Unless something new has come up when I wasn't looking, the studies trying to date
the patina on glass have proved relatively inconclusive or unhelpful.  Some people
like to leave patina, dirt, or anything else on glass in case new techniques arise that
may be able to analyze the substances in a way not available to us.  I personally
leave residue that adheres to the insides of bottles so it can be analyzed, but I like
working with clean surfaces whenever possible.  However, I mostly follow the plan of
whomever is paying me.

A few years ago, I did some studies on solarizing glass (some people consider
changing the glass color artificially as destructive; some do not).  An interesting bi-
product of exposing glass to concentrated ultraviolet radiation is that almost all of the
patina falls off.  Bottle collectors use a special acid (which actually works as a
cleanser, if you follow the instructions -- I tried it) or tumbling, but these are not
generally accepted in archaeology as far as I know.

Most patina comes off of glass surfaces with copper wool without damaging the
surface (maybe microscopically), but I would again check with the powers-that-be in
your operation to see whether they feel that is destructive.  There is a great deal of
leaway in what is condsidered destructive by different groups and individuals.  Aside
from the labeling issue, patinas sometimes hide diagnostic characteristics or disguise
color.  I have had glass shards (I like shards with glass, as opposed to sherds with
pottery) with patinas so thick that they were originally misclassified as pottery.  We
could only determine color by flaking a tiny piece (yeah, I still have my knapping tools
from prehistoric experiments).  In some cases, it makes embossing very difficult to
read as well.

A word of caution in cleaning.  Occasionally, under just the right conditions, a paper
label deteriorates but leaves the ink still lightly adhered to the bottle's surface.  These
are extremely delicate and will wash off with ANY appliction of water.  Usually
cleaning them with a light brush even destroys any legibility.  If you suspect a bottle
to have such a condition, photograph it carefully before attempting any cleaning at
all.  Paper labels also survive occasionally in archaeological settings (some at least
as old as the 1870s) and are often very diagnostic.  I have found that cleaning dirt off
with dental pics (one layer at at time) works well in most cases.

The key to me is looking at what helps with diagnosis.  Bottles or fragments can be
stored in a labeled bag if the ink will not adhere due to the patina.

Good luck,

Bill



> bloom.....sick glass...whatever you call it (but what IS the preferred
> term?)...
>
> Just wondering (as I'm perpetually known to do) if any of you clean
> /remove it in some way?  Some of our bottles (from the 1870-80s - Old
> Town San Diego) have the iridescent "fairy dust" stuff that flakes off
> (and goes everywhere) just in handling....but not in great
> quantity...leaving the majority on the bottle...which interferes with
> writing a label on the glass.  A few of our pieces are heavily
> "patinated"...with a thicker yellowish layer that scrapes off somewhat
> (I discovered accidently).  But...do most of you analysts bother to
> clean glass artifacts?  Is this Bad to do?  Can chemicals be used to
> remove it, in a bath?  or does that damage the surface permanently?
> Just wondering.  We have not removed it...but the thicker stuff can be
> annoying, especially when each fragment needs to be labeled and there
> are no unpatinated surfaces to write on...that arent subject to
> flaking off.
>
> Gee...I got to use my more typical prehistoric terms like patinated
> and flaking in this post...ha ha.
>
> Carol - the historically struggling Lab Director (or 'Goddess', as I'm
> known here)
>

Bill Lockhart
New Mexico State University
Alamogordo, NM
(505) 439-3732