Mounting Adhesives

(Excerpt from ART HARDWARE: The Definitive Guide to  Artists’ Materials, by Steven Saitzyk © 1987)

There are many adhesives that may be used in the  construction of a piece of artwork, including methylcellulose, epoxy, PVA, and  acrylic polymers. The adhesives discussed here are generally accepted as safe  and effective for general use in picture framing.

Water-Based Adhesives

There are many water-based adhesives available  for the purpose of mounting paper, paper products, canvas, and fabrics with a  natural fiber content of 50 percent or more. Animal glues, PVA (polyvinyl  acetate, wheat or rice paste, and methyl cellulose are among the pastes most  commonly used by framers.

Animal Glues are generally made from the gelatinous tissues found in the bones, skins,  and intestines. Animal glues are used because they are inexpen­sive, have a  longer shelf life than vegetable adhesives, and are usually com­paratively low  in water content (less moisture is advantageous because it means the paper tends  to buckle less during application). Animal glues are not, how-ever, considered  archival because they tend to yellow, acidify, and crystallize (become brittle)  with age. Increased awareness of these drawbacks has greatly diminished the use  of animal glues.

PVA Emulsions, commonly known as “white glues,” were the precursor to the development of  acrylic polymer emulsions and paints.’ Their advantages include low cost, a long  shelf life of several years, and easy application. Some undesirable  characteristics of PVA adhesives are that they are not water-soluble when fully  dry, and they contain a larger percentage of water than animal glues. This high  degree of moisture contributes to the buckling and wrinkling of paper during  mounting. Alcohol can be used to redissolve the adhesive within the first few  years of application. The lack of water reversibility prevents this adhesive  from being classed as archival.

Wheat Paste and Rice Paste are starch pastes, which have to be made fresh  each time before use. (A recipe is give in the section on hinging.) These  vegetable pastes are water-soluble, archival, and therefore safe for hinging,  mounting, and repair­ing paper or paper products used in making fine art.  Vegetable pastes tend to be more hydroscopic than animal pastes, which means  that vegetable pastes tend to retain more water per weight of paste, and when  the paste is used it will tend to impart that moisture to the paper or board  being mounted. This may lead to more buckling and difficulty in mounting.  Vegetable pastes form a stronger bond with paper products than animal pastes.  Some control can be exercised over the moisture by using less paste, thereby  reducing the tendency of the paper to buckle. Beware of ready-made wheat or rice  pastes, which have a very short shelf life and may become acidic.

Methylcellulose is like a vegetable paste with the vegetable part completely removed. It is pure  adhesive. It mixes easily with cold water and the resulting substance looks very  much like colorless Jello. It is nonstaining, water-reversible, pH neutral, and  completely archival. The only drawback is that methylcellulose is very  hydroscopic, and this high content of water can easily be imparted to the paper  or board used, causing wrinkling and buckling. Most peo­ple use far too much of  this adhesive during application. Since this material is nothing but adhesive  and water, very little is needed. Methylcellulose is new on the market, and  although it makes an ideal paste for archival mounting, some experimentation and  practice is needed to learn how to keep problems of moisture to a minimum.

Lineco Inc. and Seal Inc. have recently offered waterborne, water-reversible,  neutral pH adhesives that are ready to use without special preparations, and  that they claim meet archival standards. Lineco’s adhesive is made of a modified  dextrine base, which is reactivated when wet. If these products gain acceptance  they will certainly ease the difficulty of wet mounting.

Spray Adhesives

Always read the instructions on the container of  any spray adhesive before use. This cannot be emphasized too strongly.

There are many factors that can affect the  quality of the bonding when using a spray adhesive. The most important ones are  texture, moisture, temperature, and the flatness of the object(s) to be mounted.  Texture can be a problem if insuffi­cient adhesive is used and inadequate  pressure is applied. The “valleys” as well as the “hills” in the texture of the  surface have to be part of the bonding process if the mounted object is to be  held permanently.

The moisture content and temperature of the  objects to be mounted together have to be the same or they may expand or  contract differently from one another and cause adhesive failure. This simple  problem can be avoided by storing the objects to be mounted in the same location  for several hours before and after mounting. If you have to soften the object to  be mounted by humidifying it, then humidify the mount, too.

In spray mounting, the adhesive does not set as  quickly as it does with heat-activated dry mount tissue because the solvent has  to evaporate fully before the adhesive can bond firmly. Nonporous materials  inhibit this process slightly. Wrinkles and fold lines may exhibit “memory” for  those blemishes and pull away from the mounted surface before the adhesive can  bond firmly. Therefore, it is very important that the object to be mounted be as  flat as possible before mounting. The process of flattening is referred to as  “stress-relief.” Stress relief may be accomplished by using the heat from a dry  mount press, or humidifying in a vacuum mount press, or using weights or weights  with moisture.

The spray adhesives produced by 3M are currently  the industry standard; thus its products are useful for the purpose of  discussion. Recently, 3M changed the names of a number of the sprays; therefore  if you have older products, re-read the information provided on the can before  use. The two sprays that are accepted for use in picture framing have not been  changed. They are Photomount No. 6094 and Vacumount No. 6096. The other sprays  are for hobbies, graphics, and industrial uses and they are not recommended for  use in picture framing with fine artwork. Photomount and Vacumount are  nonyellowing with a pH range of 6.8 to 7.0. They have performed well under  accelerated aging tests. They are not water-reversible and are therefore not  considered archival in the classical sense. Because of this and because  accelerated aging tests may not accurately predict what will happen in fifty to  one hundred years, 3M will not recommend them for conservation mounting.

There are a number of small but significant  differences between Photomount and Vacumount. Photomount is designed to be used  on artwork 16″ x20″ or less without the use of expensive equipment. The name  “Photomount” would seem to imply that the sole intended use of this product is  for mounting photographs, but this is not the case. Photomount is a versatile  adhesive, intended for multi-purpose use. Other than the size limitation, the  only other restriction would be with the use of oil- or resin-impregnated paper,  or boards such as chipboard or newsboard, tempered Masonite, tracing papers such  as Clearprint, and genuine parchments. (I have also had poor results with  all-plastic foam boards). Photo-mount has an “open time” of two minutes. This  means that you have up to two minutes to position the artwork before the  adhesive sets.

Vacumount is basically the same as Photomount  except that its size limitation is determined only by the size of the vacuum  press you are using, and the open time is extended up to ten minutes for  positioning. Vacumount does have a higher bonding strength than Photomount, but  you cannot take advantage of this without the use of a vacuum press.

Removal of items mounted with either of these  sprays may be accomplished with the use of heat or a solvent such as  rubber-cement thinner or lighter fluid. You may also find an extra pair of hands  necessary to accomplish the recom­mended techniques. 3M recommends that you lay  the mounted object down on a table and use a commercial heat gun or a hair dryer  to heat one of the corners. The heat source should be 4 inches or more away from  the object. A thin spatula or knife can be worked carefully under the corner to  begin the process of lifting off the mounted piece. Continue to lift the corner  while heating the point of removal. The object should be lifted off during this  heating process, not peeled off.

As for the solvent method of removal, 3M  recommends that you stand the mounted object in a vertical position and drip  some solvent onto the corner. After it has soaked into the corner, begin to lift  off the object with a thin spatula or knife. Continue to drip solvent into the  point of removal and lift at the same time. After removal, you may wish to wash  off all traces of the adhesive with the solvent.

Although these methods are effective, it is  difficult to accomplish them with-out affecting the object. However, this would  be true of virtually any removal process; the challenge is to minimize that.

These aerosol spray adhesives and solvents  present a serious health hazard if used without proper precautions.