Re: [ATM] Fine scratches during polishing - Cures?

[Woodchuck <djv@bedford.net>]

On Sun, 27 Jun 2004, J Killea wrote:

> Things have been going pretty nicely during polishing of my 8" Pyrex mirror.  Tonight, however,
> somewhere between the 11th and 12th hour of polishing, I:
>
> 1. rinsed the blank in water
> 2. lightly wiped it with a turpentine-dampened paper towel
> 3. removed the turpentine by lightly going over the mirror with a clean cheesecloth.
> 4. finished removing the remaining turpentine film by exhaling on the mirror and lightly wiping
> away the condensation with same cloth.
>
> Reading this, I have the feeling some people are gasping at the risk in this procedure, but that
> CeO seems very tenacious, as is the turpentine redsidue!  In order for my laser polish test to be
> useful, I feel like I have to clean the blank VERY thoroughly.
>
> During the heavy breathing phase of this procedure, I noticed some very fine scratches near the
> center and at about 2" out from the center (25% zone?).  Some of the "scratches" acually went away
> with a bit of light rubbing with the cloth, but several did not.
>
> So I think the questions I need to ask are:
>
> A.  What is the most risk-ridden part of the cleaning process I've described, and

Touching the mirror.  In general, don't touch the mirror.

(1) is harmless and beneficial.

(2) Paper towels are not for optical surfaces.  I would call this
the most obvious possibility for trouble.  Safer would be wipes
intended for this purpose, sold to photographers.  An alternative
is an old bedsheet cut into hankies, and repeatedly washed and
thoroughly rinsed until they are as soft as cotton gets and all
loose lint is gone to the lint trap at the laundromat (don't use
starch or fabric softener).  Or a new piece of chamois carefully
selected and kept in a baggie or tin.  Another item is a big swatch
of absorbent cotton wool, the sterile kind, (not the cotton balls
sold in big bags -- these are dirty and polyester often, and have
perfume), which you make into a big "Q-tip" around a clean dowel
rod and string, or handle with chopsticks, or a pair of tongs made
of tongue depressors and a rubber band.  All these swabs and rags
shouldn't be used with much pressure beyond their own weight.  They
are a last resort.

(3) Same comments.  Instead, next time, hose down the mirror with
a chemist's washbottle and rubbing alcohol or acetone. (Observe
safety precautions).  Use a cotton swab as before, only if necessary.

(4) Same comments.

Chemists clean class by treating it with concentrated nitric acid
(and other stuff, such as conc. sulphuric or sulphuric-dichromate
and similar terrorist brews), and then hosing with distilled water.
If you don't have the proper facilities to handle nitric acid
(assuming it can even be bought in these troubled days), forget
about it.  But the general rule is that you simply do not touch it
with anything.  The problem becomes one of finding the proper solvent
for whatever is the problem.  In this case, CeO and pitch.

If you want to cut pitch residue, and your turpentine is leaving a
residue, (it probably is, and the residue is the pitch you just
dissolved in it -- you are wiping it, not flooding/draining), use
a hosing method.  Flood, drain.  Flood, drain.  Wiping just smears
residue around, or worse, transfers a new problem from the wiper
to the wipee.  Your scratches that come and go may be scratches in
the glass being filled with pitch residue, or scratches in the pitch
residue being redissolved.

Pitch is also soluble in paint thinner (Stoddard solvent) or xylene
(xylol).  These might be less likely to have a high-boiling residue
than turpentine, a natural product.  Xylene stinks and is toxic.
I don't think you need much of a solvent beyond alcohol (from the
paint, not drug, store).

A general scheme to clean glass safely [not cemented or mounted
optics, but glass that you can get at all around] is to wash it
with detergent first (dissolved a bit in water -- distilled water
preferably), without any physical scrubbing if possible, but use
something scratchproof if you have to ... synthetic sponge ("makeup
sponge" for ladies), cotton wool, old clean cotton rag.  Then rinse
repeatedly with a wash bottle.  (A wash bottle is a sort of squirt
gun, you can take an empty detergent bottle, rinse it completely
clean and fill it with distilled water as a good substitute, although
the stream is very heavy from this).  The test for cleanliness is
that the water from the rinse forms a uniform, smooth sheet, with
no droplets.  When this is achieved with distilled water, this sheet
may be allowed to drain and evaporate, and will leave no residue.
If the mirror is clean, and stood on edge to drain, there will be
nearly no water left.  Chemists often follow this with a squirt of
acetone.  Substitute alcohol in the absence of correct ventilation.
Let it drain, then touch the last drop on the edge with the corner
of a cotton rag.

If you trust your tap water, use it.  But follow it with at least
three rinses with distilled water.  Never let anything with dissolved
solids dry on an optical surface.  (Well, if you're still polishing, sure,
but...)

When I said "cotton" before, I meant 100% cotton.  Real linen (nearly
unobtainable) is a substitute.  Stay away from wool or synthetics.
I don't know about silk.

> B.  What is the safest, efficient way to clean up the mirror for a stringent polish test?  (I've
> seen de-ionized water rinses mentioned, but my problem is really with getting the CeO to detach
> from the blank's surface.  Any water is quite polar and I wonder if nonpolar solvents work better
> for CeO-on-glass.)

I think previous remarks cover this.  Finish up with detergent
followed by distilled water.  I harp on distilled water because my
tap water is a nightmare solution of iron, various forms of sulphur,
manganese, calcium, magnesium, sodium, silicates, a bunch of organics
and the occasional solid and a couple of things I have yet to
identify.  (Well water from Pennsylvania coal country -- I don't
drink this filth.).  This stuff will lay a very tenacious coating
a few waves thick on Pyrex, and I suspect it might etch glass, too,
although it's acidic.  (Perhaps there are dissolved fluorides along
with the other junk.)

The surfactant properties of detergent will help with the CeO, I
think.  (Detergent == the dissolved stuff like Joy for dishes, not
laundry powder, which has unknown binders and bulking agents in
it.)

I've given a very stringent set of recommendations, you can relax
them as you see fit.  They're almost stringent enough to cover
cleaning an optically coated surface, except there the rule "do not
touch" cannot be really relaxed, especially for the MgF2 coatings
from the olden tymes, or a young Al reflective coating.

Some say that freshly polished glass is softer, more easily
scratched than old glass.  This may be an old wive's tale, but
the surface of glass is in no way "simple" and is not chemically
inert.

Once you get used to the "don't touch" drill, you'll find yourself
using it even when grinding.  A sink with a sprayer is real handy,
or a dish sprayer over your machine or bench or what-have-you.

Dave ("Former chemist")

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