DISASTER MITIGATION & FREEZE-DRYING: general information from AOC
DISASTER MITIGATION AND FREEZE-DRYING: general information and advice.
The problems of water damage
Unfortunately, when an object has been soaked with water, it does not simply need to dry out: virtually all objects will react with water to some extent. This will cause some damage, but with correct treatment the damage will be minimised: if treatment (or correct storage) can begin immediately, there is less time for damage to occur, and the costs of restoring and cleaning objects will be less.
Water will not just leave deposits of silt on an object: there may also be oil, sewage, and chemicals present, all of which will require specialist skills to deal with. The object itself will react with the water - organic material such as wood, paper, leather and textile will absorb water and swell; water-based adhesives will fail. Metals will corrode, and this may lead to staining and damage to adjacent materials.
Sorting, prioritising, and storage
Any good conservator is well aware of the likely effects of water on a wide range of materials: some will need immediate action, others can be set aside in cold or freezer storage, until a decision is made about their future. Costs of remedial work such as cleaning and repair can be kept down if wet material is sorted and placed in the appropriate storage as soon as possible.
Drying out
Some objects can simply dry out under the supervision of a conservator, and with the aid of de-humidifiers, blowers, and so on. Some materials - notably paper and wood - would distort and shrink if left to dry in this way. Blocks of wet paper, or books, will simply stick together in an unusable mass if left to dry. This is because the size (usually gelatine, applied to a paper to give it strength and "crackle") dissolves, then acts as a glue and sticks adjacent sheets together. Many books also have water-soluble adhesive in their covers, or along their spines: in wet conditions this adhesive dissolves, then moves through adjacent material, where it will stick paper and other objects together as they dry.
Parchment presents particular problems and must be dealt with by a specialist conservator.
Freeze-drying
Removes frozen water from an object at low temperatures (usually –20o C). To persuade the ice to part company with the main ice block (and therefore with the soaked book or artefact), the freeze-drying plant has to provide two other elements. One is a constant stream of ultra-dry chilled air: the other is a minute amount of carefully-regulated heat to provide the energy needed to remove ice very gradually from the surface of the main ice block.
In certain circumstances natural resources can be harnessed to provide a low-tech version of freeze-drying – one experimental set-up placed waterlogged wood in a clear plastic tunnel, with warmth provided only by sunlight, and relied a long cold Canadian winter to do the rest.
This process although effective, is very slow, and frequently does not provide such good result s as those obtained by vacuum freeze drying.
Vacuum Freeze-drying
Freeze drying removes the water from an object, under very low vacuum, and at very low temperatures, in the form of ice vapour. The usual result is excellent, giving rise to papers and books that are usable. Many other materials can be freeze-dried, but with varied results: we have found that some resin-coated papers for example, (eg glossy "coffee table" type books) will freeze dry, but the paper is generally too badly cockled to be acceptable. With many books costing between £30 and £80 each to replace, the cost freeze-drying of a shelf of wet (but otherwise undamaged) books is far less than replacement.
The process has been particularly useful for the softer types of paper such as receipts, record books, medical records and company archives, which tend to form an unhandleable pulp when wet. Archaeological material such as waterlogged wood or leather will usually give excellent results if vacuum freeze-dried after an appropriate pre-treatment.
What you can expect from vacuum freeze-drying
Vacuum freeze-drying will remove all the water from materials such as wood, leather, paper and textiles: this will leave many items over-dry, so that they have to be slightly humidified before they can be handled safely. A book, for example, will be brittle and difficult to open immediately after treatment because the glue in the binding will still be dry and brittle; it is normally flexible again after 3-5 hours. Most types of paper - especially those with older, good quality papers - will appear unchanged (apart from any staining caused by the water); unfortunately some modern papers will stick together almost irretrievably, with or without freeze-drying. Please feel free to call for advice before you decide what to do.
