COMPARED TO CONVENTIONAL PHOTOGRAPHIC PROCESSES,
photogravure is vastly more
stable. Among the medium's other exceptional attributes, there is no doubt
permanence was a factor influencing master photographers to invest the time,
effort and expense necessary to produce high quality gravures. By the early 20th
century, when the photogravure rose to such prominence as a reproduction medium
for photographs; the basic components that form the print, oil-based ink and
paper, were well understood from the standpoint of stability. Photographers
welcomed the promise of durability as they were accustomed to grappling with
permanence issues associated with photographic prints.
Though extremely
durable, photogravures are not impervious to damage and gradual deterioration.
Every photogravure, rather every intaglio print and most other works on paper,
remain especially susceptible to deterioration mechanisms broadly classified
under the headings: chemical, physical and biological.
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The chemical degradation of paper is most often catalyzed by long-term exposure
to elevated relative humidity and light. Moisture from the air interacts with
cellulose to break chemical bonds along the polymer chain and to produce acidic
deterioration products that then serve to catalyze additional deterioration.
With time, these changes are usually manifested in the gradual overall darkening
of the paper base and increasing brittleness.
Exposure to light will have a
similar effect caused by the photo oxidation of cellulose and the accumulation
of chemical deterioration products. Temperature is a significant variable
governing the rate of any chemical reaction, including those catalyzed by
moisture and light.
Over the years, museums and other cultural institutions have
developed guidelines to take these factors into account. A common recommendation
for museum storage of works on paper is a range of 30% to 50% relative humidity
(RH) and temperature not to exceed 68°F (20°C).
As damage caused by light is
mainly due to duration of exposure and intensity, most institutions will limit
light levels to 5-9 foot-candles (54 -97 lux) and rotate exhibitions of works on
paper on and off display after several months.
Housing materials, such as mats, interleaving papers, frame backings and boxes
can greatly assist or undermine the preservation of photogravures. Paper-based
enclosures should be lignin-free (lignin is an acidic resin found in wood) and
composed of cotton fiber ("rag") or highly purified wood pulp.
The addition of
an alkaline buffer to paper enclosures is recommended to help counteract
acid-based deterioration. The importance of good quality interleaving paper for
intaglio prints in storage is essential as the linseed oil ink medium used for
these prints can be fairly acidic and cause localized staining among stacked
prints. Plastic sleeves, used in addition to or as an alternative to paper
enclosures, should be composed of chemically inert materials such as polyester,
polypropylene or polyethylene.
Plastic sleeves of unknown composition should be
discarded. Whenever possible, handling should be limited to the print enclosure
as finger oils can cause staining over time and embed surface dirt.
If prints do
require direct handling, the use of white cotton gloves is recommended.
On
display, frames should incorporate a glazing material that filters for
ultraviolet.
These higher energy wavelengths (relative to visible light)
catalyze proportionally greater degrees of deterioration.
Good quality housing
materials, including frames, museum cases and other storage boxes also can
significantly reduce the impact of fluctuations in relatively humidity. This
attribute can be a significant benefit to the private collector that is
unequipped to maintain a constant level of moderate relative humidity.
PHYSICAL DETERIORATION OF PRINTS
typically refers to problems such as tears,
folds and distortions. In most cases, tears and folds are the result of careless
handling. In general, prints without mats should be handled using a rigid
secondary support, such as a piece of good quality mat board.
Distortions,
either local wrinkles or overall warping and buckling, can be the result of
improper storage materials, but also due to fluctuations in relative humidity.
Paper expands and contracts in response to moisture content in the air. Adhesion
of a photogravure to a secondary support can restrict these dimensional changes
and cause severe local distortions.
These distortions are very common in
Camera Work and other photogravures where adhesive is placed in a small
area behind each of the four corners of the print.
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| 10X In photogravure the etching of the image takes place in one step, the depth of the etching around the aquatint grains being proportional to the depth of the gelatin relief image on the surface of the printing plate. This produces a final print with a smooth gradation of image tones. |
BIOLOGICAL DETERIORATION IS THE RESULT OF ATTACK
by flora and fauna. Paper can be
a very inviting material for all manner of insect and rodents.
Preventing such
infestations usually centers on creating as inhospitable of an environment as
possible by keeping storage areas clean and free of moisture and food. Possibly
of greater concern for the collector, various species fungi can exploit trace
inorganic components in paper to sustain their metabolism.
Such interactions can
eventually cause local red/brown stains to develop on paper commonly referred to
as "foxing." Again, the key to prevention is maintaining cool, dry, conditions
as fungal growth accelerates as temperature increases and at humidity levels
consistently above 60%.
... THE DETERIORATION MECHANISMS MENTIONED ABOVE
have been studied in depth by paper
conservators and conservation scientists resulting in a great deal of literature
from which research additional preservation guidelines and best practices can be
distilled.
From this work, numerous paper conservation treatments have emerged
for stabilizing the condition of prints. For staining caused by chemical and
biological deterioration, when washing in deionized water, may be extremely
useful to stabilize the condition of the print and reduce stains. For greater
stain reduction, bleaching treatments can be effective; though these treatments
need to be applied judiciously as there is a risk of causing additional
deterioration. An alkaline reserve, such as calcium carbonate, can be applied to
the paper to slow the rate of acid-based deterioration. Physical deterioration
like tears can be stabilized through the application of fine strips of Japanese
paper to the reverse of prints, generally using adhesives like wheat starch
paste and/or methylcellulose. Distortions can be reduced by imparting moisture
to the paper then gradually drying between smooth blotters, under weights.
Intrinsically, good quality materials used to make most photogravures is a
tremendous advantage, if conservation treatment is necessary.
For most paper
conservators, the techniques and materials used to treate photogravures are
relatively conventional and time-tested. However, conservation treatment should
never be approached casually and should be carried out by an experienced
professional.
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