|
|
|
To clean or not to clean..
as Hamlet said, that is the question.....
To restore and repair is different from conservation.
Personal tastes will prevail.
If in doubt, dont!
unless
it's
possibly
BRONZE DISEASE! 
!!!!!!!!!!!!!!
Here is a pretty comprehensive website about
from underwater sites
However do be alerted as follows:
Comment by Ben W
Although this site has some very useful information about conserving textiles and other materials, their information on conserving bronze is dreadfully out-dated.
Pearson is recommending electrolysis and shellac?! Electrolysis destroys any attractive patina or evidence of surface age, and shellac traps the bronze disease in a pressurized shell, which later has the same effect as if you put a whole egg in the oven - it expands inside the casing and ravages the interior and when the pressure increases to the breaking point, the shell(ac) ruptures and the piece has been destroyed by good intentions. I've seen it many times and it doesn't get any easier to take. It is these techinques(including Rennaissance Wax) which museums have been using, and if you go around a look at their pieces, you'll see the powdery green dots and precipitation from continued bronze disease. Renaissance wax actually encourages bronze disease. It has solvents in it. You only have to open the can to smell it.
When finished scaling a piece is then soaked in distilled water for a couple/few weeks (depending on residue surfacing) which is changed daily. Followingthat,the piece is baked to remove the humidity which has infiltrated the pourous surface of the bronze and then boiled in a very special wax (Degussa Vestowax EH100) which is a hydrocarbon/parrafin mix. It infiltrates the pores at high temperature and effectively boils out any hydrogen or oxygen which remains in the bronze. How do you know it's finished? When it stops micro-bubbling. This way, there is no more hydrogen or oxygen to activate any chlorides which may be lodged in the cuprite, tenorite or bronze itself.
I have cleaned hundreds of bronzes and I'm doing it properly, as the pieces I've done no longer have bronze disease. I've not had one piece returned to me with a problem. If you walk through any museum, you'll see the bright green dots on their bronzes that proves these other chemical and shellac methods are actually more harmful than good.
Comment by KMD
Electrolysis for bronze disease in coins?
Ben is quite right in suggesting that, in the main, electrolysis has not been the optimal technique for addressing bronze disease, or cleaning, for a variety of reasons.
Still, from the scuttlebutt on Coinzappers group at yahoo, some significant progress has been made in the recent past, on coins, at least.
Coins are struck, therefore more dense than embossed, or cast metal.
I've done a bit of plating and stripping, and yes, look at a typically electrostripped casting, old, or new, and if you look close, you can almost see the micro-crystaline lattices, which cast metal can get, in solidifying, because the thing was fried with too much juice, and more likely, the wrong solution.
I suspect that the blue tone Ben mentioned came from a contaminant in the electrolite, or in the gear, and not from the process per se.
Castings are full of holes, literally, they are, and indiscriminate and persistant atoms, if well armed, can do a whole lot of damage.
Like the soil-electrolysis that destroys mixed metals, which turn one another into mush, dust, or clay. This can take centuries, or months.
Although ancient coins were not generally as dense as modern coins, due to the difference in materials and the force applied, they rarely disolve into dust.
"Zapping", (which is a real misnomer), or electrolytic cleaning has been hailed as a cure for bronze disease, at least in coins.
This supposedly takes one session, which does not damage the patina.
This is what I have gleaned, but it wouldn't be the first time I done
ate my foot on this a here list.
Likewise, modest electro-repatination efforts are bearing fruit, which may in time satisfy even the most discriminating Wallis, er wallet.
(Nine out of ten Doctors recommend it, as long as you don't swallow.)
A bunch of guys and gals have been contributing to the gentle art of zapping coins, (and other stuff) with low current, in a fairly weak sodium carbonate solution, (or another), over longer periods, and amazing things are being done, due to stubborness, a sweet & pure love of patina, and an urge to tinker.
Galvanic current, mixed with caustic electrolytes : electro-stripping.
This basically rips the metallic surface or skin off the casting or coin, making it frosty in texture, almost as if it were sandblasted.
We've seen these brassy overcleaned items on the market. I wince.
