Preservation Basics: Timber and Metal by Andrew Pearce
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Preservation Basics: Timber and Metal by Andrew Pearce
- 1. Preservation Basics: Timber and Metal Andrew Pearce: National Museum of Australia
- 3. Weathering of timber mast
- 4. Delamination of plywood in late 1800’s pram
- 5. Rot damage to Walter Burley-Griffin door
- 7. Wear damage to anchor chain
- 8. Steel pylon bolted to aluminium surface of helicopter
- 9. Star picket (iron in saline environment) and “bronze disease” on coins
- 10. Fingerprint etching on silver
- 11. Corrosion of lead by timber vapours
- 12. Timber Preservation Basics • • • • Protect from moisture Protect from light Protect from biological attack Protect from impact and abrasion • Reduce the loads
- 13. Metal Preservation Basics • • • • Protect from moisture Keep salts away Limit overpolishing Be aware of bimetal corrosion and reduce moisture if possible • Watch the vapors and pollution: air transfer and tissue wrapping • Wear, impact, abrasion and vandalism • Don’t forget the non-metal bits!
Hinweis der Redaktion
- Preservation basicsThe basic care of wood and metals.To understand the care of wood and metals, first we need to have an understanding of the deterioration forces that may be at work.
- The deterioration of timber can be in the form of weathering due to exposure, splitting and delamination, biological damage from rotting or insect attack, or may be due to mechanical damage, that is, damage caused by wear, use or by load
- Here is an example of weathering of timber. This is a portion of a radio mast that was erected in Antarctica. As you can see, the timber has been deeply weathered in the softer areas of the grain. Obviously this is an extreme example, but I expect you’ll all be fairly familiar with weathered timber.A few points worth mentioning: If the timber is positioned horizontally with crevices due to weathering, moisture due to rain or condensation can collect in the fissures. This can cause localised swelling, or in colder areas, can freeze, and splits can rapidly become far more severe.Light exposure is also a problem and can cause bleaching and significant colour change.
- Here is a good example of delamination of timber. The base of this pram is plywood and dates from the late 1800’s. As you can see, the edge of the plywood has delaminated and the timber has begun to separate. Given the objects use as a pram, it is quite likely that the separating edge was then highly vulnerable to the picking of little fingers.Once an adhesive failure such as this has occurred, changes in humidity will cause the individual layers of the plywood to expand and contract differently to each other, and the delamination will grow inwards, spreading across the surface of the plywood.
- Obviously as an organic material, timber is highly vulnerable to rotting and to insect attack.. Here you can see the lower end of a timber door which has been left damp and in contact with the ground. You can see quite extensive losses to the timber
- As with timber, there are numerous causes of deterioration in metal objects. These include moisture, physical damage due to use or wear, corrosion due to the presence of multiple metals joined together and finally the presence of chemical agents such as salts or organic acid vapors.
- Here you can see extensive wear damage to the inside of the links on this anchor chain. In this instance, the damage is evidence of use, but wear can be anything from a polished area where patina has been lost, through to extensive wear to the point of total failure.
- Here is a lovely example of bimetallic corrosion. In bimetallic corrosion, two dissimilar metals function as a batter in the presence of moisture, with a flow of electrons causing corrosion of the anode (in this case the aluminium surface), while the iron components remain largely intact (albeit with a little minor surface rust).
- Here we see some nice examples of the effects of salt on metals. The slide shows a star picket (now down to less than 8mm across the centre in some sections) that was retrieved from being buried in high salt content soil. The second example is of a pair of bronze coins being attacked by “bronze disease” where the chloride irons present are highly aggressive in their attack on the copper.
- I’ll be talking about handling later on, but really no lecture on deterioration of metals would be complete without talking about fingerprints. Here is the surface of a silver plate which has been etched by acidic oils from human skin. Polished copper, silver and brass are the prime recipients of fingerprints, and on unlacquered surfaces they can be very disfiguring.
- Here is my last slide showing damage to metal, and it’s one that takes a few people by surprise. Lead (found in cast metal figurines, historical ammunition, some very early Roman coins, and also in the cast rigging fittings of some model ships) is highly susceptible to attack by organic acid vapors.Unfortunately I was unable to track down the shot I really wanted to show you, which was of a priceless Roman lead coin collection which had been lovingly stored in oak display drawers by its owner, who opened them one day to discover that the entire collection was now nothing more than puffy white powder. Vapors from oak, teak, craftwood, cedar and a number of other timbers can quite aggressively attack lead items.
