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Monday, 11 August 2008

Lead Rot 101





Lead Rot 101

There is one thing that wakes the ingenuous collector from his peaceful sleep and fills his heart with dread. It instills these who fill their attics with shiny leaden objects they are never going to paint with sheer terror. A phanomenon so vile that only people who stepped bare footed on a metal Night Goblin Spear Regiment seem to have nurtured such evil.
Imagine to open your display cabinetts to have a look at your priced possesions. Notice the small amount of white powdry stuff on your favourite miniatures. Act now or all that remains will be a pile of dust as your collection has withered away.

Know your enemy

The process in question is called lead rot and a well known process when it comes to antiquities like organ pipes that contain a lot of lead and are exposed to all kind of climatic conditions. The reason for relativly new lead miniatures to crumble away mostly goes back to the boom in the miniatures industry in the mddle of the 80s. Faced with increasing prices for lead the manufacturers of the so called Nottingham lead belt used polluted lead alloys to cast miniatures. These alloys contain a lot of other questionable materials and seem to be the ones that are most likely to rot away.
In the following paragraphs I want to give some brief information about the chemical context and ways to prevent or stop lead rot.
Most of the stuff is taken from miniature fora and acts as a kind of summary.

Organic compounds are the chief category of substances acting harshly upon lead, and Acetic acid is among the most destructive of these carbon compounds. Acetic acid acts upon lead and transforms it into lead carbonate which is the white, granular, powder we frequently see on lead ship model fittings. The museum objects conservation community has been aware of the phenomenon for several decades and the chemical process that causes it is well-understood.

The chemical process is: Acetic and some other acids, in the presence of carbon dioxide, catalyze with lead to produce lead acetate and lead hydroxide. Lead acetate and lead hydroxide together react with carbon dioxide and form lead carbonate. Lead carbonate then releases acetic acid and the process becomes self-sustaining. It is important to recognize that the formed lead carbonate is not just a substance clinging to the surface of a casting, it is the surface of the casting transformed to powder. For practical purposes, a portion of the lead is gone and lead carbonate is left in its place. The lead carbonate releases acetic acid which can continue the process until the lead part is progressively consumed from the outside.

Wood exposed inside display cases with relatively stagnant atmospheres will create an acetic acid-laden micro-environment where lead artifacts will corrode even without being in physical contact with the wood. In addition to materials forming the surrounding exhibit case, the model itself may be made primarily from wood.

All woods will emit acetic acid to some measurable amount, but the following woods sometimes used by modelers are known to be harmful to lead:

Very Harmful
Unseasoned oak (white and red),Plywood and chipboard,Teak,Basswood Sweet chestnut
Fire-proofed woods,Rot-proofed woods

Moderately Harmful
Seasoned oak, Larch, Ash, Birch,Beech
Red cedar

Less Harmful
Sitka spruce,Douglas fir,Pine,Elm

A general rule of thumb can be applied: Hardwoods emit more acetic acid than soft woods. But any wood will fall into at least the minimally harmful category.2
Please note that also paints, glues, and other miscellaneous materials may cause lead rot to a lesser degree.

Solutions

Prevention
The single most important element is to provide your cabinets with plenty of ventilation at least complete air circulation twice a day. I use bags of SILICA to keep the moisture down and can be purchase in most office outlets.If the cabinet are air tight you might want to drill a one inch diameter hole in the case and keep the cabinet out of the direct sun.

If you intend to seal a mini in its bare metal state, I've found the  absolute best material to do so is shellac. And no, I'm not talking about a spray can of clear lacquer paint or a pot of wood shellac from the hardware store. I'm referring to genuine shellac chips from the lac beetle dissolved
in denatured alcohol. It's easy enough to find if you just tell someone you need shellac chips for French polishing. Pop into an antique store and it's likely someone there can tell you where you can get some. The Shellac then seals the mini away from oxygen, and you'll never have to worry about it again. Best of all though is that if you ever decide to unseal the mini, you don't have to soak it in any sort of corrosive compound to remove the shellac, just drop it in a tin of alcohol and in about 5 minutes the shellac coating will be dissolved away. Rinse in clean alcohol and your mini is clean of the sealant.

Affected miniatures
First of all you have to clean the miniature, this can be done by using a short soak in white vinegar followed by cleaning the miniature with a toothbrush with toothpaste.
For heavily affected areas you can use a dremel or something similar with a cleaning and/or polishing bit. Do not forget to let the miniature dry on a paper towel or use a hair dryer thoroughly.
After cleaning and before any repair, conversion and painting work you have to prime the miniature. do not use any water based primers/paints for this. the best way seems to use a polyurethan (PU) paint because this will seal the miniature hermetically. you can get this paint in any colour and even clear, if you prefer to display your miniatures in bare metal form.

http://www.dt.navy.mil/cnsm/lead_01.html

6 comments:

Anonymous said...

What I don't understand: If acetic acid is the main cause of lead rot, why use vinegar to clean the object? Vinegar is watered down acetic acid.

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