this is a letter written to Road & Track magazine, Technical
Correspondence column, for the July 1996 issue:

White Stuff
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I used to think the white stuff that plugs up radiator tubes came from
impure water, but after using only distilled water and ethylene glycol
and still getting plugged tubes, I called a radiator shop. A spokesman
called it "lead bloom."  He didn't seem to know where it came from,
though.  My older cars and tractors don't seem to have this problem.
Could it be from the alumninum used in newer motors?  I can't find
anybody who knows.  And what can be done to minimize its formation?
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Mike McGuire at Unigard was also unfamiliar with the term "lead bloom"
but gave several clues as to the source of your "white stuff." Running
straight water or old, worn-out antifreeze or antifreeze diluted by
topping off with straight water corrodes aluminum and yields a pasty
white goop.  Mike pointed out that the agents in antifreeze that protect
against aluminum corrosion -- silicates -- wear out, and replacing the
antifreeze every 30,000 miles or two years is the only cure. So, while
you may have freezing protection if your coolant is old, it won't
protect against aluminum corrosion.

Another source of white contaminants is silica dropout. This occurs in
all-iron engines running high-silicate antifreeze, where there is no
aluminum for the silicates to chemically work with.  If you have an iron
engine, inspect the antifreeze label for a phrase about compatibility
with heavy-duty engines.  It is a good sign that antifreeze is a
low-silicate blend.  This is because the long-haul engines from Cummins,
Detroit Diesel and others are always all-iron, and require a low-silicat
antifreeze.

Just for the record, distilled water can cause its own problems because
it has such a low pH.  Water will seek its own balanced pH, and when
distilled water is placed next to aluminum, it leaches the minerals it
wants from the alloy and black soot forms in the coolant.  That's why
silicates are put in aluminum-compatible antifreeze; they're
sacrificial mineral deposits.