Problems with degraded leathers
Many leathers and parchments - especially if they are in good condition - will be little affected by vacuum freeze drying; tooling and gilding generally appears to be unaffected by the process. However leathers which are in poor condition (for example, those which are over-dry, lacking in lubricants, poorly tanned, or suffering from red rot) will be damaged by the initial wetting, and this damage cannot be reversed.
The age of the binding is not necessarily significant - the overall condition of the leather is all-important. Deteriorated leathers may become jelly-like in the presence of water, and during vacuum freeze-drying will shrink extensively: they are unlikely to be usable. We can pre-treat particularly important bindings, so that shrinkage will be minimised: it may then be possible to re-use the binding if it is removed and backed with a stronger, modern leather. By this means at least the original will be retained and returned, so that a new copy of the binding can be produced.
Vacuum freeze-drying will not remove soiling from the material: on water-damaged paper you will often find a "tide-mark" of brown, which marks the level to which the water penetrated the book. This brown stain contains water-soluble material which forms over the years as the paper's cellulose slowly breaks down. (This breakdown is a natural and largely unavoidable process).
If the water which marked the books is dirty - for example it has soaked through the ceiling, or is muddy flood water - then this soiling will not be removed by vacuum freeze-drying, and further cleaning may be required.
Colour-runs from the binding, from bookmarks or from hand-written notes or dedications, will not be removed by vacuum freeze-drying.
Vacuum freeze-drying will not remove contaminants from the material: mud, soot (eg if books have been water-damaged as a result of fire-fighting), and mould will be left on the pages. Mould-growth after wetting can be extremely rapid: the spores will mark the pages, and the paper will be stained by fungal by-products. Additional cleaning may have to be arranged, especially if any of the contaminants pose a hazard to the reader.
Vacuum freeze-drying will not necessarily kill off mould spores: they will be inactivated during the freezing process, and should remain so, provided the book remains too dry for the spores to germinate. Many fungal spores can survive indefinitely at temperatures well below zero.
Bookworms and silver fish: adult insects, but probably not eggs, will almost certainly be killed off by the low temperatures, and by the lack of oxygen. (If you wish to use the vacuum freeze-drying process as a means of insect control, please let us know - an additional freezing cycle 2 - 4 weeks later will be needed to kill off any insects which have hatched in the interim, and we will be happy to arrange this for you.)
AOC: Assessment and estimating
We can arrange to have at least 2 conservators on site within 24 hours of any incident: we will sort (consulting with the client), pack and take to storage all vulnerable material, pending the decision of the loss adjuster. We will arrange appropriate storage for material, and produce an assessment report outlining a series of treatment options, together with their costs, to allow the client to reach a decision about the most appropriate course of action to take.
Facilities
We have a vacuum freeze-dryer that can take up to 0.25 cubic metres at a time, or items up to 1m long. Material can be held either on our premises (in freezers or cold stores) or else by arrangement with a local blast-freezing company, at reasonable (weekly) rates.
Although we do not have a paper conservator on the staff at present, Gretel Evans has completed a conservation course that dealt with post-disaster care of photographic materials.
Security
Our building is open during normal office hours; it can be opened at other times by arrangement. Our premises have fire, smoke and intruder alarms, and a secure car park. If you need to contact us outside office hours in an emergency, you should be able to reach me at home, or else through a relay service at AOC's normal number (0131 440 3593; e-mail ).
Staff
We have at least three conservators in house, all of them familiar with freeze-drying processes. We can also call on additional staff (3-8), experienced archaeologists, who can assist with retrieval and recording in the event of a major disaster and recovery operation.
Client consultation
We have regular contacts with our clients, and are happy to offer advice on packing, storage and transportation of material. If required, a fixed budget can be agreed for immediate salvage work prior to assessment for freeze-drying and conservation.
The cost of vacuum freeze-drying
This varies tremendously: it depends, for a start, on how wet the material is when it is received, how dense the material is, and the thickness of each wet or frozen block.
