Salvaging Wet Collections

Introduction

There are four techniques that can be used to dry collections.

  1. Air drying
  2. Desiccant air drying
  3. Vacuum freeze drying
  4. Vacuum thermal drying

In deciding which method to choose in a particular situation, the following factors should be taken into account:

  • What is the volume of materials?Air drying requires a great deal of space, a large supply of absorbent materials, and requires prodigious amounts of time and attention. This can be unrealistic if a large number of objects are involved. However, if the materials are frozen, small numbers can be removed from the freezer and dried over time.
  • Which method will yield best results?Some kinds of materials may be damaged if frozen, or so thoroughly soaked they are impossible to air dry successfully so another drying method should be chosen – see below
  • Is there need for access to the collections during drying?Some drying methods will necessitate restricting access for weeks or months.

1.  Air drying

Brief description of treatment

Air drying is the most obvious and immediate way to dry almost any wet museum specimen or collection. Air drying requireslittle or no special equipment. It is a suitable choice to dry small numbers of damp or slightly wet materials (baskets, historic artifacts of wood, leather, books, metal, etc.) However, air drying is extremely space and labor-intensive as a space is outfitted with fans and dehumidifiers and wet objects are laid out on absorbent surfaces. This technique is therefore most suitable for relatively small quantities of damp or slightly wet artifacts or specimens.

What collections materials can be treated this way?

Most collections can be air dried.

Exceptions, which cannot be successfully air dried, are the following:

  • Coated (glossy) paper, as found in some books and magazines
  • Collections that are too large or too wet to dry before they will become moldy.
  • Objects with stuffing, such as study skins, upholstered furniture, and some taxidermy mounts are not good candidates for air drying

General Procedures

  • Set up a drying space of appropriate size with tables covered with absorbent materials – blotting paper, newsprint, toweling, etc.
  • Aim to have temperature and relative humidity as close as possible to “normal” room conditions. (This will vary according to local conditions, but in the northeast 70-75 degrees F at 50-55% RH or below would be considered reasonable target conditions.)Introduce dehumidifiers and heaters as needed to achieve these conditions.
  • Ensure good air circulation. Try to pull dry air into the room with fans, and push humid air out. Ensure that air currents are gentle so that they do not damage specimens or disrupt specimen labels.
  • Spread wet items on tables with absorbent coverings. Gently blot off as much water as possible from the object’s surface.
  • Change absorbent materials whenever they become significantly dampened.
  • Pad out different layers of three-dimensional objects to increase air circulation.
  • Re-shape shaped objects, and stuff them out with nylon net or polyester batting so that they will retain their shape as they dry.
  • If possible, remove stuffing materials that will slow air drying. If not feasible, consider another drying method.
  • Robust items such as documents and records (rather than specimens or artifacts) can be hung with clips to a laundry line or string to save space.

Pros and Cons of this Treatment

Pros

  • Gentle
  • Low “out-of-pocket” cost (existing space, simple equipment)
  • Easy to monitor
  • Collections stay at the institution
  • No risk of over-drying

Cons

  • Risk of mold
  • Very labor intensive – a cost that should not be overlooked in insurance settlements
  • Requires large spaces
  • Extensive rebinding of very wet books should be expected, and expanded shelf-space will be needed.
  • Some materials cannot be successfully air dried, as listed above.

2.  Desiccant air drying

Brief description of treatment 

Desiccant air drying is done in a controlled drying enclosure which a specialist company can assemble onsite, or wet materials can be transported to a chamber at their premises. The chamber has a controlled temperature, controlled relative humidity, and very good air circulation. Large quantities can be dried, largely because the room set-up is very efficient and the air circulation effective enough to support a great density or wet material Objects in the drying enclosure can be manipulated during the process to ensure even drying.

This method is essentially an advance on air drying, in that environmental conditions are closely controlled and a much lower RH can be maintained than would normally be possible without a special chamber. Note that desiccant air drying is not the same as desiccant dehumidification, a method often used to remove moisture from building structures.

What collections materials can be treated this way?

Any collection that can be air dried can be desiccant air dried. Conversely, collections that cannot be successfully air dried are not good candidates for desiccant air drying.

General Procedures

  • A professional service will provide, at your facility or theirs, an enclosed space
  • Controlled temperature (usually 74-80 degrees F)
  • Controlled RH (usually under 20%)
  • Active air circulation
  • The service will ensure that objects in the chamber are moved or rotated to promote even drying

Pros and Cons of this Treatment

Pros

  • Like air drying, this method is gentle
  • Collections can stay at the institution, and access is possible
  • Large quantities can be dried
  • Drying is much faster than plain air drying
  • Somewhat wetter materials can be successfully dried, as compared with air drying

Cons

  • Mold is still possible if care is not taken; Collections that are too large, or too wet, to dry before they will become moldy.
  • Objects are manipulated during drying, and some skill is needed in manipulating objects during drying for a successful result
  • Some materials cannot be successfully air dried, as listed under air drying.

