Volume 14, Number 3, Sept 1992, pp.30-31

Technical Exchange

Walter Henry, column editor

Seismic Protection for Shelving

After the Loma Prieta earthquake, an event which tossed several hundred thousand volumes to the floors of Stanford University's libraries, a Stanford graduate at the Center for Design Research developed the Seismoguard, a device to prevent books from falling from shelves during a quake. It makes use of a steel retention bar on brackets, which fit onto ordinary library shelving.


The retention bar is suspended out of the way of the books and held in place at the shelf just above it, allowing easy access to the books. During an event, the bar drops down in front of the books and prevents them from falling off the shelf. After a quake, all that is necessary to do is to return the bar to its "ready" position.

The device has been tested on a sinusoidal shaking table at Stanford's Blume Engineering Earthquake Center and shown to activate at 0.007 g's, before the point at which books are shaken from the shelves. The Seismoguard was also shown to withstand prolonged extreme inertial loading with no damage either to itself or the books. Only a severe force will trigger the Seismoguard so there is little risk of accidental activation.

The device is not yet available commercially. For information on manufacturing and sales licence get in touch with:

Luis Mejia
Office of Technology Licensing
Stanford University Stanford, CA 94305
415 723-0651

Reactive Polymers

At AT&T Bell Labs, John P. Franey has developed Intercept, a reactive polymer system incorporating scavenger molecules to neutralize corrosive gases passing through a polymer. As gases migrate through semi-permeable films and resins, they react with "a copper-like material that is bound up in the polymer".

According to an item in Science News [41(20): 321-336 May 16, 1992]

"...After five years in a reactive polymer bag, a silver spoon looked as if it had been polished that day, while a similar spoon not bagged for that time was black with tarnish.... The polymer darkens as pollutants exhaust its scavenging ability, but Bell Labs' studies show that a bag no thicker than a plastic garbage bag would last about 30 years before needing to be replaced."

Plastic Media Blasting

PMB (Plastic Media Blasting) is based on using removal media designed to be harder than the coating on the surface but softer than the surface. Example: paint on a steel auto body. The steel is approximately 5-6 on the Mohs scale of hardness. Anything harder than 6.0, such as quartz (8.0), will scratch the steel auto body. So the problem is to select a media which is under 5.0 in hardness.

Before the early 1980s, the most popular blasting media were walnut shells (2.5-4.5 on the Mohs scale), glass beads(5.0- 6.5),and sand (quartz) (8.0). Having walnut shells varying so greatly in hardness, they can damage fiberglass and aluminum substrates. It was during this period of time that the aerospace industry voiced concerns about paint removal on their products. A full line of engineered plastics have been evolving during the last 7 or so years and are in use as plastic media blasting.

The different plastic media are more expensive than the old blasting media. Delivery equipment was created to allow storage, reclaiming, and dust separation. The number of recycles depends on the nature of coating being removed; 8 and 20 recycles are common.

Principal differences between sand blasting and plastic media blasting are that sand is shot at 60-100 psi nozzle pressures and PMB is shot at 10-40 psi. Sand is very aggressive; it can build up heat causing metal warpage. PMB is less aggressive (i.e., it is softer than the intended substrate) and will not cause warpage. PMB uses high volume air compressors having a minimum of 50 hp, capable of delivery rates of 170 cfm.

Besides aerospace and automotive applications, techniques have been demonstrated to conservators for use in sculpture restoration. Paint, patina, environmental accretions can all be removed even in detail areas for the sculpture, leaving the metal untouched and looking as it did after casting. This equipment has been designed to work on delicate materials. A trained operator can make control adjustments as the removal process uncovers delicate areas on the workpiece. Because the plastics do not etch harder metals, they are not a good candidate for deep rust removal. However, a fully PMB System is without doubt an excellent service for removing coatings as an alternative to hazardous chemical washes or abrasive sanding processes. The only drawback is the cost of the equipment. At this time, a small glove box unit costs around $10,000 and a large car-sized booth is over $50,000. Additional information can be obtained from: Scott Bentley, SPEED STRIP, 2623 San Fernando Rd., Los Angeles, California 90065; 213 221-0000.

Some of the More Popular Types of Plastic Media Blasting





Type I



Normal paints on soft Al, Cu

Type II



Normal paints on hard Al, steels

Type III



Powder coatings on steels

Type IV



Normal paints on fiberglass, Al

The Mohs Scale

The Mohs scale of hardness is a scratch hardness scale. It consists of a series of minerals, each of which can be scratched by the mineral above it on the scale; i.e., quartz will scratch glass and diamond will scratch all of the materials.




2.5 - 3.0

Aluminum (soft)

2.0 - 2.5

Silver, Copper

2.5 - 3.0

Brass, Bronze

3.0 - 4.0

Aluminum (hard)

4.0 - 5.0

Wrought Iron

4.0 - 6.0


4.5 - 6.5

Cast Iron

6.0 - 8.0

Steel Alloys*

5.0 - 8.5













Walnut shells

2.4 - 4.5



Polyester Type I


Urea Type II


Melamine Type III


Glass Bead

5.0 - 6.5

Silica sand (quartz)






Aluminum Oxide


Silicon Carbide


Some alloy steels such as #235 abrasive resistance steel are high in Mn, hard, tough, and wear resistant, giving up to 10 times the life of mild steel.

Scott Bentley

Textile Dry Cleaning Course

Several textile conservators and I are trying to ascertain whether there would be interest in a course on dry-cleaning, to be held on the West Coast and geared toward textile conservation. If you are interested in participating in a course, please get in touch with me. No commitment is necessary at this time. Please contact:

Sarah Gates, Textile Laboratory, M. H. de Young Memorial Museum, Golden Gate Park, San Francisco, California 94118, 415 750-7611.

Vandalized Rock Art Restorations

Vandalism of rock art sites is a widespread and disturbing problem. The types of damage sustained by these sites includes spray-painted graffiti, scratched initials, incised additions to ancient motifs, and bullet holes. Removing or repairing the damage from vandalism is problematic. Agencies responsible for the care of cultural property have resorted to a myriad of methods with--predictably--mixed results. Some treatments have been recommended by conservators; however, the majority currently in use can be best described as home remedies. Having been asked increasingly over recent months to assess vandal damage to rock art and to propose treatments, I have come across the results of various kinds of attempts to undo damage and make repairs. In an effort both to assist agencies responsible for the care of rock art sites and to improve conservation awareness among those who maintain rock art sites, I am hoping to review as many of the applied methods as possible and publish the results. In order to accomplish this, I have drawn up a short questionnaire designed to gather the necessary information which is gradually being sent out to persons responsible for the maintenance of rock art sites. I would be grateful for any information regarding this topic (both appropriate and inappropriate methods), and for the names and addresses of organizations known to have tackled such problems who would be willing to complete a questionnaire.

J. Claire Dean
Archaeological Field Conservator
University of North Dakota
UNDAR - West P.O. Box 669
Belfield, North Dakota 58622


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