(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.
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.
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.