What you now have with 'the new', electrolysis, as I understand it, is that rather than latching onto some bronze molecules, and removing them, galvanically, as in stripping, what happens is that oxygen and hydrogen molecules (or something), are whippin around in the solution, soccer-jostling one another to make contact with the surface of the electro- conductive coin, statue, or whatever, and eventually, they manage to elbow much of the encrustation out of the way, bit by bit.
Knowing when to stop helps, too.
No doubt I got it wrong, but Jerry and the best of the zappers didn't
This incessent, inexorable, molecular action lifts off the gunk, without pushing any copper molecules around, turning the coin into re-fried cheese, OR tunneling into some nasty corroded, or porous piece of the surface, which hand work, or stripping might destroy.
I'm talking about removing major crusties, without the major risk of stripping, or altering the patina.
Though similar, a coin is a different animal, than a cast fibula, a cast Osiris, or one of them darn gorgeous bronze fishes.
(What are they called, anyway?)
Ben, from what I've absorbed, you basically kick ass restoring ancient bronzes, judging from some of the Osiris and other once encrusted cast figures I have seen, and I won't ask you your secret, (or should I?), but what's your secret?
Ancient coinage, by and large, is struck from a cast pellet, or flan, whereas modern coin blanks are punched from rolled and milled sheet metal.
This does not remove voids, if any, but it flattens them, so they have less volume. (If they're flatter, they don't suck as bad. QED)
Most cast metal objects have features which shrink, and solidify at different rates, mostly depending upon chilling, pattern thickness, and metal supply.
Modern CAD patternmaking has exactly calculated features, which cool together, reducing shrinkage and distortion, but back in the day, although there was some pretty good casting, but even today, where thick meets thin, you get voids in the casting.
Thicker sections are more likely to get spongy, or even become crystalline, especially if there's less metal in the pouring cup, than in the piece.
That's why you need to pour 2 to 4 times the metal in the object.
If in the mold, you don't have an ample molten reservoir pressing down to feed the piece, as it shrinks, bizzare voids and textures result.
A porous casting can come stright from the mold with mangy fur where you'd rather strived for some smoooth bronze skin.
Today, as in ancient times, rather than scrapping a porous casting it's normal to weld, plug, chase, even burnish over the BAD metal.
Spongy castings suck, but suckier still, are ancient spongy castings, literally, soaking up whatever it has been exposed to, year after year, after year, decaying slowly, steeped in all that surrounds it.
Little grottos form in unsound metal, where changes occur, and keep occuring, so the term bronze disease fits. Rather like temites.
Once I worked in a bronze foundry, where large commissions were cast.
One day, right after the major bronze casting of the month had been sandblasted, a visiting client's dog ran by and wheee, took a leak on the foot of this large, freshly-skinned, really impressive piece of statuary.
Freshly sandblasted, to better absorb the patina chemicals.
Well, the job called for a light brown, buffed waxed patina, sorta like dark butterscotch? Two days later, the foot, right leg and base were found to have turned green, 2 hours before the client's arrival.
Franco got it scrubbed off, re-sandblasts and patinas that whole side.
Well, two days after the patina is on and waxed, (yeah, 2 day patina!) the leg is bright green again, and it came back, UNDER THE WAX.
This weren't no modest mossy green, nooo... Shrek green maybe?
This time it was sandblasted, scrubbed, steamcleaned, and again, sandblasted. This went on, every 4 or 5 days for almost 3 weeks, until the client finally relented and went for the green patina.
Just another rude example of how diseases can be spread.
I guess that's culture for you!
From Roger.
28th November 2008
This unusual slideshow is the first of my thematic projects.
It is not focused on the single bronze artefact (they will follow) , but a collection of images of the different corrosion surfaces seen on ancient bronzes.
Nearly 120 close-up and macro photographs of bronze artefacts from many European Museums with some special attention on very particular and beautiful corrosion products and patinas.
A lot of these corrosion processes, patinas, and surfaces have been discussed on this group so I think some members might find it interesting in comparing TRUE aged patina and corrosion surfaces to the FAKE patinas seen on many Ebay listings.
A few days ago I also added a slide show on the cleaning and conservation of a Roman Bronze Gilded Plate brooch “tricked” with gold paint by a British dealer.
Do you know this website?
Really an fantastic resource.
The dreaded 
bronze disease !
About Collectors' Resources pages | What's New
Search | Site Map | Contact Us