Guidelines for Storage of Archaeological Metals
In these guidelines, we have translated the results of our laboratory research into adaptable options for creating and managing the most effective desiccated microclimates for storage of your archaeological metal collections.
The work behind these guidelines is ongoing and we will update the advice continually as new evidence emerges. If you would like advice on your particular storage requirements, please contact us.
A paper detailing the research behind these guidelines has been accepted for publication in Studies in Conservation, published by Taylor & Francis. You can download the preprint version here:
Immediate post-excavation actions
Once excavated from the burial environment, metal artefacts are exposed to plentiful oxygen to support corrosion processes. Key to ensuring the survival of these objects is swift action to dry them, removing any water to stop corrosion occurring.
HOW TO DRY:
Method 1 - Air dry
Place objects on a tray with space to allow airflow around each artefact.
Leave the tray in a warm, dry environment.
If possible, weigh objects periodically until weight loss ceases at which point they will be dry.
If weighing is not possible, leave for two weeks after which they are likely to be dry.
Method 2 - Oven dry
Place objects on an oven-proof tray or dish with space to allow airflow around each artefact.
Place tray in an oven.
Heat oven to 90C.
If possible, weigh objects periodically until weight loss ceases at which point they will be dry.
If weighing is not possible, remove from oven after 48 hours.
CAUTION: Do not attempt to dry objects in an enclosed environment with a desiccant
Packaging for storage: principles
Whether planning for the short or long term, there are key principles to adhere to when designing packaging for metal artefacts. Any packaging must protect objects from physical impact, should be accessible to allow objects to be handled and should minimse the chance of deterioration in condition of artefacts. Importantly, although any free water in the objects has now been removed, they must be prevented from corroding by desiccating the air around them. Achieve this by:
Selecting an airtight storage box.
Including the optimum amount of desiccant (most likely silica gel).
Monitoring the moisture levels inside the box with a humidity indicator card.
Regenerating the desiccant as required to maintain a dry environment.
As simple as these instructions may seem, our research has shown that minor differences in box type and size, amount of silica gel and the external environment have big implications for preservation of objects.
Choosing a storage box
With a vast range of commercially produced storage boxes available, how do you decide which will keep your objects dry for longest? Ideally, we would test the leakage rate of every box but, as that is not practical, here are some golden rules for making that decision:
Choose a box with a snap seal or a locking lid.
Boxes with locking lids may perform more consistently.
With locking boxes:
Look for a large lock to seal ratio where the locks run the length of the box side.
Choose boxes with locks on all sides.
Select boxes with a gasket inside the seal.
Ensure the gasket can be compressed easily, achieving a tight seal.
A stiff mechanism that is difficult to close may indicate a better seal.
Visually inspect all boxes to check for defects or gaps between box and lid.
Consider the size of box you need for the number of artefacts and your storage.
Many boxes may fit these criteria. On the basis of our research, we recommend Addis Clip & Close, Lock & Lock, Stewart Sealfresh, Stewart Gastronorm and Araven. We cannot recommend Sistema boxes for this application.
Desiccant: method
Without a desiccant in your box, the internal humidity will match that of your ambient store environment and will offer no protection against corrosion. We recommend using dry silica gel as a desiccant to absorb moisture entering the box via leakage of air. After a period of time, the silica gel will have absorbed enough water that it is no longer able to keep the box contents sufficiently dry. At this point it must be removed from the box and dried again in a process known as regeneration. Our recommended procedure for using silica gel is as follows:
When purchased, silica gel will be dry. However, for maximum protection of artefacts, follow the regeneration procedure before initial packing of box.
Use the graphs provided below to determine how much silica gel is required for the size of your boxes, your ambient store environment and your maintenance regime.
For each box, take 4 grip-sealing polythene bags and prick with holes (at least 12 per side).
Divide the amount of regenerated silica gel for one box between the 4 bags.
Place one bag along each side of the box.
Place a humidity indicator card facing outwards on one side of the box.
Place artefacts into the box.
Close the box ensuring that the seal is / clips are engaged firmly.
Desiccant: amount
The following recommendations are based on extensive laboratory testing modelling ambient store environments of 50% and 80% relative humidity (RH).