It is cheaper to treat several items or blocks of material, than single items. This is because it takes a certain amount of time to set the machine up, stabilise* the contents and
*Stabilisation: after freeze-drying, items are frequently too dry to handle safely, and are slowly brought up to ambient RH and temperature.
then clean and reset the machine after use. A small amount of time is also required each day for maintenance, so these costs are standard for every run.
The length of time required to remove all the moisture varies with the thickness and the density of the wet material: a single sheet of parchment, for example would only require 1-2 days, while a volume 3-4 inches thick could take up to 6 weeks to dry completely.
Every job is individually estimated on assessment. It is advisable to discover exactly the type of material in question: some materials will freeze-dry better than others, some materials will require specialist packing to prevent expansion and distortion, and once we know these details, we can supply a quotation for the work.
Vacuum freeze-drying schedules and deadlines
We usually set realistic deadlines for the freeze-drying service: the process cannot be speeded up, and the client should be aware of the time each batch is likely to take. Every effort will be made to adhere to agreed schedules; clients generally like to prioritise their material for treatment. We will give the client an estimated cost, and completion date, before work begins.
Recommendations
Previous experience leads me to suggest that you should note the following points, in the event of preparing soaked books and paper for freezing and/or remedial conservation:-
if feasible, blocks of wet material should be no more than 4 - 5 cms thick , or even less (otherwise it takes an extremely long time to freeze-dry): each of these blocks should be separated by 2 sheets of polythene or PTFE tissue (so that when separated, each block has a separating sheet at top and bottom).
For this you could obtain a stock of the thin (PTFE) tissue used in butchers and delicatessens; you can purchase it relatively cheaply in a variety of sizes, up to about A2, from packaging suppliers, or wholesalers specialising in the food industry. Alternatively, a caterers' supplier can provide large quantities of polythene bags of varying sizes, which would be equally effective. PLEASE do not use anything such as bubblewrap, Jiffypack or any materials that have air trapped within them, such as thin foams: under vacuum, the air bubbles expand hugely, and could cause individual items to tip up and fall.
Please do not seal items in water- or air-tight bags, for the same reason.
You should note that any items with air trapped in the binding (e.g. laminated material; plastic-coated ring binders) will have to be pierced to allow air and moisture to escape.
Try to avoid handling or separating wet materials unless absolutely necessary - they will stick, and less damage will be caused if they can be separated after freeze-drying.
Wet materials can be held - and then transported - in refrigerated vans; check your Yellow Pages for refrigerated self-drive van hire or else have a list of haulage contractors with freezer vans available. It is better to transport the material in cold conditions, rather than frozen: some materials may require to be reshaped before freezing. Impress upon your carrier the need for urgency - mould growth can be spectacular after only 24 or 36 hours.
If you intend to pack or transport wet documents, isolate any which appear to have non-fast inks or dyes: thin PTFE sheet would probably suffice.
Material should be packed flat - do not stand wet books up on end for transportation, this will damage the softened corners.
If you pack wet material in cardboard boxes, line the boxes with at least one bin-liner - otherwise the cardboard will get wet, soften and collapse in transit.
Do not over-estimate the amount of material each box should take - wet books and paper are very heavy.
Recommendations
If possible, ensure that none of the volumes have inclusions (bookmarks, letters etc.) that may stain the adjacent pages in the event of a flood. If such material has to be included within a volume, it should be placed in a thin archival quality polyester envelope.
It may be worth considering storing some little-used volumes in polyester envelopes or bags, if they are likely to be particularly vulnerable to water damage.
Check where water might penetrate book storage areas (eg from down pipes, water pipes, outlets etc.): it may be advisable to place polythene sheet across the top of shelves that are situated beneath water-carrying pipes.
PLEASE FEEL FREE TO ASK FOR ADVICE: WE ARE HAPPY TO DISCUSS ANY QUERIES YOU MAY HAVE.
AOC Archaeology Group, 4 Lochend Road, Leith, Edinburgh EH6 8BR
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