arts
(Isaiah 3:16). Given therefore its pluri-millennial origins, it is not
so strange that the trade of manufacturing varnishes retains in
its crannies (despite the innumerable solicitations it modernly
receives from kindred techniques) rudiments of customs and
procedures abandoned for a long time now.
So, returning to boiled linseed oil, I told my companions at
table that in a prescription book published about 1942 I had found
the advice to introduce into the oil, toward the end of the
boiling, two slices of onion, without any comment on the purpose
of this curious additive. I had spoken about it in 1949 with
Signor Giacomasso Olindo, my predecessor and teacher, who was
then more than seventy and had been making varnishes for fifty
years, and he, smiling benevolently behind his thick white
mustache, had explained to me that in actual fact, when he was
young and boiled the oil personally, thermometers had not yet
come into use: one judged the temperature of the batch by
observing the smoke, or spitting into it, or, more efficiently,
immersing a slice of onion in the oil on the point of a skewer;
when the onion began to fry, the boiling was finished.
Evidently, with the passing of the years, what had been a crude
measuring operation had lost its significance and was transformed into a mysterious and magical practice.
Old Cometto told of an analogous episode. Not without
nostalgia he recalled his good old times, the times of copal gum:
he told how once boiled linseed oil was combined with these
legendary resins to make fabulously durable and gleaming varnishes. Their fame and name survive now only in the locution
“copal shoes,” which alludes precisely to a varnish for leather at
one time very widespread that has been out of fashion for at least
the last half century. Today the locution itself is almost extinct.
Copals were imported by the British from the most distant and
savage countries, and bore their names, which in fact
distinguished one kind from another: copal of Madagascar or
Sierra Leone or Kauri (whose deposits, let it be said parenthetically, were exhausted along about 1967), and the very well known
and noble Congo copal. They are fossil resins of vegetable
origin, with a rather high melting point, and in the state in
which they are found and sold in commerce are insoluble in oil:
to render them soluble and compatible they were subjected to a
violent, semi-destructive boiling, in the course of which their
acidity diminished (they decarboxylated) and also the melting
point was lowered. The operation was carried out in a semi industrial manner by direct fire in modest, mobile kettles of four
or six hundred pounds; during the boiling they were weighed at
intervals, and when the resin had lost 16 percent of its weight in
smoke, water vapor, and carbon dioxide, the solubility in oil was
judged to have been reached. Along about 1940, the archaic copals,
expensive and difficult to supply during the war, were supplanted
by phenolic and maleic resins, both suitably modified, which,
besides costing less, were directly compatible with the oils. Very
well: Cometto told us how, in a factory whose name shall not be
uttered, until 1953 a phenolic resin, which look the place of the
Congo copal in a formula, was treated exactly like copal itself—
that is, by consuming 16 percent of it on the fire, amid
pestilential phenolic exhalations—until it had leached that
solubility in oil which the resin already possessed.
Here at this point I remembered that all languages are full of
images and metaphors whose origin is being lost, together with
the art from which they were drawn: horsemanship having
declined to the level of an expensive sport, such expressions as
“belly to the ground” and “taking the bit in one’s teeth” are
unintelligible and sound odd; since mills with superimposed stones
have disappeared, which were also called millstones, and in
which for centuries wheat (and varnishes) were ground, such a
phrase as “to eat like four millstones” sounds odd and even
mysterious today. In the same way, since Nature too is conservative, we carry in our coccyx what remains of a vanished tail.
Bruni told us about an episode in which he himself had been
involved, and as he told the story, I felt myself invaded by sweet
and tenuous sensations which later I will try to explain. I must
say first of all that Bruni worked from 1955 to 1965 in a large
factory on the shores of a lake, the same one in which I had learned
the rudiments of the varnish-making trade during the years 1946–47. So he told us that, when he was down there in charge of the
Synthetic Varnishes Department, there fell into his hands a
formula of a chromate-based anti-rust paint that contained an
absurd component: nothing less than ammonium chloride, the old,
alchemical sal ammoniac of the temple of Ammon, much more apt
to corrode iron than preserve it from rust. He had asked his
superiors and the veterans in the department about it: surprised and
a bit shocked, they had replied that in that formulation, which
corresponded to at least twenty or thirty tons of the product a
month and had been in force for at least ten years, that salt “had
always been in it,” and that he had his nerve, so young in years
and new on the job, criticizing the factory’s experience, and
looking for trouble by asking silly hows and whys. If ammonium
chloride was in the formula, it was evident that it had some sort of
use. What use it had nobody any longer knew, but one should be
very careful about taking it out because “one never knows.” Bruni is
a rationalist, and he took all this very badly, but he is a prudent
man, and so he accepted the advice, according to which in that
formulation and in that lakeshore factory, unless there have
been further developments, ammonium chloride is still being
put in; and yet today it is completely useless, as I can state from
firsthand experience because it was I who introduced it into
the formula.
