Volume 2, Number 2, May 1980, pp. 1-3.

Technical Exchange

Various authors

UFO's--Unidentified Filtering Objects

Most members of WAAC recently received a mailing of information on plastics from Gem-O-Lite of North Hollywood, CA. This mailing was requested after a meeting between the company and Rohm and Haas representatives with members of the LACMA staff. Gem-O-Lite is seeking input from conservators as to which plexiglas products it should stock in order to make small quantities available to individuals. During discussions we asked for an explanation about the deletion of the old standby, UF-1 Plexiglas. By studying the ultraviolet transmittance curves we suspected that we might simply be faced with a shift of designations, i.e. UF-1 becomes the new UF-3 and old UF-3 becomes the new UF-4, especially as the new UF-3 is clear and the new UF4 has yellow coloration resembling old UF-3. The Rohm and Haas representative claimed that the formulations were all new, but to date, they have not provided more detailed technical data as requested.

We have encountered conflicting reports on the existence and efficacy of other manufacturers' UV filtering plastics and are currently seeking samples to run comparative tests. Please contact the Conservation Center at LACMA if you have information, sources, or samples on UV filtering plexiglas substitutes.

Optically Coated Glass

For conservators who may have heard of new products for glazing paintings which promise non-reflective or reduced reflection capabilities, but whose experience has been limited to "frosted" or etched surfaces that distort and obscure the image, products such as Denglas and Invisiglas do, in fact, work.

Optical coatings have been in existence in the glass industry for many years. Virtually all camera lenses in the moderate to high price range are coated. Because the coating technology was developed for small surface areas, it is extremely expensive to adapt methods of vacuum deposition for use on larger surfaces as required for glazing. One manufacturer, however, has been able to produce large sheets by chemical deposition, essentially dipping the glass.


Whenever light must pass through two transparent materials of differing refractive indices (n) some amount of reflection must occur. The index of refraction is the basic measure of light in a medium. In a vacuum it has been given the value of 1.00, and is essentially the same in the atmosphere. But when we look at a material such as glass, the index becomes 1.51 meaning that light through glass travels about two thirds the speed of light in air. We can actually calculate the amount of reflectance by employing the following formula:

    (n.s  -  n.o) x (n.s  -  n.o)
    _____________________________      =     R

           (n.s  -  n.o)

    n.s equals the index of glass and
    n.o equals the index for air

By substituting the values for air and glass, we arrive at the percent of reflectance being four percent from the first interface of air/glass and four percent from the second interface of glass/air resulting in a total eight percent reflectance.

This 8% becomes very important when we begin to put glass or plexiglas over darker and darker paintings. The 8% value remains constant; however, darker and darker pictures will generally reflect less incident light so that the percent of light lost in reflectance off a glazing material becomes percentage-wise greater in comparison to the total amount of light reaching the human eye. In areas that are almost black, a nearly equal amount of light will be reflected as will be visible from the painting surface. In instances such as this, viewing may be hopeless.

If you can put a coating upon glass which is equal to in amplitude but opposite in phase to the reflection, the two reflections will cancel each other out and reduce the total perceived reflection. (Figure 1 illustrates this. <graphic status=omitted>)

For reasons that go beyond this report, we cannot explain all the physics, but we can say that for a single coating the characteristics of phase and amplitude can be ideally met with glass with a material having a refractive index of 1.23. But no double materials exist with this index.

Fluorides have been used in single-layer coatings and magnesium fluoride (MgF2) has the closest index to the theoretical ideal, 1.38. If the proper thickness is deposited, the overall reflectance throughout the visible spectrum is about 1.7%. The eye, however, is very sensitive and a reduction down to 1.7% is not satisfactory in picture glazing.