3.  Vacuum Freeze Drying

Brief description of treatment

Vacuum freeze drying is a particularly useful drying method for wet collections. Frozen, wet material is placed inside a vacuum freeze drying chamber at a specialist facility and placed under vacuum while temperature is closely controlled.

This drying method takes advantage of the fact that, at a certain temperature and pressure, ice sublimates directly into a gas without going through a liquid phase. This means a wet artifact or specimen that has been frozen will not become wet again during the vacuum freeze drying process, because the water that is bound in ice turns directly to vapor.

It is this transition from ice directly to vapor that protects the cellular structure of the material being vacuum freeze dried. The lack of a “liquid water” phase avoids additional swelling or distortion, beyond what happened before the materials were frozen, and also stops inks, dyes and other water-soluble media from further bleeding during the drying process.

What collections materials can be treated this way?

Any objects or specimens that can safely be frozen can be vacuum freeze driedincluding leather, textiles, and bird or mammal skins, but these will need careful attention and may need more cleaning after treatment. Vacuum freeze-drying is the only satisfactory method for saturated books and coated papers.

The materials that are generally not appropriate for be vacuum freeze drying include:

  • Acrylic and oil paintings on canvas and panel paintings
  • Materials with friable media (pastel, charcoal, some paints).
  • Research collections, if it is a concern that DNA may be affected by freezing
  • Artifacts with very complex structures incorporating many materials, such as musical instruments
  • Japanese lacquer ware
  • Wet ivory and bone
  • Some plastics may not be appropriate to freeze as they may become embrittled
  • Within photographic collections, if time allows, check for the following materials, which risk permanent damage if frozen: collodion wet plate negatives, cased photographs (daguerreotypes, ambrotypes, pannotypes, tintypes, ferrotypes], glass color transparencies (Autochromes), and lantern slides
  • Although research is lacking, there is some feeling that magnetic media and audio grooved media (cylinders, discs) may be damaged by freezing
  • Herbarium specimens prepared with high moisture acrylic adhesives may become tacky upon freezing and sheets may stick together
  • Bones and wood should generally not be freeze-dried because splitting and cracking may occur. Air drying is the preferred method for these materials.
  • There is concern about freeze-drying photographic prints, although these can be safely frozen. Air drying is the preferred method for these materials.

General Procedures

Wet collections that have been frozen are placed in a commercial freeze drying vacuum chamber. Vapor pressure is reduced inside the chamber to below the triple point of water (4.57 torr/0.6092833 kilopascals). A small amount of heat (around 40.5oC or 105oF) is introduced intermittently into the chamber, causing the ice within the still frozen objects to sublimate out as a gas.

Pros and Cons of this Treatment

Pros

  • This method is suitable for most materials and is especially suitable for large numbers of wet books and records.
  • Because there is no additional wetting, there is no additional shrinking or distortion, and no additional bleeding of unstable colors.
  • Coated papers do not stick to each other
  • Surface dirt tends to fall away

Cons

  • Higher up-front costs than other methods – labor intensive
  • Collections are not accessible during treatment
  • Some distortion still occurs. Physical restraints can be used to limit distortion during drying of books, but these methods are not standard, and do require considerable additional time.
    • Extensive rebinding of very wet books should be expected, and expanded shelf-space will be needed
  • Collections must be transported off-site

4.  Vacuum Thermal Drying

Brief description of treatment

Like vacuum freeze drying, vacuum thermal drying is carried out in a vacuum chamber within a specialist facility. However, vacuum thermal drying is carried out at an elevated temperature (above the triple point of water). For this reason, ice does not sublimate into gas, as in vacuum freeze drying, and the object is exposed to the destructive actions of water as it dries. As a result, physical distortion is likely to occur, and unstable media (dyes, inks, paints) will continue to bleed until the substrate is dry.

What collections materials can be treated this way?

Vacuum thermal drying is not a suitable drying method for museum collections or for permanent retention records. It is a reasonable method to use for materials such as business records, where information must be saved but the material itself is replaceable

General Procedures

Wet or frozen materials are placed in the chamber, and a vacuum is drawn. Heat is introduced raising the temperature to a moderately high level (often around 60oC or 140oF or higher), and humidity within the chamber is gradually lowered as materials dry.

Pros and Cons of this Treatment

Pros

  • Relatively quick
  • Relatively inexpensive, because faster
  • Large amounts can be processed
  • Can help remove smoke odor

Cons

  • Additional wetting and heating allows renewed swelling, bleeding of colors and severe distortion
  • Coated papers stick
  • Water-soluble paints, inks and dyes may bleed
  • Over-drying may occur
  • Organic materials are aged by heat
  • There can be enormous distortion in books. Extensive rebinding of books should be expected, and expanded shelf-space will be needed.

Additional Resources for Stabilizing Wet Collections

Visit the Response & Salvage page of the Museum SOS website

The Conservation OnLine Disaster preparedness and response page provides a useful collection of resources