The graphs below give you the mass of silica gel (kg) required for the size of your storage box (litres). This assumes that you are using the recommended storage box brands or boxes that perform similarly.
Select the correct graph for your management regime, i.e. the maximum RH acceptable for your collection and the frequency of gel regeneration.
For stores at 50% RH or below, follow the recommended amount for 50% RH.
For stores with RH above 50% or where the humidity is not known, follow the recommended amount for 80% RH.
If in doubt, increase the amount of silica gel.
To double the regeneration cycle length, double the amount of gel.
Use the graph below as a guide if:
You want to maintain 15% RH or lower for 12 months
OR
You want to maintain 20% RH or lower for 24 months
Use the graph below as a guide if:
You want to maintain 15% RH or lower for 6 months
OR
You want to maintain 25% RH or lower for 12 months
Desiccant: regeneration
Spread silica gel thinly in oven-proof dishes.
Set oven to 105C/220F.
Heat silica gel in the oven for 7 hours.
Switch off oven and allow to cool.
Repackage gel into polyethylene bags for immediate use or store in airtight container.
Monitoring
The hardware and procedures recommended in these guidelines are designed to maintain humidity levels as stated. However, small variations in method, defects in boxes or other unforeseen circumstances can reduce this lifetime. Simple monitoring procedures can ensure that failures in dry storage are noticed before damage to objects occurs.
Humidity indicator cards:
We recommend that you include a humidity indicator card in your storage boxes. There are different types of humidity indicator card available but all will exhibit a colour change that denotes the internal humidity of the box. Choose a card on which it is easy to read the humidity in the RH region that you need to monitor.
If you have included humidity indicator strips in your boxes, ensure that you establish a routine for checking them fortnightly where ambient environments are above 50% RH and monthly where ambient RH is below 50%. If indicator cards have been positioned facing outwards in the boxes and are visible from the outside, this should be a relatively fast process for a trained individual. For very large collections, a sample of the boxes could be monitored on a rotational basis.
Self-indicating silica gel:
The silica gel used in this research is orange-green self-indicating silica gel supplied by Gee-Jay Chemicals. Working with this silica gel daily, both dry and conditioned to all humidities up to 90% RH, we know that the colour range it exhibits is wide, from orange (dry) to green (mid-range RH) and on to blue (high RH).
A very experienced eye could perhaps estimate RH from gel appearance to the nearest 10% RH but even this would not be sufficient to pick up a rise from 10-15% RH, for example. Therefore, crucial corrosion thresholds for vulnerable objects could be missed. It would be near impossible for an inexperienced individual to judge the subtle colour differences reliably.
Although we cannot recommend self-indicating silica gels as the primary method of monitoring internal RH of boxes, they do offer a useful indication of a dangerously high RH. If silica gel appears green, it is a sure sign that there has been a failure of storage hardware or monitoring/regeneration regime and that intervention is required immediately. At this point, the box and its contents should be checked for condition and the silica gel regenerated.
Sustainability
Plastics have been used for fabrication of storage boxes for many years. They can be produced with the properties of rigidity, flexibility, transparency, longevity and airtightness that we require for storage of archaeological artefacts. With the movement to reduce consumption of plastics, the catering industry (the source of most boxes in use for desiccated storage) is looking towards alternatives. As these become available, we will research their airtightness and ageing properties in order to recommend them where possible.
The longevity of polypropylene and polyethylene is a cause for great concern in environmental terms yet it means that, far from being single use plastics, storage boxes in archives may serve their purpose daily for decades. We strongly recommend that once these boxes reach the end of their useful life, they be recycled.
Training Materials
Training in Desiccated Storage v1 2020
Need help with your storage?
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Acknowledgements
These guidelines are the result of years of experimentation by an ever-changing team of researchers. We would like to express our thanks to the Cardiff University Conservation students, past and present, who have contributed to this work:
Emily Franks, Jake Howe, Natalie Ioannou, Rebecca Keeling, Elizabete Kozlovska, Marcin Krzewicki, Mary Lawrence, Yichen Liu, Eleanor Moxon, Jack Newcombe, Jodie Pike, Kathryn Schronk, William Smith, Meredith Sweeney, Jenna Taylor, Stephanie Whitehead.