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Textile conservator, Gwen Spicer of Spicer Art Conservation at work

Wednesday, October 2, 2013

What is acid-free?

by Gwen Spicer

October is "Archives Month" and we at Spicer Art Conservation, LLC are celebrating the month by discussing some storage topics. In an earlier post we talked some about environment (see "It's Spring and it's Dry?").  This post will focus on selecting the materials which are within closest proximity to your artifacts.  Whether it be storage, support, or display materials, these materials will almost always be in contact with an artifact and should always be acid-free.  

Care is needed when selecting materials used for storage. The materials selected can either assist in the preservation or deter it. Poor quality materials can cause acidity, off-gas, and discoloring (among other effects), that deteriorate collections. The results of poor quality materials are not immediately observable and so their damaging affects will not be apparent until it is too late and the damage has been irreversibly done.  To prevent these types of damage, good quality and acid-free materials should be used.
archival materials,support of artifacts, art conservator, acid free storage, museum collection care
An acid-free box with "artifacts" waiting to be stored.  The tray is made of ethafoam, the lifting handles are made of twill tape.  All items are acid-free, of course.

So what is acid-free?  They are materials that are a neutral pH of 7.  There are two methods to reach this pH 7. One, is to begin with naturally, inherently neutral, pH materials, like 100% cotton fiber.  The second method is to begin with a wood pulp or other acidic material and buffer it until it reaches a pH 7. The disadvantage of buffered wood pulp or any other acidic material is that the buffering eventually becomes exhausted and then the product returns to being acidic. Buffering can be done in more than one way, yet each way will always result in temporary buffering. Calcium carbonate or other buffering agents can work, but how long do they last?  Is the length of its buffering ability effected by the environment that it is stored?  Too many unknowns exist in using this method, best to not use it at all.

Yes, archival materials are expensive, which is a result of their being made from pure-virgin materials. As framers, collectors, and other private owners of art become aware of the benefits of high-quality materials, the buying audience becomes larger, thus the price does drop. The down-side to true archival,  pure-virgin materials is that is not conducive to the new "green" society that we are moving to where recycling is encouraged. To find out more on sustainable practices read about it on the AIC wiki page by clicking the link.

But back to storage using archival materials. An important part of safe storage is using safe materials that do not cause further damage. For example, direct acidic contact causes yellowing and localized brittlement. The shelf that the box rests on, if wooden, can also affect the archival integrity of the box which houses the artifact over time.

pH indicator pens can be purchased from any archival supplier.
The use of a pH pen is a good idea for testing your shipments of archival materials or the materials you already own.  The reason for this is that one can not be sure of purity, particularly of archival materials that are old.  The paper and boards used are absorbent and over time are effected by their environment.  So, if an acid-free box is sitting on a uncoated wooden shelf, over time the bottom of the box will have absorbed the acidity of the wood and will no longer be acid-free. Even the inherently acid-free nature of cotton rag cannot stay acid-free in that environment.  Therefore, the pen will give you an indication of when boxes or other storage materials might need to be replaced. Unfortunately, no FDA or other “watch-dogs” exists for archival materials. So, it is the buyer who must beware. I always use the motto of “if it is too good to be true, then it is.”
Wood remains a common material in storage construction, due to its cost and ease of use. However, wood has inherent volatile compounds which off-gas and cause damage to organic materials. Each wood specie has a different rate, hard woods emit (especially oak, for example) more than soft woods. Newly milled wood emits more than older wood. But it is important to remember that wood emits acid throughout its life, until it is  fully degraded and no longer viable for any purpose. If new wood needs to be used, only Poplar is recommended. It is the lowest acid-emitting wood available. Wood is not just in furniture, be aware of it's use in other ways: the jacket below for example.

historic textiles, garments, art conservators, why to use padded hangers, wooden hangers are bad
The evidence of the wooden hangar that this coat hung from is unmistakeable.

As stated earlier, no liquid applied barrier fully prevents acid migration, only an applied film will do so. Mylar and Marlvelseal can easily be used to line wood shelves or the inside of wooden drawers.
Marlvelseal is a foil composed of layers of nylon, aluminum and polyethylene. It provides a good barrier that resists transmission of water vapor and gases. The polyethylene side or dull side can be heat-sealed and can conform to curved shapes with a tacking iron. Use Marlvelseal to seal questionable materials for display and storage.
Mylar is a polyester, transparent film, that is a strong, inert, and dimensionally stable film that is very clear. It comes in different thicknesses. Look for virgin polyester, type-D made by the DuPont Company. It is helpful to note that Mylar does create a static charge. It is most commonly used to encapsulate and separate materials by interleaving.

Marvelseal, archival materials, art conservation of textiles, prevention of acid migration,
A wooden drawer lined with a layer of Marlvelseal. It is a true barrier
 against the migration of acids from the wooden drawer. All of the
sides and bottom of the drawer are covered.  The tape used to secure it to the drawer is linen tape.

Now onto the drawer itself and the rest of the storage containers and furnishings. Whether you use drawers, shelves of a combination of both, the selection of materials for these units or any other storage furniture is performed with consideration of materials that do not off-gas or cause other damages to the collection. Safe materials for furniture include aluminum with a powder coating, anodized aluminum, and steel with powder coating.

proper storage of artifacts, heirlooms and collectibles, art conservation of objects.
Sometimes acid-free tissue is not enough. Compare the knives side by side above.  the one on the right was wrapped in acid-free tissue, the one on the left was wrapped in Pacific Cloth, a cotton-flannel with scavengers (more on scavengers later) which remove the sulfur dioxide from the air and prevent tarnishing.

