Polygel® Mold Rubber: Perfect
for Fieldwork and Fortified Gypsum Castings
Back in Texas at the Ships of Discovery headquarters in the Corpus Christi
Museum of Science and History, we tackled the thorny problem of finding
the right material and technique to mold and cast the inscriptions. The
molding material would have to be flexible but thick, durable, immune
to shrinkage, and capable of producing multiple casts with excellent detail.
The casting material would have to be rigid and have the appearance of
stone, but not be brittle or soft. We experimented for several months
with different types of materials until we found a liquid, room-temperature
vulcanizing rubber system that could be brushed or poured on the surface
to be molded. Depending on which liquid rubber we used, we could make
it as thick as peanut butter or as thin as honey. It was Polygel®
40 and 50, a patented self-thickening liquid rubber for flexible molds.
We then turned our attention to finding a hard, durable, fast-setting,
relatively inexpensive casting material. The old standby, plaster of Paris,
would be too soft and brittle. The various epoxies we tried did not have
the look and feel of stone. Then we discovered Forton MG from Ball Consulting,
Ambridge, PA, a truly remarkable material used by architects to make decorative
exterior moldings. A gypsum-based plaster, it is reinforced with a hardener,
a resin, a polymer, and chopped fiberglass to make a hard, resilient,
weather-resistant material much tougher than the soft carbonate rocks
on Sapodilla Hill.
The real work on Sapodilla Hill started in December 1998 when Canadian
archeologist Jon Moore and I arrived on Provo with our whole kit and caboodle
to start the molding process. Armin Thiele, proprietor of the Mariner's
Inn, conveniently located only a few hundred meters from the hill, graciously
permitted us to stay at the hotel and provided warehouse space for the
truly staggering amount of necessary equipment and materials: paint brushes,
Ziploc bags, plastic mixing spoons and cups, bags of rags, petroleum jelly,
mineral spirits, acetone, paint remover, dental tools, photographic equipment,
spray bottles, coolers, a beach umbrella, and eleven barrels of molding
and casting chemicals!
We knew we did not have the time or materials to mold all the inscriptions,
so the first order of business was to select the most notable examples:
the oldest, the largest, the clearest, the most unusual, the most informative.
Assisted by local volunteers, and with in-kind donations from members
of the Provo Rotary International Club, we prepared each of the candidate
inscriptions. Using soft paintbrushes, we gently brushed away the sand
and dust, but there was nothing we could do about the light growth of
lichen that covered most of the stones. Next, we sprayed on a mixture
of petroleum jelly and mineral spirits to prevent the mold material from
sticking fast to the stone. Then came the tricky part: combining four
different chemicals in the right proportion and at the right time to make
the flexible polyurethane Polygel® "face" mold—the mold in contact
with the stone. We discovered that the warm conditions on top of Sapodilla
Hill made the chemicals react much faster than they had in the lab. Because
we had only two or three minutes to pour the four chemicals, mix them
together, and get them on the stone before they began to congeal, we mixed
only relatively small batches. Large inscriptions required as many as
four or five pours, while small ones needed only one or two. Clay dams
were built around individual inscriptions on bedrock, to keep the liquid
molding material in place until it set.
Poly 15-6 Was Thickened to Create Mother Molds
We let the molds set up overnight, then peeled them off the next morning.
We were delighted to see that the mold material picked up even the most
minute details, but, because it was flexible, we had to make a second
mold of hard plastic—a mother mold—to help the flexible face mold keep
its shape. This necessitated mixing enough of Polytek®'s Poly Fiber
into Polytek®'s Poly 15-6 Liquid Plastic to make a nonflowing, brush-on
mold shell, which could be "buttered" onto the often vertical cured Polygel®
molds. The Poly 15-6 hardened quickly to form a thin, light, but rigid
polyurethane shell. (Through unpleasant experiences better left out of
this account, we learned that this "skookum," as Jon called it, would
also, given an opportunity, form a light but rigid shell on your forearm,
your shoes, or anything else it came into contact with.) The mother mold
set hard in a few hours, after which we could separate it from the back
of the face mold and then remove the face mold itself from the surface
of the stone.
Each morning we carried out supplies and equipment up the hill, and each
night we came back down again with two or three completed molds. This
we did, over and over, day in and day out, for eleven days. It has probably
been more than a century since anyone spent so much time on Sapodilla
Hill.
Stable Polygel® Molds Allow Forton Casts to Be Made Later
In March of this year, back at the National Museum, I concentrated on
making casts of the inscriptions from the rubber molds. This turned out
to be more complicated than making the molds in the first place. First,
the face molds had to be carefully positioned in their mother molds, then
cleaned and lubricated with a silicone spray so that they could be separated
from the plaster cast without damage to either. Flat molds had to be embedded
in a box of wet sand so that the resulting cast would have the proper
thickness.
The Forton MG consisted of four powdered chemicals that had to be mixed
in precisely the right proportion, then added to a liquid polymer and
carefully blended. Finally, a liquid catalyst was added to accelerate
hardening. The resulting liquid Forton was poured through a sieve onto
the mold. After the face mold containing the inscriptions was completely
covered, I added a second layer of Forton, this time reinforced with chopped
fiberglass to give the cast additional strength. After a few hours, the
mother mold could be removed, and the face mold peeled away from the final
cast.
But that was just the beginning.
