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So you say your ancient bronzes have a fungus and you don't know what to do about it? .
*******************
From: Charlie Karukstis
Regarding treatment of coins with bronze disease, I'd like to suggest
a
process that should be accessible to just about everyone. It's in
the
middle in terms of effectiveness, length of treatment, and
difficulty.
Place the coins in a glass container and fill with a 5% solution of
sodium
sesquicarbonate. Let them soak for about fourteen days, replace the
solution, and soak for another fourteen days. Then, place the coin
in
distilled (or deionized, it truly doesn't matter) water for about a
week.
Stabilize with Renaissance Wax or Incralac.
OK, now the notes on this treatment:
(1) Don't have sodium sesquicarbonate lying around? You can make it
with
equal molar amounts of sodium carbonate (Na2CO3 - also called soda
ash) and
sodium bicarbonate (NaHCO3 - yes, you know what that is). For
example, a
5% solution would then be 10.6g of carbonate and 8.4g of bicarbonate
in
100ml of water.
(2) a 5% solution WILL REMOVE any "patina" on the coin! If there is
an
exceptionally aesthetic "patina" to be preserved, try a 1% or 2%
solution.
Be warned, though, that it will take three times as long, and has a
higher
risk of being ineffective.
(3) I was quoted out of context in the recent Celator article
regarding
distilled/deionized water. Yes, it can remove chloride ions from
bronze
(the cause of the "bronze disease"), but in an excruciatingly long
period
of time. The rinse is meant to remove any sesquicarbonate and
byproducts
after treatment.
(4) An optional step before dealing the coin is to repatinate it; we
can
start a separate thread/tirade about that one. Suffice to say, I
don't
like converting a coin back to basic bronze unless it's necessary to
save
the coin.
(5) Incralac is a lacquer containing benzotriazole (a chelate which
will
inhibit corrosion). Benzotriazole is a carcinogen (there - you're
warned),
and Incralac is difficult to use properly (I airbrush a thinned
solution -
very tricky to get the proper surface). Renaissance Wax is a very
decent
microcrystalline wax that, applied properly, will act as an effective
sealant. Obviously, if you don't get all the chloride ions out, you
don't
want to be doing this.
(6) The container must not have air in it - air will break down the
solution prematurely.
(7) Multiple coins are OK unless one has an inordinate amount of
chlorides
present. My trick to separate them? I use those nylon thingies
electricians use to strap wires together. Pull it around the coin
and snip
off the end - the coins won't touch each other (a bad thing). Plus,
they're
inert.
I think this is a much better approach than any mechanical method,
which
probably won't remove all the chlorides, or heating the coin, which
simply
removes water necessary for the reactions to take place - which will
reoccur shortly thereafter anyway (especially in Houston!). This
process
is not that difficult to do (honestly!), and is actually a very
powerful
treatment available to us mere mortals.
*****************************
From: Robert Kokotailo
The bronze disease question comes up from time to time, and about two
years
ago was handled in depth on Numism-l.
What came out of the Numism-l discussions was what I believe to have
been a
fairly good discussion of the cause and solution to the problem
(mostly
because of David Hendin's postings). The current discussion of it on
this
list show that many of the same mis-conceptions about it are still
around.
Unfortuntatly, I did not save all of it and have to work from memory.
There is a Numism-l archieve that you can consult to find the
original
postings.
First it is important to understand that bronze disease is caused by
chloride ions in the bronze, and no amount of heating or coating of
surfaces will stop the problem for any length of time. The chlorides
require moisture to to react with the copper so heating will, by
driving
out the moisture, temporarily stops the problem and coatings may stop
it
for a longer period, but it will return. Coating the coin with
anything
will just lock in the chlorides, and probably make the problem worse
in the
long term.
There is no need to keep coins apart (other then from direct
contact), as
the chlorides do not jump from one coin to another. This practice
was
started some time ago because it was once thought that bronze disease
was a
bacterial infection that could spread from coin to coin. This was
proven
to be incorrect long ago when coins were heated to very high
temperatures
(to kill all bacteria) with no long term results.
Paul got to the heart of the matter when he wrote :
>Regarding bronze disease;
>
>First soak the coin in deionized water (critical--not tap or other),
changing
>it every few days.
>
>Do so with at least a quart of water.
>
>Second, dry with hair drier.
>
>Third, watch to see if powder returns.
>
>If it does repeat.
This works because the chlorides are water soluble and over a number
of
days or weeks of soaking in deionized water, they will eventually
come out
of the metal. Note that it is critical to use DEIONIZED WATER as
other
sources of water many introduce Chloride ions. Unfortunately, if
your
coins have been coated with anything (ie. wax, nail polish etc), it
will
have be removed for this to work.
