NATIONAL PARK SERVICE Ruins Stabilization in the Southwestern United States Publications in Archeology 10

This section describes well-controlled, accelerated weathering tests with various commercial sprays to evaluate water repellency on adobe walls at Fort Union National Monument. The findings herein are primarily the results of research conducted by Rex L. Wilson (1956-1960, Mss., vol. I, pp. 11-17).

Pueblo Bonito, Chaco Canyon National Monument, Bloomfield, New Mexico. Four distinct masonry styles through two main building periods are evident in this, the largest prehistoric "apartment house" north of the Valley of Mexico. Modern repair and stabilization maintain this ruin at the original profile and elevation of walls and features uncovered during two major excavations, performed in 1897-99 by the Hyde Exploring Expedition, and in 1921-27 by the National Geographic Society.

Silicone Preservatives

There are more than 153 thousand square feet of standing adobe walls at Fort Union. Mindful of this figure, considerable thought entered into the purchase and application of silicone preservatives. Small-scale applications of two well-known products, Dewey and Almy's Daracone and Dow Corning's Silaneal 772, indicated that we could expect one gallon of a silicone solution to cover anywhere from 75 to 100 square feet. Daracone, priced about $5 per gallon, seemed extremely expensive in view of the number and size walls that required treatment. Therefore, many silicone manufacturers were contacted in an attempt to find a product that could give similar protection for less money. After our experiments, we concluded that Silaneal 772, when diluted in water in the ratio of 1 to 9, would give about a 5 percent silicone on the walls for around 50 cents per gallon. Purchase of this product, therefore, seemed justified.

Nearly all Fort Union adobe walls have been generously sprayed with Silaneal. It will probably require several years' observation to make definite conclusions as to which of these products affords the best protection, but it seems safe to conclude, on the strength of present evidence, that Daracone and products with similar characteristics and prices are not ten times better than Silaneal for waterproofing adobe.

Most applications of silicone were made on completely dry walls with a homemade spraying device under sufficient pressure to provide a rundown of from 4 to 6 inches. The solution must necessarily reach and penetrate all cracks, large and small, to do an effective job and give fullest protection. Experiments have shown conclusively that constant rainfall will attack the minutest cracks, work behind the silicone crust and finally force it to fall away, leaving unprotected spots vulnerable to further erosion.

No preservative will withstand the abuse of a hailstorm of magnitude. Falling hail stones are known to have flaked off chunks of adobe up to 1/4th inch in thickness, a depth to which no known silicone product will penetrate. Because hail is common at Fort Union, we should plan to touch up or respray entire walls on a year-to-year basis. Periodic inspections for hail damage should be made immediately after a rain when damaged areas show up dark against the light background of protected adobe.

Experience with Silaneal 772 has shown that when the product has been applied and allowed to dry, it is absolutely impervious to further applications of the same product. It will not, however, shed certain other products, e.g., Daracone and Dow Corning's 770 Water Repellent, which contain wetting agents superior to water. Both these products and others like them could be used in respraying hail-damaged walls. We have had some success at Fort Union with spraying the Silaneal 772 high on a damaged wall, allowing the solution to run down and find its way into cracks and chipped spots, thus re-establishing complete protection.

Costs of silicone preservatives and labor were not included in the reports for the individual rooms, but are considered building-by-building in table 3.

TABLE 3. Costs of silicone preservatives at Fort Union National Monument.

Experimentation with Silicone Preservatives

Two experimental walls were built in the corral section east of the warehouse district. The rock foundations are 1-1/2 feet wide and are about 6 inches above the surface of the ground. The longer of the two sections is 40 feet and the shorter is 20 feet. Ten sections of adobe wall were built upon the longer foundation, and five sections were built upon the 20-foot foundation. Each of the sections of adobe are 3-1/2 foot long, 1 foot wide, and five adobes (ca. 2 feet) high. The wall sections are 6 inches apart, and are numbered 1 through 15 with paint applied to the foundations. Sections 1, 2, and 3 were capped with soil-cement adobes; all others were left uncapped.

To facilitate measurement of weathering over a period of time, 20d nails were driven into the walls at various places, with their heads flush with the outer edges of the adobes. Then, using a brush and allowing considerable rundown, the walls were treated as follows:

Using a pump powered by a Jeep and mobile tank, water was applied with a garden hose sprinkler under sufficient pressure to simulate a gentle rainfall from the south at an angle of about 45 degrees from horizontal. Two or three sections of wall were treated in this manner at the same time. Neighboring sections that could not be well-flooded were covered with tarpaulins during the testing. Although the original plan for testing the experimental walls to the point of failure was not concluded, valid conclusions can be made. Table 4 indicates how far experiment had progressed up to August 8th, 1960.

TABLE 4. Normal and simulated precipitation (in inches) on selected test sections of walls at Fort Union National Monument.

Several light hailstorms were recorded at Fort Union between July 8, 1959 and August 8, 1960. They resulted in considerable abrasion to the test walls but with no appreciable precipitation. Therefore, precipitation falling as hail is not included in the table.

All Silaneal 772 applications, regardless of strength, appear to have held up equally well. The ready-mixed solutions of Daracone, Klear Film, Water Repellent, and Hydrocide Colorless SX also survived the experiment with results roughly equal to that of the Silaneal. It would appear that one product gives no better protection than the others.

Two of the solutions broke down completely under the effects of normal rainfall: Hydrocide Colorless 101 on Test Section 13, and Polystyrene applied to Test Section 14. As of the date of this writing, any protection once afforded by the former product has completely vanished and little remains of the latter.

In addition, tests were made with individual soil-cement and regular adobes to determine the depth of penetration of the several silicone solutions applied to test walls. Each adobe was liberally brushed with a solution in the same manner as it was applied to the test walls. After being allowed to dry for several days, each adobe was broken and immersed in water. Depth of penetration of the several solutions was then measured. Most of the preparations tested penetrated soil-cement and regular adobe to an equal depth of about 1/8th inch. Specific results are given in table 5.

TABLE 5. Depth of penetration (in inches) of various products in soil-cement and adobe.

Despite claims made by manufacturers that their silicone products will not discolor adobe, it is quite evident that some do alter the original color. Hydrocide Colorless 101 temporarily bleached the adobe test wall. A very rich mixture of Dow Corning's Silaneal 772 likewise left a whitish deposit on the test wall which eventually disappeared. When applied at the specified ratio of 1 to 12 or at the richer 1 to 9, Silaneal did not appreciably change the appearance of adobe. Polystyrene, not a silicone but a plastic, darkens adobe substantially and eventually turns a yellowish color.