- Having talked about the basic deterioration processes for timber and metal, what I now want to do is run through some basics for how to protect collections: Firstly timber:Really a lot of this is common senseProtect timber as much as possible from moisture. This can cause warping and dimensional changes, delamination, failure of glued joints and staining. It can also lead to conditions where rotting can occur.Protect timber from excessive light, which could cause bleaching of the timber and deterioration of any varnishes and lacquers.Protect timber from biological attack. This means not only storing things where they won’t remain damp and begin to rot, but also keeping things as protected as possible from insects and vermin. You also want to remember that many timber objects are not solely timber. For instance, many antique chairs for instance have cushions which are cloth covered and stuffed with horsehair, which is highly attractive to insect attack.As a general rule, timber is soft, and impact or abrasion will cause permanent damage. Try to use and to store timber objects in a manner where abrasion and impact are kept to an absolute minimum. Even small steps like placing small felt pads under chair legs are worthwhile, or placing carpet cups under the legs of heavier pieces of furniture in order to protect floorboards in historic houses.Finally, try and avoid long term loads that can cause distortion of shape over time. Timber under load tends to gradually conform to a shape that reduces that load. If this is not desirable, then doing something to remove or reduce the load as much as possible is highly recommended.
- On to metal now.Metal is far less prone to damage from light, but you still need to bear in mind that many metal objects are painted or have coatings that can be adversely affected. Don’t fall into the trap of just seeing the metal part of an object, look at all the other materials and surface finishes, and see what is most at risk.With metal the damage can be far reaching and cause quite catastrophicAs much as possible, protect your metal objects from moisture. The chemical processes of corrosion almost all require moisture in order to take place. Keep things dry if at all possible.Salts are a real killer on most metals. Seaside environments are problematic, but don’t forget the salty oils on your skin and the damage that handling can do to uncoated silver, copper and brass objects. Cotton, latex or nitrile gloves are your friend here.While I’m on the topic of polished items, every time you polish an object you are abrading the surface and removing metal. The polish residue that finds its way into cracks and crevices is porous and can harbor moisture which can then cause localised corrosion. If something must be polished, try to do it once and then use or store it in a manner which protects it from requiring frequent re-polishing.Some objects will suffer from bimetal corrosion, and there is little which can be done in many circumstances, but the process becomes much faster in the presence of moisture. If you know that you have objects with combinations of zinc, aluminium, iron, copper or brass, then try to keep the environment as dry as you can. In some cases it may be possible to partially dismantle an object and place an impervious barrier such as a thin layer of Mylar sheeting between the dissimilar metals, but this is getting into more complex treatment and interventionOrganic acids and airborne pollutants: As I’ve said, timber vapors can really destroy succeptible metals such as lead. If you do have valuable lead and lead/pewter objects, keep their storage environments open to air exchange, don’t leave them cooped up inside timber enclosures with no air circulation. I’m not saying that you should throw the baby out with the bathwater, this doesn’t mean they cannot be in a timber framed glass showcase, it simply means that the case should be designed with an open base or back and pathways that encourage air circulation and exchange.Which then brings me to airborne pollutants, and also another thing that can build up in showcases. Silver tarnishes rapidly in the presence of sulphur vapours. Sometimes these can be in plastic materials housed within showcases, but in many parts of Australia, the prime source of sulphur in the air comes from industrial pollution. For non-display objects, the best barrier to sulphur pollution is to wrap objects with acid free tissue paper as a barrier preventing the sulphur from getting to the silver. Acid free tissue wrapping is a simple, cheap and highly effective method of protecting many decorative metal objects. It keeps moisture, humidity, airborne salt spray and pollutants away from the surface of objects, greatly limiting the onset and progress of corrosion.Wear damage, impact, abrasion, vandalism. As with timber, metals wear, and this should be avoided if possible. Impact and abrasion will cause damage which will cause fairly extensive intervention to repair. Finally, be aware of vandalism. Sadly, polished metal surfaces seem to attract the lowest common denominatorFinally, I want to recap on the point I made at the beginning. Don’t forget that with many objects in collections, the metal components will not be the most fragile components.