DO NOT WAX OR OTHERWISE COAT YOUR COINS AFTER TREATMENT. If the
treatment
was successful in removing the chlorides, the problem is gone (in
which
case the wax will not actually hurt), but if the treatment was not
successful and some chloride ions remain, the problem may return and
any
coatings applied sill make additional treatments much more difficult
and
probably less effective.
******************
From: "Dave Welsh"
----- Original Message -----
From:
To:
Sent: Thursday, July 01, 1999 4:11 PM
Subject: Re: [Moneta-L] Bronze Disease
> From: AngShan@aol.com
>
> In a message dated 7/1/99 5:45:01 PM Central Daylight Time,
> pauler10@ix.netcom.com writes:
>
> > Second, to add to the discussion, I am pretty sure the second
coin does
> > indeed have bronze disease. It didn't have this a few months
ago. What
do
> I
> > do? I know this has been discussed before but I didn't pay
attention to
it
> > at the time. Help me save this coin!
>
> Well...I'll tell you my procedure for curing bronze disease;
For coins in the beginning of bronze disease infection (ie; light
infection);
> 1) I put the coin in a jar of olive oil, let it soak for a week.
This is completely useless.
> 2) Scrub it very good with a medium bristle toothbrush under warm
water.
> 3) Wash off the remaining oil residue with a bath of tri-sodium
phosphate.
Ditto.
> 4) Wash that off and bake them in the oven. 150 degrees for AR's,
250
degrees
> for AE's.
This will help. See below for why.
> Deeply pitted bronze disease;
> 1) Well, I use a rotary nail buffer with a small emory bit to drill
out
the
> diseased pits.
Some form of mechanixcal removal of the corrosion products is indeed
necessary.
> Then I perform the steps listed above for light infestation.
Only one of which will be effective.
> One thing that I DON'T do is coat them with nail polish or a wax!
Absolutely right. Don't seal a coin with bronze disease until you're
certain
it is stabilized. Sealing isn't necessarily a bad idea, but not in
this
case!
THE CAUSE OF BRONZE DISEASE:
This is now understood to be a complex chemical process depending
upon the
presence of free or available chloride ions and moisture. The
chloride ions
combine with bronze constituents to create chlorides and other salts
which
then break down in the presence of moisture to liberate chloride ions
and
continue the corrosion. An aggressive case of bronze disease can ruin
a coin
in less than a month.
ITS PREVENTION:
Keep coins in a very dry environment. Bronze disease cannot propagate
without humidity.
Stabilize the surface of coins after cleaning and/or conservation,
so as to
remove all chloride ions.
Do not store coins in any material that contains chlorine as a
constituent -- PVC for example.
ITS CURE:
Bake the coin at 250 degrees F for enough time to thoroughly dry it
out.
Remove the choride ions by excavation and/or dissolution in deionized
water.
Dry the coin thoroughly and bake again to remove all moisture.
Soak in deionized water again.
Seal the coin or let it stabilize at normal ambient humidity.
Another way to get rid of surface chloride ions is to immerse the
coin in a
10% solution of sulfuric acid. This is a very fast process and
requires
skill, but if done properly will get rid of surface chloride ions
without
totally removing the patina. This is the only approach I know that
works
when the coins's whole patina is polluted by chloride ions (malignant
patina). Have a DI water bath handy to rinse the acid off when the
desired
effect is reached.
*****************************
From: Charlie Karukstis
Just a few observations from our discussions of last night:
(1) While de-ionized water is not distilled water, for our purposes
they
are interchangeable. What is critical here is the so-called "common
ion
effect". If you're looking to remove chloride ions from a coin, and
the
water you're using is already saturated with chloride ions, then
you're not
going to remove many ions (and theoretically could transfer more to
the
coin). Unless you're using Russian tap water, this is not likely to
be a
problem. Distilled water is perfectly fine for our needs (at least
the
distilled water available in the US is)
(2) While boing water will speed up the process of chloride removal,
unless
you're prepared to boil the coin for a week this will not matter.
Chloride
removal using water only is a very lengthy process. If I can ever
find
some of Ian MacLeod's articles from Studies in Conservation from
years
back, I could post the actual time differences for the various
processes
being discussed (water, sesquicarbonate, dithionite, citric acid,
etc). My
point here is that while distilled/de-ionized water is a good thing,
it is
not a panacea for "bronze disease".
(3) Whoever said that sulfuric acid won't remove "patina" (oxides,
etc.)
from a coin? Not in my neck of the woods, at least. Perhaps in very
small
concentrations, perhaps, but at that point I'm not sure why one would
utilize it. A lot of the bulk coins coming from source countries
that have
been initially "cleaned" have been done using battery acid - suffice
to say
if you see what these coins looks like it's a substance to use
carefully on
bronze alloys.
