Recently I acquired some old miniatures and I faced the dreadful curse of all metal minis collectors: oxidation, commonly known as “lead rot”. I’ve done some research and I’m trying different methods for cleaning the minis and restoring them.
LEAD ROT – HOW AND WHY
This plague affects, in its worse form, old miniatures made with an high lead concentration alloy (or pure lead). Different alloys and newer miniatures, made of “white metal” and the like, are less affected or totally unaffected.
The metal will be covered by a white or grey substance, that will range in its form form a powder to a crust, depending on the gravity of the oxidation process. Under this layer the metal will be damaged -it transformed into that superficial layer-, in some cases the damage could be so severe it looses its structural integrity and the poor miniature could literally fall into pieces or disgregate.
A bad case of lead rot
Primed or painted minis are less affected or immune, but some factors can cause the lead rot and/or accelerate it anyway: from research and experience wood and moisture are a deadly combination. Specifically it seems that woods (especially hard woods) release the tannins attacking the metals and bad ventilation/air flow will increase the concentration, accelerating the rotting process.
More generally speaking, keep in mind that “rusting” is caused by acids: the metal reacts with the acid, becoming a salt (and usually releasing hydrogen). A lot of common substances have acid PH or can release acids under specific conditions. Let’s at least try to not store the minis in wooden cabinets or boxes, avoiding cold and/or damp places.
SOLUTIONS?
To restore the miniatures affected by lead rot is important to remove the oxidation layer and all the “converted” metal. Then, if the damage is severe, it will be possible to restore parts by reattaching them with standard procedures, filling holes and crevices with putty and doing some re-sculpting, if needed.
So I started doing some testing and research…. Removal with “manual” methods is difficult at best, it’s easy to damage “good parts” and almost impossible to completely remove the oxide.
So I started asking myself: how do archaeologists do? I read some articles here and there and the solution seems to be almost always the same: electrolysis and, more specifically, electrolytic reduction! That’s not hard, it’s easy to setup and the required equipment and materials are relatively simple to salvage or acquire.
Minimalist Setup
After some more articles, blogs and books abstracts, I started thinking about some changes for adapting the process to our small friends: we need to work primarily on pewter and lead alloys and very small volumes, so this is what I used:
–Small Plastic Container, better if with a lid or cover that could be cut and adapted to keep the “electrodes” in place, it’s important they don’t touch each other. This will contain the solution and the electrodes (miniature and anode/s)
–Demineralized Water for the solution(we don’t want water containing substances that could modify our chemical reaction)
–Sodium Carbonate, Na2CO3, for the solution too. You can commonly found it as washing soda (irritant for the eyes, better not to touch it too much with bare hands). It’s also possible to obtain it by heating baking soda, I still haven’t tried it, since it’s so cheap and common, however there’s plenty of resources if you want more info on that…
–DC Power Supply, to induce the chemical reaction. Remember, it must output DC, direct current, NOT household AC (that is completely useless for our purposes here)! 12 to 24 V, more are probably worthless, less works too but they will take a bit more time. If you want to speed up the process it’s better to use a power supply able to withstand 2-3 Amps of current continuously, however smaller currents are working too (100-200 mA), they just take longer if there’s a lot of oxidation to remove. The better solution here is, at least for me, a standard PC power supply.
Old AT power supplies have a nice switch to turn them on, while today’s ATX can be easily powered on by shorting (if your’s standard) the green cable from the main power connector with a black (ground) one. If you plan to use it for other purposes (nice for electronics projects and some less-sane stuff) you can convert it permanently to a sort-of lab power supply (plenty of info on that too).
–Crocodile / Alligator Clips or some other cables, to wire the power supply to the electrodes. If you stay on the upside of the 3 A range it’s better to have a decent diameter to not overheat them, the smallest cables used for electronics experiment are too thin. It’s also possible to use more cables in parallel.
–Multimeter / Amperometer if you want to check the current flow.
–Larger Container with Water, an old Toothbrush, and possibly some Soap. Those are useful to rub away paint remains or similar stuff from the miniatures after the process, if the current is high enough the paint will be nicely removed with the oxide. The toothbrush can be any kind of small-scale brush with plastic bristles, not too smooth.
–Acetone or some other extremely volatile, water-miscible liquid that doesn’t leave residues. I usually immerse shortly the miniatures in it, after the process and the manual cleaning, to remove any moisture left. This is another possibly irritating substance it’s better not to touch too much, plus its vapors aren’t good for the respiratory system and are highly flammable.
–Anode, the electrode (or electrodes) that will be connected to the power supply’s positive pole (in an electrolytic cell the anode is connected to positive voltage, to attract negative ions, while the cathode is connected to the negative pole, to attract positive ions).
