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Sealing Wood for Storage and Exhibition

Marc A. Williams, a Fellow of the American Institute of Conservation, recently posted some useful advice on the Museum-L list-serve.  Many smaller institutions cannot afford enameled metal shelves and cases for storage.  Instead they use plywood.  If any type of wood is necessary, it must be sealed to prevent the off-gassing of harmful acidic vapors (formaldehyde, for instance, is a common lumber additive) that will damage many types of artifacts.  With his permission, we are sharing Marc’s suggestions here, along with his knowledge about additional sealant materials gleaned from our correspondence with him.   Of course, an individual consultation with a conservator is always ideal, as each institution has a host of specific problems to consider.

 “Assume you have a typical mixed collection found in many historical museums:  You have two general issues.  1) Off-gassing into the air that circulates around.  2) Transfer of harmful materials from contact.”  This can cause acid migration, or staining, for paper and textiles and  corrosion for metals.  Proper storage in acid-free boxes prevents acid migration.  But most often, not all collection items are boxed.  In an exhibition case, the pollutants circulate in a micro environment and can damage artifacts even more quickly and noticeably.  Marc recommends the following “fairly simple, inexpensive and adaptable procedure:”

  1.  Seal the wooden components, be they wood, plywood, particle board, pressed board or anything made of wood.  Seal all surfaces, including undersides, ends and edges.  Apply a minimum of three coats of sealant.  What is the best sealant?  To the best of my knowledge, no studies have been done on sealants for off-gassing.  However, in the 1980s, the US Forest Products Laboratory did a comprehensive study of vapor barrier coatings.  My assumption is that if H2O (water vapor) can not easily penetrate, as it is a rather small molecule, other offending off-gasses will not easily either.  USFPL’s conclusion was that in general, liquid sealants slow down diffusion of gasses, but do not stop them.  Three coats are always better than two coats, which are always better than one coat.  If an opaque coating is acceptable, the best is aluminum flake pigment in almost any binder…Generally, inclusion of a pigment improved performance over a transparent coating.  Shellac proved to be a better barrier than acrylic coatings or oils, which were terrible.  Therefore, for an off-the-shelf product, I would suggest white-pigmented BIN, made by the Zinsser Co…it will be in denatured alcohol as the solvent.  This has the advantage of drying rather quickly, so most of the solvent will be gone a week after application of the final coat.  This is opposed to a month or more for a coating that is in paint thinner as a solvent.
  2.  After three coats and a week of drying, a different paint can be applied if desired as a topcoat.  Use a latex so that the solvent released will be water and the drying will be faster than with paint thinner.  If top coated, wait at least a week before moving objects onto the shelves.
  3. Pad the shelves with 1/8″ polyethylene foam, commonly known as ethafoam.  This can be purchased in long rolls, generally 550 feet, in varying widths.  I like to purchase the perforated foam (at every foot) for easy tearing and installation on shelving.  It saves the hassle of measuring, cutting and getting straight cuts, saving a lot of time and wasted material.  It is relatively inexpensive from places like Consolidated Plastics or Uline.  The foam not only is a barrier from contact, but also pads objects on the shelves, preventing abrasion.

On the relative merits of Marvelseal:  “Nothing beats aluminum film for vapor impermeability.  So, Marvelseal is great as a material.  However, its application is problematic.  How is it applied to bottoms of shelves?  How is it applied to edges and ends?  If these surfaces are not sealed, significant vapors enter the air.  This is even worse in enclosed spaces such as cases…I believe Marvelseal is better than BIN for top surfaces of shelves, except that it does not seal perimeters, edges, ends, and undersides.  Also, it is rather expensive.  If you want a ‘perfect’ solution, use BIN, three coats on all surfaces, then Marvelseal on the tops of shelves, then 1/8″ ethafoam for padding and an additional barrier.  But many smaller institutions can not afford this.  So, if money is the issue, I would skip the Marvelseal.”

 On the relative merits of water-based polyurethane:  “Water-based polyurethane has about the same sealing properties as shellac according to the Forest Products Laboratory research.  However, it does not have hundreds of years of testing to determine stability, as does shellac, and is in water rather than alcohol, which has a much lower incidence of grain raising and a much faster drying time (alcohol, that is).  Shellac is also MUCH more reversible, although that may not matter for shelving.  Plus, the pigment in the BIN provides some degree of additional barrier over the resin alone.  My vote goes for the shellac.”

Please keep Marc’s advice in mind and on hand, as it is equally useful for both exhibit fabrication and for setting up storage areas.