(4) Mechanical action on a coin is the preferred method for removal
of
encrustation (i.e. a scalpel under a microscope), but it's virtually
impossible to successfully remove chloride ions this way. A chemical
process is the only effective method I know of that will remove all
traces
of chloride ions.
************************************
From: Robehrlich@aol.com
I have no experience in cleaning or preserving coins but pass on some
information gleaned from a book I came across in a course on Greek
bronze
sculpture.
An interesting article by Arthur Beale, from the conservation
laboratory at
the Fogg Museum of Harvard, is in the book, "The Fires of Hephaestos"
by
Carol Mattusch (Harvard University Art Museums, 1996). He describes
the
development of techniques for dealing with bronze sculptures as they
evolved
in last century. While some of the concerns are not relevant for
coins, like
the need for solder repairs, care to not change the metal composition
or
preservation of deliberate, ancient patination, others are relevant.
The
major initial corrosion product in bronze sculptures, particularly
those that
spent 2000 years deep in unxygented salt water, is cuprous chloride.
This
itself is not threatening. When it comes in contact with air above
46%
relative humidity the copper oxidizes to the cupric form which is
light
green powder. The problem is compounded when the corrosion layer is
overlaid
by copper carbonates.
He goes on to mention the techniques of prolonged extraction with
distilled
water followed by mechanical removal or superheated steam treatment.
The use
of sodium sesquicarbonate was discovered by an English chemist
working in
Egypt in the 30's. Both these techniques may take months.
The treatment that is most accepted appears to be use of a complexing
agent,
benzotriazole, sometimes in conjunction with sodium carbonate. An
unpublished method is use of silver nitrate as discussed in this
group but it
apparently has not been adopted. Two references in the journal
Studies in
Conservation (vol. 32, 25-40,1987m and vol 40, 110-119, 1993) use
sodium
dithionite (used in photography) and powdered zinc respectively. I
have not
looked at these references. The drawbacks to dithionite as well as
sodium
sesquicarbonate are changes in the color of the patination.
Besides the problem of "bronze disease" associated with chlorides,
the
article touches on the dangers of sulfur and organic acids. Mahogany
cases
might not be a good idea as they could give off organic acids (i.e.
tannic
acid) unless the wood is thoroughly sealed. Any sealed container
could be
troublesome if the seals depend upon rubber gaskets which contain
sulfur.
The recommended storage is in powder coated steel cabinets. Coatings
such as
silicone have been found to be somewhat effective in preventing
sulfur damage
but less effective against bronze disease.
Another reference that someone who has access to it might look at is
"Ancient
and Historic Metals: Conservation and Scientific Research" published
by the
Getty Conservation Institute in 1994.
*****************************
From: Charlie Karukstis
Rob has some excellent information in his post. To amplify some of
his
comments:
(1) I currently am of the opinion that the sodium dithionite
treatment is
the most effective for (specifically) removing chloride compounds
from
bronze alloy coinage. I have used this process for nine years, and
have
had excellent results. Why did I not suggest it to the group in our
earlier discussions? Because the trend of the thread was towards
"accessible" treatments for "bronze disease", and unless you have the
proper facilities for the dithionite process, it is something I
cannot
recommend to all numismatists. That is why I suggested the
sesquicarbonate
treatment (a fair balance between accessibility, effectiveness and
length
of treatment, as I mentioned in my post).
(2) If you have access to the journal ,
there have
been some excellent articles by Ian MacLeod (of the Western Australia
Museum, I believe) concerning the relative merits of available
treatments
for chlorides in bronze alloys. The Getty book (Ancient and Historic
Metals) is geared more towards metallurgical issues, and is less
helpful
for conservation issues currently under discussion.
(3) As Rob mentioned, chlorides are not always found in isolation on
coins.
More generally, they will indeed be found in conjunction with
carbonate
compounds and even copper acetate (verdigris). It's not always easy
to
definitively identify the presence of chlorides. I will simply add
from
empirical experience that they are present (but usually stable, such
as in
the cuprous form) on more coins than we might wish.
(4) Benzotriazole treatments are tricky to administer effectively.
As it
is also a carcinogen, this might not be the best approach for your
rainy-day conservation sessions. Incralac (the lacquer containing
benzotriazole that I mentioned earlier) is an excellent sealant with
the
benzotriazole acting as a chelate, but is not suited to removing
chlorides.