Denglas has employed a three layer coating consisting entirely of titanium dioxide and quartz. (Some ultraviolet filtration is measurable--possibly due to the TiO2). This three layer coating permits reduction across the visible spectrum to less than 1% reflectance. (Figure 2. )

There are some negative results which have been reported due to the fact that a slight color cast results, but this color distortion is not any worse than old UF-3 plexiglas (new UF-4) and is generally more pleasing. (As yet, plastic materials with the exception of polycarbonates, have not been demonstrated to be able to withstand the kiln firing to at least 350 degrees centigrade used in the process of making Invisiglas. As far as I know, Denton Vacuum is not actively pursuing plastic coatings.

Denglas would appear to have two practical advantages over Invisiglas: It is significantly cheaper and it is available in larger sizes. (Denglas sizes: 50" x 84", Invisiglas sizes: 30" x 40"). We have had no problems with cutting either type of sheet (the total thickness of the coatings is about 3000 Angstroms) nor in cleaning them. Care must be taken in cleaning however, because sloppy cleaning is not "forgiven" as is the case with normal glass. Delamination has not been observed nor has any color changes been recorded.

At the Norton Simon Museum we have extensively tested both Invisiglas and Denglas on about 50 paintings for four years. The use of these materials routinely fools observers who question why we cover some paintings and not others. We feel that optical coatings present us with an excellent compromise between protection of the works of art and maximum visibility.

Jim Druzik

Tableaux Paper

The Conservation Center of LACMA has been using a paper for a number of years in non-direct paper conservation applications, such as wrapping honeycomb panels and as a dust liner on the back of frames. This paper, known as Jefco Tableaux Paper, has proven to have great wet strength and durability in these applications and raised our curiosity as to its long term stability, fiber content and possible future use in more contact applications for paper art works.

As reported in earlier Newsletters, Jim Druzik of the Norton Simon Museum in Pasadena has generously offered to test papers and boards for interested parties--and so we sent him sheets of Tableaux paper, untreated and treated in a variety of ways. The following is a report of his analysis:

Technical Papers
29 Franklin
Needham Heights, MA 02194


Lining panels, secondary supports, any application where a support paper requires an unusually high wet strength capability.

(Reference ph=5.65 for distilled H20)
  SET 1                SET 2             SET 3
  5.95                 6.10              5.05
  5.90                 6.10              5.05
  5.90                 6.15              4.95
  5.85                 6.00              4.85
  5.85                 6.05              4.80


Negative for alum, rosin, starch, PEI, DAS.
Positive for formaldehyde.


Magnesium Methyl Carbonate ph=10.10
Magnesium Bicarbonate ph=9.60


The Tableaux Paper has been found to fall within the range satisfactory in the conservation of paper, if pre-treated with either magnesium methyl carbonate, magnesium bicarbonate, or magnesium acetate. The excellent wet-strength capability renders it useful for water bath support, in that it transmits water quickly remaining fully supportive. To further examine the proper mode of deacidification prior to use, four samples were prepared:

  1. Soaked two hours in tap water, air dried, brushed with Wei T'o
  2. Soaked 2 hr in distilled water, air dried, buffered w/ Wei T'o
  3. Soaked in magnesium bicarbonate solution only
  4. Buffered with Wei T'o only

The four samples were then subjected to IC analysis and the results plotted as logarithmic functions. The method of ICA is to prepare nine samples of HCl of specified hydrogen ion concentrations. The "buffer capacity" of a paper being defined as the degree a buffered paper sample can neutralize increased aqueous acidities while its mass remains constant.

The graph shows that all three samples of Wei T'o treated Tableaux behaved almost identically regardless of other treatment. All four papers were seen to maintain their internal alkalinity through 2.8 with Wei T'o maintaining alkalinity through 2.3. (It is thought unlikely that more severe conditions than this would exist in reality.)

Some concern exists in that SET 3 cold extraction values are significantly lower than the other two sets (taken from a different roll). However, all samples responded identically to deacidification.

Carbonate percent by weight was 1.1% for magnesium bicarbonate and averaging 3.5% for samples treated with Wei T'o.

