The Cleaning and Preservation of Metals - Troubleshooting

WHAT APPEARS TO BE THE PROBLEM?

When you are first presented with a metal object, what you believe to be metal, but it is so encrusted that only your experience tells you that metal is there somewhere, five questions should spring to mind:

Is it brittle?

Is the surface mainly corrosion product?

Is the surface mainly scale product?

Is the object suffering from a combination of the above?

Who's played with this before me?

Answering the above to any degree of accuracy needs a little in the way of explanation as to why the questions are important. Collectively, the answers enable the cleaner/conservator to select the most appropriate steps forward, or at the very least, to avoid the steps that lead to tears.

Brittleness

Even heavy coins can be so corroded that they would crack if dropped on a hard floor. The cause of brittleness is usually a consequence of corrosion - either the metal has, effectively, entirely mineralised or selective dealloying has taken place. In the first case, corrosion has completely converted the metal 'matrix' to corrosion product, in essence this corrosion process has returned the metal to a natural mineral state, ready to be smelted again! In the latter case, corrosion has selectively penetrated the metal leaving a matrix too riddled with microscopic cracks and holes (even corrosion product within the matrix) to retain any durability.

Apart from the necessity for gentle manual handling, there is a less obhvious need-to-know: that is the resistance of the material to ultrasonic cleaning. Wonderful that the process is for cleaning modern jewellry, high frequency vibrations in the metal, providing a host surface for microscopic implosions, simply shake old metal to bits.

The question 'who's played with this before?' is intended to be a flag-waver here. Several years ago, Polymathematica restored an ancient (and very expensive) silver Greek coin using the electrochemical process of reductive consolidation. The owner was delighted, but to improve the coin a little more gave it 'just a second' in a n ultrasonic bath. Not since the time of Dr Crippen has a bath done so much damage - the treated surface of the coin collapsed under the ultrasonic barrage. BE WARNED.

Having rasied the spectre of loss at 1,000,000 cycles per second, brittle material can be cleaned uder low power/short exposure conditions providing the surface is even more durable than the matrix. Anglo-saxon pennies can be a good example here. Regular inspection under a binocular microscope, in between ultrasonic blasts is essentail to ensure that the surface of the coin isn't breaking up. Be aware of tricks of the eye though. As a surface is slowly breaking up, incremental damage is often missed so don't give it 'one last blast' whether you think there could be a problem or not.

Finally, spare a thought for the cleaner metal surface that is hosting the same ultrasonic attack that the tenacious crud on the surface is appearing to resist. If the ultrasonics fail to remove material, it is because it is well and truely part of the matrix. Cleaning by hand cannot be replaced by the killer bath!

Corrosion Product

This subject can occupy a 1 year MSc study in Corrosion and Protection, not on coins and artifacts per se but on the multitude of types of corrosion product on the multitude of alloy types under a multitude of burial conditions. Suffice to say, the POlymathematica site will deal this in less than 300 words and direct you to some good books and sites on the subject (see Background Information and the Corrosion site at UMIST and NACE).

All corrosion is is the process of metals hard won by extractive metallurgy to revert back to metal ore minerals. Just as there are many different types of metal ore, therwe are as many different types of corrosion product. Unless you have a reasonable idea of the enemy upon which you are about to splash chemicals and pick at, a good chance of failure beckons.

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Last Update, 1-July-96