This guide was produced by York Archaeological trust for the Portable Antiquities Scheme. The full set of conservation advice notes can be found on the Portable Antiquities Scheme website.
Protecting your finds
Metal finds need to be cared for. Excavated finds may have lost much of their metal content in burial. Some may be fragile, thin, brittle and easily broken. Others might be very heavy, and need extra padding. Different metals corrode in different ways; gold hardly corrodes at all, whilst iron corrodes quickly.
Some objects are made of more than one metal or contain other materials like glass, enamels, wood, bone, ivory, horn, leather, or textiles. You will need to think about specific requirements for these objects.
Your FLO can demonstrate some of the special packaging and storage materials you might need and can show you a DRY BOX to protect your finds from corrosion.
Bags, boxes, padding, markers and labels
Conservators have tested a range of supplies to see how well they withstand long term storage and handling. Materials which do not fade, fall apart, or produce acids or gases as they age have been selected. These materials are sometimes called archival.
Using robust containers and permanent black markers is important.
Mini-grip bags with write-on strips and a Jiffy foam insert are good for most finds. Fragile finds are best packed individually in small clear plastic boxes.
Acid-free tissue paper or polythene foam is recommended for cushioning your finds. Avoid cotton wool, it may stick to the corrosion and be difficult to remove, and foam rubber can give off sulphur and tarnish silver. Textiles like velvet and felt can do the same.
Recommended packaging materials:
Polythene grip-top bags with write-on strips – remember to punch holes to ventilate!
Spun-bonded polythene labels (e.g. Tyvek®)
Permanent black markers (e.g. Artline® pens)
Airtight polythene boxes (e.g. Stewart®)
Clear polystyrene boxes for individual finds
Polyethylene foam (e.g. Jiffy ® foam) – not household foam rubbers
Closed-cell Polyethylene foam (e.g. Plastazote®)
Contact your FLO for a list of suppliers
Storage environment - Relative Humidity
Corrosion requires oxygen and moisture to progress. If you can exclude either of these, metals will survive for a long time. It is easy to create a dry environment for your finds.
On site: Make sure the plastic bag has holes to prevent condensation. Dry out your finds gently when you get home. Avoid radiators and ovens, as the sudden change can damage your finds.
Storing your collection: Pack the dry find in a perforated bag with Jiffy foam to protect it from knocks, and store the bags in an airtight container with silica gel and an indicator strip.
Silica gel is a crystal that absorbs a certain amount of moisture. Once it has absorbed that amount it has to be dried out again to keep working. Don’t leave the "Dry Box" open, or your gel will quickly become exhausted. How can you tell that it needs refreshing? An indicator strip will turn pink when the gel isn't working. It shows the current level of moisture in the air (relative humidity,RH). Iron needs to be kept very dry (less than 15%RH), so no pink should be seen at all. Other metals will be fine at that level too, but it is too dry for glass and organic materials like leather and wood. You can dry the gel out yourself following the instructions, but your FLO may be able to arrange to have the bags dried for you. There may be a charge for this service.
Storing Different Materials
Iron corrodes most easily. It has to be kept dry at less than 15%RH to prevent rusting. Active corrosion is indicated by bright orange powdery crystals or little droplets of weeping iron. In this photo the iron object was not kept dry; the corrosion absorbed moisture from the air and formed an acid liquid weeping iron which attacked the label, staining it and causing it to disintegrate. Orange iron-stained tissue paper is a common sight and indicates an urgent need for dry storage.
Copper alloy should also be kept dry. Corrosion can begin again above 35%RH. Bronze disease, a particularly damaging form of corrosion, can be kept from getting worse by dry storage, but it progresses quickly when moisture is present. If you notice pale green powdery corrosion spreading, the find may need chemical stabilisation. A conservator can check this for you, and treat if necessary. You can arrange this through your FLO.
Lead and Pewter form white powdery corrosion in contact with paper, cardboard and wood. Even the gases given off by paper and card can cause corrosion, so keep these metals away from cardboard boxes. Lead is a poison which can be absorbed by your skin wear gloves when you handle lead finds. Dont brush off the white powder; it is bad for your health!
Silver turns black (tarnishes) when sulphur reacts with the surface. Air pollution, handling, and chemicals in some textiles can make this worse. Wear gloves if you handle silver. Excavated silver can be very brittle and cracks easily; handle with care.