(5) Rob correctly notes that benzotriazole and sesquicarbonate
treatments
will alter the "patina" on a coin (actually. I'll go further than
that -
they will REMOVE the "patina" from a coin). Actually, any process
short of
distilled water will alter the complex layer of oxides, sulfides,
carbonates, etc. on the surface of any bronze alloy coin - removing
chlorides is a bit like chemotherapy: you can save the patient, but
the
patient will go through some changes.
(6) I have always been somewhat suspicious of wood cabinets for
storage of
coins, precisely because of the possible emittance of various complex
organic acids such as Rob mentioned. I really don't know much about
them,
and am still fascinated by them because of their elegance and
utility.
Would Paul, or Art Rubino (who sells these things) or anyone else
care to
discuss some of these issues with us?
***********************************
--------------------------
From: Jim Gossett
Rose Marie,
Yes, there really is something called "bronze disease." Obviously,
it's
not a disease in the way one usually thinks of "disease." It's a
catalytic
process through which copper (a constituent of bronze) is turned into
a
fuzzy, green, soft mess. The coin is literally eaten away. It's
catalytic, in that chloride ion, oxygen, and water initiate the
process,
but chloride (in the form of hydrochloric acid) is released again
along the
way, free to react with more of the coin's copper.... released
again...and
on and on. The fuzzy green stuff is a copper-hydroxide-chloride
compound.
It differs from the "good", hard green and blue-green coatings --
like
malachite and azurite (hydroxy-carbonate compounds of copper).
Bronze disease is treated by removing the fuzzy matter, and then
treating
to remove chlorides and water from the porous structure of the coin.
Protective coatings are often applied thereafter to keep water from
the
surface.
***************************************
From: "Charlie Karukstis"
Rose Marie:
The difference is that verdigris is copper acetate, while "bronze
disease"
is generally applied to cuprous chloride present in the flan of a
coin.
While copper acetate is in general fairly stable, cuprous chloride is
anything but. It is quite unstable and active: in the presence of
water and
oxygen, cuprous chloride will convert to cupric chloride, while
producing
hydrochloric acid as a by-product; the HCL further attacks the flan,
and the
process continues.
Cuprous chloride can be empirically identified (usually) by its
powdery
nature (because of its instability, it generally will not consolidate
like
copper acetate). While it tends to be of a lighter shade of green
than
copper acetate, don't use this as a hard and fast rule.
There is not a topic in numismatic conservation that raises disputes
like
the causes and treatments of "bronze disease", and there is a lot of
apocryphal information being disseminated (at least among amateurs).
There
is nothing, for example, of microbiologic process involved, and yet
myths
persist about treatment of the coin simply by heating the flan.
While this
will temporarily remove the water available for reaction, it is not a
permanent solution. Likewise, mechanical removal of the chlorides
cannot be
successful owing to the usual porosity of ancient flans.
The current best treatment for chloride removal, in my opinion, is
the use
of sodium dithionite, but as this is not a chemical for casual use, I
generally recommend the use of sodium sesquicarbonate. If you can't
obtain
this directly, you can make it using sodium carbonate and sodium
bicarbonate; we discussed this on this or another list a few months
ago -
email me for details. The usual admonishments about stabilization of
a flan
after treatment apply.
**********************************
From: "S. Kazmi"
Hi Rose Mary,
Yes, the dreaded "bronze disease" is for real!
It differs from the desirable "patina" as when gently scratched with
a
wooden toothpick it results in a powdery residue. One of the simplest
methods to remove the bronze disease is to gently remove as much of
the
powdery stuff as possible, and then soak the coin in distilled water
for
at least three weeks (changing water every two days). Then "baking"
the
coin at 325 degrees for 10 minutes to remove all moisture and finally
sealing it with a matte finish "hobby" lacquer or Renaissance wax.
As this list seems the have a few "nuclear physicists" as members, I
am
sure some one can give a more detailed reply.
****************************************
From: "Charlie Karukstis"
One comment on the distilled water thread at the CNG site - while it
will
eventually remove most chlorides from the coin (through common ion
exchange), it will take a very long time, and require quite a few
changes of
water. If the coin has active chlorides, the sodium sesquicarbonate
process
I mentioned a few months ago might be a better approach, and is not
difficult.
I agree with Ken on the importance of sealing a <> coin; I
use
either Renaissance Wax or Incralac, depending. One caveat to
remember is
that if there are, for example, active chloride ions remaining in the
flan
after sealing, the coin might very well continue to be attacked -
it's
important, therefore, to stabilize (through treatment and rinsing) a
coin
before sealing. Another risk is if an small area of the flan is not
sealed,
it could possibly function as an anode.
One other thing - a dehydrator is just fine, but I almost always dry
a coin
using acetone. No effect on the surface (unless of course waxes,
paint or
the like are present) and much quicker.
*************************************
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