Sky & Telescope - January 2020 - 74

ASTRONOMER'S WORKBENCH by Jerry Oltion

An idea whose time has come . . . again.
WHEN SKY & TELESCOPE Contributing Editor Howard Banich decided it
was time to recoat the mirror in his
28-inch telescope, he received a rude
shock: It would cost about $2,000 to
have it aluminized. Connecticut ATM
Zane Landers faced a different problem:
His local coater moved away, and he
was reluctant to subject his pride and
joy to the vagaries of the mail.
Both of them arrived at the same
conclusion: Silver-coat the mirrors
themselves.
Silver-coating used to be the only way
it was done. But silvering was a ﬁnicky
chemical process, and when aluminizing in a vacuum chamber came along,
the convenience of having someone else
do the coating, and more importantly
the increased durability of the coating,
quickly supplanted home silvering.
When you're looking at a cool two
grand for a coating, though, the equation quickly becomes weighted toward
economy. Plus, silver has one distinct
advantage: It reﬂects 98% of the light
that hits it, as opposed to aluminum's
89%. Howard and Zane and dozens
more like them have begun to reevaluate silver as a coating of choice, and

after some initial setbacks they have had
some resounding success. You can too!
Silvering a mirror is a multi-part process, but each step is pretty simple. First
you strip off the old coating, then you
clean the mirror. Next, you sensitize
it with a tin-based solution and ﬁnally
coat it with a two-part silver solution,
using two sprayers simultaneously.
Cleaning is by far the most difﬁcult
part, but it's not all that hard. You just
have to be painstaking. You must get
every bit of oil, dirt, or anything else
off the surface of the glass or the silver
won't stick. Howard starts by scrubbing with talcum powder mixed with a
little distilled water, then rinses that off
with distilled water. Then he uses the
glass cleaner from the silvering kit and
rinses again. He repeats those two steps,
changing into a new pair of disposable gloves before each step. Be sure to
never touch the mirror with bare skin,
because even freshly washed skin is
covered with oils.
The most common mistake is to not
clean the sides of the mirror enough.
Oils from your ﬁngers as you were
handling the mirror will work their way
back onto the surface as you rinse off

q Left: The mirror must be squeaky clean before coating it. Middle: The twopart silvering solution is applied simultaneously using two sprayers. Right: With
a little practice, you can achieve a near-perfect silver coating.

JANUARY 2 02 0 * SK Y & TELESCOPE

CLE A NING MIRROR A ND NE A R PERFECT COATING: HOWA RD BA NICH (2);
SPR AYING SILV ER: JERRY OLTION

Silvering Mirrors

the top. You want the surface and the
sides to literally be squeaky clean, and
water should sheet off it rather than
bead up when you rinse it.
Once you've got the mirror absolutely clean, you sensitize it with the tin
solution. This goes on as a spray. You
then spray on the two-part silvering
solution. Within seconds of beginning
the latter, the silver coating starts to
form. If all goes as planned, you'll get a
smooth, even silver coating.
Your ﬁrst few attempts will probably not go well. It's hard to believe how
much you need to clean the glass in
order to get a perfect coating. But it's
easy to remove the silver and try again,
and each coating costs probably $10-$20
in chemicals. Practice makes perfect.
The expired chemicals aren't particularly dangerous, but you do need to
collect and dispose of them properly.
Fortunately, the same company that
sells home silvering kits (Angel Gilding,
angelgilding.com) also sells a claybased cleanup and disposal kit that's
safe and easy to use.
There's one last problem with silver:
It tarnishes. You can only get a year or
so out of an unprotected silver coating
before it needs to be recoated. As cheap
as the chemicals are, that's no big deal,
but an anti-tarnish cloth made for
silverware draped over the mirror will
extend the life a bit. Even better, the
Oregon ScopeWerks, a group of ATMs
that Howard and I belong to, has been
experimenting with protective coatings

http://www.angelgilding.com

Sky & Telescope - January 2020

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