Certainly not everything in a collection fits neatly into boxes, on standard sized shelves, or into drawers.  These items, rugs, large flags, tapestries, and large textiles to name a few, often require rolling onto tubes and storage of those tubes on acid-free supports. At times, acid-free tubes are not used due to budget restraints, in this case, a layer of Mylar or Marvelseal over the standard cardboard tube is required. Mylar is secured to the tube with double-sided d-tapes, (3M #415). Marvelseal can be heat sealed to any surface. These materials are barriers against the migration of acidic products from the standard tube to the artifact. A word of caution, while they may seem like a good idea and cost effective, liquid shellacs and paints do not sufficiently provide the same amount of protection, and should be avoided. Think of the old adage: "Do it right the first time".

art conservation using safe materials, decomposition of plastic over time, dry cleaning bags are bad for artifacts and storage of garments of historic costumes and textiles
An example of ordinary plastic dry-cleaners bags over time. These pieces are yellow
and a crumbling due to their brittleness. 

Above I mentioned a diversion from "green" practices, when we discussed virgin materials. I will make up for that here by suggesting a very green practice: Substituting acid-free tissue with washed cotton muslin or cotton sheets. Cotton sheets make excellent handling slings, they provide support and diminish the direct handling of an object during transporting. The benefit of their use as handling slings is that they can be washed and reused, whereas the tissue is just thrown out.

Another great material is Tyvek. Tyvek is spun bonded olefin which is a high-density polyethylene fiber bonded under intense heat and pressure, manufactured by DuPont. This product has many good qualities: it is strong, non-buffered, has resistance to water and mold. Tyvek both protects artifacts from dust while allowing for air circulation. It is smooth with no binders, fillers or buffers. It is low linting, and resistant to water and chemical-aging. It comes in a range of weights. The weight that is commonly used is Type 1443R. This material can be washed in the washing machine several times. It is good for dust covers and wrapping collections. Can be seamed or glued. However, while it is a good acid-free material, it is not a good barrier for preventing acid migration. Also, some solvents cause swelling. Tyvek used in archival applications should never be confused with Tyvek Housewrap, used in the building industry, as house wrap may contain ultra-violet stabilizers and coatings.

Gwen Spicer is a textile conservator in private practice.  Spicer Art Conservation specializes in textile conservation, object conservation, and the conservation of works on paper.  Gwen's innovative treatment and mounting of flags and textiles is unrivaled.   To contact her, please visit her website.


  1. Can i use tyvek home wrap to wrap a tube? Our tubes are not acid free.

  2. Hello thewitcher. If you look again at the last sentence of the article it specifically cautions about using "house wrap". That type of Tyvek is not the same and it typically has coatings and stabilizers applied to it which would make it very NON ARCHIVAL. Go online and look for Tyvek Type 1443R from a reputable conservation material supplier: Gaylord, University Products or Talas are those that I would recommend. Happy tube making!

  3. I know you said there is no liquid coating that will block acid from wood, but is there a coating or varnish that will do at least an ok job?
    I made a small wooden box (with walnut) for valuable paying cards stored in plastic sleeves.
    I'd like to prevent them from being damaged by acid. Although, maybe the plastic sleeves are good enough protection on their own?

    1. Nate,
      You could use 3 coats of water-born polyurethane or shellac. The better news of wood choice is that you used walnut, which is not the worst acid emitter (oak is the worst). The best barrier to the cards is mylar or marvelseal (see our post on glossary of safe materials if in doubt) , but you are using some sort of sleeve as a barrier and that is always a good idea.

  4. Dear Gwen, it is very helpful to read your recommendation of POPLAR as the least-acidic wood available for cabinetry work. I have been searching online (for a few hours) for a trustworthy pH comparison chart of wood lumber, because I need to know about several other contenders for a forthcoming project that involves an edition of 60 wood containers. So far, the best source is a list of a few woods (http://www.npl.co.uk/upload/pdf/corrosion_of_metals_by_wood.pdf), but I'm looking for something more comprehensive. I need to make the best choice for the wood container of a fine press artist's book of the Song of Songs.

  5. Hi Gwen,
    I'm working on a large mosaic using mat board tiles and I need a stable surface to glue the tiles to. I was thinking of using wood, but now I realize that all wood contains acid. Will the acid eventually deteriorate the glue? Is there another type of board I could use that is acid-free and not flexible? Thanks

  6. Thats a really good question. I tried fiberglass over acid free framing mat, but you need a lot of it to make it as rigid as wood. If you read the pdf in the above comment it implies that acid can be removed from wood by storing it cold and moist fir 2 years, or just normaly for 20 years.

  7. hi, what an interesting entry you have written.
    i have a question which might be kind of dumb but here goes...
    i just purchased some acid free paper sheets to wrap linens in, and wanted to iron it to get the creases out, so the packages were as attractive as possible.
    will ironing the paper alter the ph in any way?
    thanks very much -

    1. Dear Heidi,
      I have not thought about ironing tissue paper. But my guess is that if the paper is made from 'rag' that the pH would not change. I would have the iron set at a lower temperature.

      At another time, you might want to think about wrapping your linens in muslin or old sheets.