Technical Note: Latex Bonding of
Model Build-Ups
Herbert G. Smith, CPO
Introduction
Modifying plaster-of-paris models is a time-consuming process, affected by the weather,
dryness of the plaster and models, and the
quality of the plaster of paris. This process
can be complicated further when the plaster
added for build-ups to the original model
flakes or breaks off.
Traditionally, casts were poured just prior
to modification to help reduce non-adherence of secondary plaster build-ups. Practitioners thought that by not allowing the
model to dry thoroughly, and by acting before the chemical reaction was complete,
secondary build-ups would adhere more
readily.
While this procedure did improve adherence, it did not solve the problem completely,
particularly with thin veneers of plaster. Also,
it was not always possible to completely modify the model promptly, due to other demands
from the practice. If complete modification
could not be performed before the model
dried, the model would be soaked in water
until the build-up and smoothing process
could be continued. Still, some flaking and
separation of added plaster occurred.
A New Solution
By using latex as an adhesive to bond added
layers of plaster to the original model-even
very thin layers-flaking and broken plaster
can be avoided. The material used is a 60
percent concentrate of liquefied latex, traditionally used by masons to adhere new layers
of mortar to existing cured mortar. This
product should be available through a local
mason supply house.
Procedure
Soak the dry plaster model in water briefly.
Once the model is removed from the water,
pour a mixture of 20 percent latex and 80
percent water over it, concentrating on areas
where plaster is to be added.
When mixing the plaster of paris for the
build-ups, a mixture of 30 percent latex and
70 percent water is used to moisten the plaster. The water and latex are mixed just before use, and the concentrated latex is always
kept tightly capped when not in use.
A Word of Caution
The proportions of latex and water used suit
the climate, plaster and models of this facility. Care must be taken to adjust the proportions to meet prevailing local conditions.
Too much latex will cause the resultant model to be more difficult to shape and smooth.
HERBERT G. SMITH, CPO, owns McCall's
Prosthetic & Orthotic Laboratory, 3412 49th St.,
St. Petersburg, FL 33710.
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