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Articles by Chris A. Paschke, CPF GCF CMG
"Adhesive for Dry Mounting"
July 1994
Once the decision has been made to eliminate all conservation
procedures and enter the world of mounting or gluing onto a substrate, the need
for a better understanding of mounting adhesives becomes imperative.
Mounting is probably the least glamorous and most
frustrating of all aspects of custom framing. There are so many options,
adhesives and procedures that knowing what to do and when to do it can be
frightening. A successful framer should strive to understand basic procedures,
as well as attempt to keep up with new product development, industry updates and
technical advances.
Adhesives include wet, spray, pressure-sensitive
or heat activated materials. The chemical base or make-up of an adhesive will
often categorize and in turn determine its adaptability or appropriateness for
any given type of mounting. Some
adhesives best adapt to mass production, where speed and permanence is desired,
while others allow for specialized attention during a slower mounting process
and are removable.
ADHESIVE
BASES AND OVERVIEW
Water-based adhesives include, vegetable starch
and PVA, and may be found as both wet and spray glues. Removable vegetable starch glues remain
removable with distilled water even after long term mounting. Non-removable
polyvinyl acetate glues (PVA) are water soluble when wet, but like any acrylic
are permanent once dry.
Rubber or acrylic based solvent adhesives are
found as both pressure-sensitive and spray glues. These are often considered
permanent depending upon the mounting application and technique used.
Then there are the dry mount adhesives. As little
as five years ago production of commercially available dry mount tissues was
held to a small cluster of major manufacturers, while today numerous generic
and private label brands have been introduced into the market. Often the only
differences we consumers detect involve price, but like any competitive
product, a closer look might reflect thinner core tissues, variations in
adhesive colors and perhaps time/temperature adjustments.
HEAT
ACTIVATED ADHESIVES
Dry mounting adhesives are easiest to understand
when broken into specific categories. By taking the time to analyze the various
available tissues, you will be better prepared to select the proper adhesive to
fit your needs.
All heat activated adhesives are identified by: 1)
type of bond (permanent or removable); 2) degree of porosity (breathable or
non-breathable); and 3) physical composition (tissue-core or film). Acidity
levels need to be mentioned but don't require a separate category.
1) TYPE
OF BOND
One of the keys to successful dry mounting is
remembering where the bonding actually occurs. A permanent adhesive bonds
within the press. All layers of the mounting package must reach the required
bonding temperature and remain there during the required time allotment of time
to set the adhesives.
A removable adhesive bonds at the conclusion of
the mounting process, once removed from the press, while it is cooling under a
weight. It becomes removable through the reapplication of heat which reactivates
the adhesive, making the art separable from the mounting substrate. Once again,
permanent adhesives bond inside the press once they reach bonding temperature,
while removable adhesives bond outside of the press as they cool.
Weighting all dry mounted items is an extremely
good "habit" to get into, whether using permanent or removable
adhesives. It establishes correct mounting technique regardless of selected
adhesive, expedites the actual cooling process, and will help reflatten
any newly mounted items during cooling.
Whether an adhesive has a permanent or removable
bond will also determine its appropriate applications. This brings me to the
issue of porosity.
2)
DEGREE OF POROSITY
Porosity is the level of which an item, in this
case an adhesive, is permeable by moisture or air. This is an extremely
important designation when selecting a tissue for compatibility with all
selected mounting materials.
If a non-porous/non-breathable material, such as a
photograph or heavily lacquered print is to be mounted, the adhesive must
remain breathable to allow for air to be forced out and/or through the mounting
layers. By using a non-breathable adhesive with a non-breathable photo, there
is much greater potential for air to be trapped between the two non-porous
items creating bubbles in the completed mounting.
If a breathable piece of art is to be mounted
however, essentially any tissue or film may be used regardless of its porosity
factor, simply because air will always be able to be compressed out through and
around the porous art.
3)
PHYSICAL COMPOSITION
Dry mount adhesives are available in both roll and
pre-cut sheets, and come in two basic compositions, tissue-core and film.
Tissues have a center core or carrier of either breathable tissue (ColorMount,
TM-2, TriMount) or non-breathable glassine (Seal MT-5, TM-1), with adhesive
applied to either side of the carrier for mounting.
Since both sides are identical there is no top or
bottom. They are clean, dry, non-tacky, relatively opaque white in color, and
are also extremely time effective when considering production use. Tissues
adapt extremely well to oversized mountings, float mounting or multiple bite
procedures.
Pure film adhesives (ie:
Fusion 4000, TM-3, Flobond) are a 100% adhesive with no carrier or tissue in
the center. This makes them translucent when unmounted and clear when mounted.
Films may also be pieced or overlapped because of the lack of central carrier
paper, which allows for greater creativity and less waste.
They are perfect for mounting fabrics, wrapping
mats, embossing, controlling color ghosting, tiered matting and numerous other
creative layering techniques where maximum bonding, flexibility and lack of color
is important. Since they are removable, they bond as they cool under a weight
so never attempt to check the mounting by peeling apart the items until they
are cooled.
4)
ACIDITY LEVEL
Nearly all dry mount adhesives are inert, meaning
they contain no harmful acids capable of damage. It is actually the carrier
sheet which needs to be checked for pH levels.
Many manufacturers have developed tissues using acid-free or archival
carrier papers in conjunction with dry mounting adhesives (ArcivalMount, TM-4,
Drichival). These tissues mount at lower temperatures, are breathable,
removable and neutral pH. They are considered more delicate but do not meet
conservation standards.
Using heat activated adhesives can never be
considered archival because the very act of dry mounting art to a substrate
breaks all conservation guidelines. Adhesives travel toward the heat as they
are mounted and the very act of dry mounting encourages a certain percentage of
non-reversible adhesive to penetrate the back of the artwork, even with a
removable tissue. This is NEVER ARCHIVAL regardless of the name of the tissue!
ADHESIVE
SELECTION
So what does all this mean? With dozens of dry
mounting tissues alone on the market how does one ever select the correct
adhesive for his/her specific needs? By researching the basic components or
classifications of assorted tissues and comparing them to the items most
generally mounted in your shop you can determine the best adhesive for your
applications.
Target your specific market by types of art,
porosity factor, sizes, creativity etc., then select two basic adhesives. The
first you will use 80% of the time, the second 20% of the time. No doubt you
will also maintain a repertory of assorted mounting procedures, for those
one-of-a-kind situations. And lest us never forget Japanese hinging.
Always consider the basic factors of removability,
porosity, and composition. By simply
paying closer attention to the items being mounted, their desired appearance
and the substrate selected, each project will tell you which adhesive you
should be using.
So perhaps that paper embossed mat should be
mounted by hand with a water soluble wet glue this time and not a heat
activated adhesive after all!
END