The rest of the chapter is Levi describing how he had been assigned the task of investigating why a whole pyramid of paint mix was screwed up, when the paperwork looked all correct. It turns out that instructions for ‘2 or 3 drops’ had gotten dirty and started to look like ’23 drops’, both ruining the paint and fooling the chemical tests; he discovers a clever way to fix the stockpile, which uses the ammonium chloride to neutralize the responsible chemical and which is added as a preventive; except a decade later, they stopped using the responsible chemical at all; thus, then the ammonium chloride kept being added entirely unnecessarily:
Since the storeroom contained several shipments of perilously
basic chromate, which must also be utilized because they had
been accepted by the inspection and could not be returned to
the supplier, the chloride was officially introduced as an anti-livering preventive in the formula of that varnish. Then I quit my
job: ten years went by, the postwar years were over, the
deleterious, too basic chromates disappeared from the market,
and my report went the way of all flesh: but formulas are as holy
as prayers, decree-laws, and dead languages, and not an iota in
them can be changed. And so my ammonium chloride, the twin
of a happy love and a liberating book, by now completely useless
and probably a bit harmful, is religiously ground into the chromate
anti-rust paint on the shore of that lake, and nobody knows
why anymore.
On the whole, I read Levi’s chapter as indeed anti-Chesterton’s Fence. The legacy mechanisms are often useless, potentially highly wasteful (16% of the resin is wasted in that phenol story, for absolutely no reason), confusing, risk problems down the line (one can easily imagine the onion or ammonium screwing with later changes or ingredients, as Levi notes that it is apt to increase rusting), and can obstruct improvements (if you use the onion unthinkingly, you will be less inclined to get a better way of measuring temperature like a thermometer). Investigation into anomalies or blackboxes may be expensive, and can require deep theory of little apparent practicality, but can pay off big (like rescuing an entire stockpile of paint from futility) and should be done and as much rendered legible as possible. And if one doesn’t wind up able to present a complete history and explanation as a nice little case-study that Levi can write up, because it’s that dumb a thing (the smudged recipe having already been thrown out, say), then oh well, one should throw out the extraneous thing and investigate it that way, with no particular respect for the onion, which deserved no special veneration or status and was exactly as foolish as it looked, our fathers being no wiser, and often less wise (particularly in chemistry or technology), than ourselves.
The C2 wiki presents only a very abbreviated version, and the chapter in question actually tells a whole bunch of stories. Here’s a reasonably full excerpt:
The rest of the chapter is Levi describing how he had been assigned the task of investigating why a whole pyramid of paint mix was screwed up, when the paperwork looked all correct. It turns out that instructions for ‘2 or 3 drops’ had gotten dirty and started to look like ’23 drops’, both ruining the paint and fooling the chemical tests; he discovers a clever way to fix the stockpile, which uses the ammonium chloride to neutralize the responsible chemical and which is added as a preventive; except a decade later, they stopped using the responsible chemical at all; thus, then the ammonium chloride kept being added entirely unnecessarily:
On the whole, I read Levi’s chapter as indeed anti-Chesterton’s Fence. The legacy mechanisms are often useless, potentially highly wasteful (16% of the resin is wasted in that phenol story, for absolutely no reason), confusing, risk problems down the line (one can easily imagine the onion or ammonium screwing with later changes or ingredients, as Levi notes that it is apt to increase rusting), and can obstruct improvements (if you use the onion unthinkingly, you will be less inclined to get a better way of measuring temperature like a thermometer). Investigation into anomalies or blackboxes may be expensive, and can require deep theory of little apparent practicality, but can pay off big (like rescuing an entire stockpile of paint from futility) and should be done and as much rendered legible as possible. And if one doesn’t wind up able to present a complete history and explanation as a nice little case-study that Levi can write up, because it’s that dumb a thing (the smudged recipe having already been thrown out, say), then oh well, one should throw out the extraneous thing and investigate it that way, with no particular respect for the onion, which deserved no special veneration or status and was exactly as foolish as it looked, our fathers being no wiser, and often less wise (particularly in chemistry or technology), than ourselves.