Finishing the Castings
Those casts destined to be mounted on a wall had to be given sufficient
depth to conceal wooden mounting frames in their concave backs, and each
of them had to be colored to resemble the original stone and to bring
out faint inscriptions. Building up the sides added another step or two
to the process, including lengthy sessions carving off excess material
using a small, but very loud, pneumatic chisel. Back in Texas, Roy Garrett,
the Corpus Christi Museum's exhibits designer, helped me work out a clever
way to mount the inscriptions securely and make the accompanying labels.
Duplicating the mottled appearance of the original stones was another
challenge. At this point the cast was just a dazzlingly white puddle of
plaster with writing on one side. First we tried rubbing graphite
powder into the surface of the casts, but the results were much too even.
Finally we settled on powdered water color pigments in black, white, yellow,
and brown. I tried applying powdered watercolor pigments directly to the
surface of a test cast, but it did not have the right look. Next, I tried
mixing the colors with water to make a thin "wash" that could be brushed
on, but the water beaded up and refused to fill in the inscriptions. In
desperation, I tried adding a tiny amount of an industrial surfactant
(a chemical that makes water "wetter") routinely used in the lab for other
purposes. What a difference! Not only did the wash now fill the inscriptions,
it also brought out lines and marks not previously visible. Clearly, the
best way to make out faint details on the Sapodilla inscriptions is to
use the colored Forton casts, not photographs or drawings.
Even with the assistance of Museum regulars Grethe Seim and Sherlin Williams,
casting one complete set of molds to keep in the Museum's permanent collection
and eleven more for display in the Provo airport took the better part
of two weeks.
At the Museum on Grand Turk, volunteer Dr. Randy Davis and I finished
coloring the last of the casts, fabricated the mounting brackets and test
fitted each to a mock-up wall, and carefully packed each cast and assembled
a trunk of mounting tools and materials.
On installation day, we were met by Airport Manager Bill Johnston. Randy
looked at the blank wall and, pushing up imaginary sleeves on his bare
arms, pronounced, "We're not quitting till it's finished." Less than four
hours after our arrival, we were standing in front of the finished exhibit,
wondering if we could have made the castings even more realistic. A passerby
stopped, scrutinized the exhibit for a moment, and, mistaking the casts
for the real thing, asked indignantly, "Is there anything left to see
up there on the hill?" We explained that the exhibit was made only of
casts, exact reproductions of the original stones, all of which were exactly
where they have always been on top of Sapodilla Hill. After the man sauntered
off satisfied, Randy said, "I guess they're realistic enough."
The Past and Future at Sapodilla Hill
Although the inscriptions on Sapodilla Hill are not unique, they are remarkable
for their num,ber and variety and the length of time they span. What is
the reason for this concentration of hundreds of individual inscriptions
on Sapodilla Hill? The answer to this question is obvious to anyone who
stands at the crest of the hill as I did back in 1982 and takes in the
view. Looking to the west, one sees the only (relatively) safe, (relatively)
deep-water passage capable of providing large
ships access to Provo. Known as Sandbore Channel today, it is used by
virtually all of the cargo ships that call at South Dock, immediately
to the east of the hill. It is easy to imagine ships' officers overseeing
loading and unloading in Sapodilla Bay from the crest of the hill, where
there is usually a cooling breeze. Perhaps the two ship inscriptions on
the hill represent actual vessels once anchored in Sapodilla Bay. Or perhaps
passengers waiting for a ship to arrive or depart passed the time on this
high vantage point from which they could spot their vessel as soon as
it appeared on the horizon. In either event, they were people with time
on their hands, soft stone at their feet, and an urge to leave a record
of their passing.
Most of the inscriptions are quite neatly executed—obviously the work
of people accustomed to writing. Perhaps the most artistic and symmetrical
inscription is that of Daniel Brown, dated 1817, carefully framed inside
a lovely cartouche. By contrast, a graffito dated 1996 that almost touches
it is ragged and clumsily executed. Compared to the careful, skillful,
painstakingly exact inscriptions of visitors in the 18th and 19th centuries,
the graffiti of recent visitors is invariably crudely painted or hurriedly
slashed into the rocks—a silent, sad commentary on modern society's fixation
on "more" instead of "better."
While some of the inscriptions contain dates, many others do not. Still,
there are sometimes hints hidden in the way in which the letters are formed
and in other writing conventions. These epigraphic clues include, for
example, the backwards S; the A with a V-shaped crossbar; tiny X's between
initials; and the use of the letter I in place of J. Each of these writing
conventions was popular for a limited period of time.
The Sapodilla Hill project is far from finished. Undoubtedly, there are
undiscovered inscriptions on the undersides of some boulders and in places
where the bedrock has been covered by soil and roots. Only a few of the
most obvious inscriptions have been studied and fewer still have been
molded. Although the hilltop has been declared an Area of Historical Interest,
it is privately owned. There is still the task of ensuring that it does
not become yet another victim of development. Already the site has been
bulldozed, defaced, painted over, and perhaps even robbed. It is hoped
that an effort spearheaded by Rotarian Jon Thompson to place signs on
the site advising visitors that it is protected by law and suggesting
rules for behavior will reduce the impact of visitation while at the same
time keeping the site open to the public.
So take a moment to reflect on the fact that even simple recording and
preservation projects like this one require the expenditure of staggering
amounts of time and resources and the cooperation of dozens of people,
aided by quality moldmaking and casting supplies from companies like Polytek®
Development Corp. and Ball Consulting. Are the results worth it? Only
time will tell. |