Here the material the anode is made of is very important, because the chemical reaction will rust it and alter it quickly. If you don’t have some spare solid gold or platinum, the best alternative is a carbon rod (graphite), it will last long and the only collateral is the deposition of some carbon particles on the minis. Another alternative is stainless steel, but the reaction will produce small amounts of very bad substances (possibly hexavalent chromium), so I don’t recommend its use. Carbon rods can be bought for science experiments or on Ebay, salvaged from motors (brushes) and from some batteries (zinc-carbon), also some large mechanical pencils use thick leads that are practical for our purposes.
Carbon rods after a few hours of use as anodes*
THE PROCESS AND SETUP
Setup isn’t so complex, the miniature to clean will act as the cathode, thus it must be connected to the negative pole of the power supply. The other electrode is the anode and it’s connected to the positive.
They’re immersed in the solution: 5-6% (in weight) of sodium carbonate in water (stir it).
Important things to keep in mind:
-Be sure to have the minis connected to the negative (-) pole of the power supply and the carbon rods, or whatever conductive material you’re using as anode, to the positive (+) pole. If the polarity is inverted, the minis will get oxidized and damaged.
-Be also sure the anode and cathode never touch: that will short the circuit, causing a very high current “spike”, damaging the power supply, or the anode, or the cathode (you miniature), possibly all of them! For this I use some kind of plastic “grid” or spacers over the solution container.
Solution container with cathode and anode support and spacers*
-If you want to monitor the current flow, the amperometer or multimeter must be connected in series to the load (so the current will pass through it while working the “circuit”), so use it as part of the cable going to the anode OR to the cathode. Don’t connect it in parallel to the load (our solution), it will short the circuit, probably destroying itself.
-The current flow it’s important, having an high current (2 Amps and up) will heat all the elements, make the solution evaporate faster, etc. If you don’t have a current-regulated power supply (most aren’t), a good way to control the current is by immersing the anode more or less in the solution. Less surface immersed = less current flow (the conductive area is smaller).
-I recommend to keep the cathode (your miniature) completely immersed in the solution during the process, that will keep the oxidation removal uniform. To do this it’s possible to use a metal clip that will get partially immersed in the solution. Remember, the current must pass through the miniature, so there has to be an electric connection from the negative pole of the power supply, all the way to the miniature itself. Metal cables and clips/clamps are the easiest way to obtain this. Just choose something that will not exert a too strong force on the miniature, damaging it!
A modified clamp for using the miniatures as cathode*
-It’s possible to use multiple anodes to have a higher current and/or to have the miniature cleaned faster and more uniformly. This can be achieved by placing the anodes equally spaced and at the same distance from the miniature. A tube- or ring-shaped anode can also “contain” the miniature and be connected in different parts to the positive pole of the power supply.
-The time it will take to clean a miniature will depend on a series of main factors: current intensity, miniature and anode/s total surface, oxidation depth. Usually 2-3 hours are enough for most pieces and 1-2 A currents.
SOME AFFECTED FIGURES AND THE RESULTS
Depending on the manufacturer, different alloys are used in the production of the miniatures, some of the most affected seems to be old Grenadier’s figures and Citadel from the 80s. I’ve personally opened a perfectly-preserved 1983 Grenadier shrinkwrapped box, and found a figure (and only one, lucky me!) affected by a bad lead rot case…
Here you can see some different minis and the results obtained
In the next article I will examine more miniatures that underwent the process, the results are generally good, cleaning is complete and in-depth.
For the worst cases , where oxidation is actually keeping together the parts, I will do some more experiments with changes on the amount of electrical current and type of solution used, I’ll write an article on that when I’ll have extensive data… I will also do some further research on some materials that can be used for the practical restoration of the miniatures (apart from the standard green stuff and epoxies).
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UPDATE 10/2019: As Tardigrade pointed out in the comments, it’s a good idea to also add some idea for cleaning a bit the minis after the process, mainly to remove the solution and part of the dark patina.
I use an old toothbrush and a water solution with just a bit of soap, to scrub the “subjects”.
After the scrubbing I use denatured alcohol, while Tardigrade uses 91% isopropyl alcohol, to remove the water, by immersing the figures in it.
This will lessen the darkening of the miniatures due to the process, and will also make them shinier.
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*the white stuff you see in the pictures is washing soda, left by the solution after evaporation.
I’m offering a miniatures cleaning and restoration service, in case you’re interested.
Part two here
Copyright © 2015-2019 Francesco Perratone
Further readings and research:
– Metal restoration and electrolytic cleaning –
http://nautarch.tamu.edu/CRL/conservationmanual/File14.htm#ELECTROLYTIC%20REDUCTION
http://www.metaldetectingworld.com/control_voltage_current_p23.shtml