All four samples were taken to 360 hours of accelerated aging in a dry oven at 100 degrees Centigrade. This approximates a minimal duration of 150+ years. All samples showed yellowing with aqueous pretreatments though showing less yellowing than Wei T'o buffering alone. Fold endurance loss was just detectable.

Subsequent aging with untreated Tableaux to 300+ years indicated that after an initially small brightness reversion, color reversion for all treatments stabilized.

VDP analysis (Volatile Decomposition Products analysis) will take some time to complete. but initial indications seem to suggest that all total volatiles (acidic) equal the acidity of the starting sample (accumulated acidities have not yet been demonstrated for small enclosed spaces).

The sizing agent is still somewhat in doubt since colorimetric determinations are always subject to some personal interpretation. However, the presence of a high concentration of reducing groups within the paper furnish points very strongly towards the existence of formaldehyde.

Experimentally, three papers were opened in a blender and treated with potassium permanganate. One paper had known alum/rosin sizing, one paper was unknown and the third paper was Tableaux. All three papers reduced the permanganate to a brown manganese dioxide within ten minutes. Upon addition of more KMnO4, rosin was no longer available in the two papers containing it and dilute oxidation of cellulose proceeded at a slow rate. However, the Tableaux Paper continued reducing the permanganate rapidly until color distinctions were hard to make.

graphic here shows the potassium permanganate reaction

Presuming that no other strong reducing agents occur in the paper to be tested: the oxidation of formaldehyde to formic acid is a fairly good method. This method does not carry a TAPPI designation.

We find Jim Druzik's results very supportive for the use of Tableaux Paper, after deacidification using either an aqueous solution such as magnesium bicarbonate or a nonaqueous treatment such as Wei T'o . The advantages of Tableaux are its great wet strength, flexibility due to the long bast fibers, and the fact that it comes in rolls of 40 inches wide (any length up to 144 yards). LACMA orders it from McManus and Morgan, but it is available through other art supply stores. Prices: 40" x 1O yards--$9.95, 40"x 25 yards--$23.75, 40"x 50 yards--$45.00.

We would like your comments on this product and welcome your correspondence.

New Tool for Edge Stripping

For those WAAC members who use the black poly strip which is attached to a painting's tacking edge as a spacer, I have devised a jig for cutting the width of this material to accommodate thinner paintings. It is a simple tool that most people can easily build. If you will send me a self addressed stamped envelope, I will send you a drawing of the device. Write to: Jack Lucas, Lucas Conservation Laboratory, 2015 Todd Road, Vancouver, Washington 98661.

ASIFA--Hollywood Offers Special Service to Public

The International Animated Film Society--ASlFA-Hollywood has announced the availability of their "Search and Rescue" team to the general public, according to Ron Stark, Vice President in change of animation art. "Many collectors and galleries have called upon us to find ways of restoring animation art to its original state. And because many of our members are experienced ink and paint craftspeople, we are now able to offer this same service to everyone." Stark explained. Animation art, called "cels," is produced on a clear acetate sheet and either inked or xerographed on the front and painted on the back.

"The composition of the inks and paints in use has changed over the years," said Charles Solomon, ASIFA's archivist. "Years ago india ink and water-soluble paint was used to transfer the drawing to a cel. Now, vinyl inks and paints are used. The water products would, after a few months, crack and flake off. The vinyl products we use will not." The price of restoring animation artwork is dependent upon the amount of work needed to bring it "back to life." "We are fortunate to have as our contributing sponsor the Cartoon Colour Company. With their help we are able to provide such a service at minimal cost," Stark said. The Society is made up of members of the animation industry and allied arts as well as devotees of the medium. The International Animated Film Society is a non-profit organization dedicated to encouraging excellence in and universal understanding through the art of animation. Those interested in the services of the Society should write or call ASIFA-Hollywood, Search and Rescue Team, International Animated Film Society, 357 N. Sprucewood Avenue, Agoura, California 91301 (213) 991-9955.

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Timestamp: Thursday, 11-Dec-2008 13:02:25 PST
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