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Making
Large Bells
This
describes how the Liberty Bell and other large bells were made in
the 1700s. The basic steps involve designing the bell, creating
its mold and pouring molten metal into it.
Design
Bell
casting began by making a full-size drawing of the bell. The mathematical
equations governing bell designs were developed over the centuries
and were well know in the 1700s.
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 Create
templates
The
design was transferred to a thin wooden board and a full-size profile
of the bell was cut out. Two templates (called strickle boards) were
made - one for the inside and one for the outside of the bell.
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Mount
the "inner" template
After
mounting the strickle boards to a pole, the templates looked like
giant drafting compasses. The template of the bell's inner shape
was installed in the center of the mold pit.
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Build
the core
Workmen
dug a pit deep enough so the top of the finished bell was 6 inches
lower than ground level and wide enough to work around mold structure
that would be built there.
 The
inner mold (or "core") was a hollow brick structure made
in the shape of, but slightly smaller than, the inside of the bell.
It was hollow to allow it to be heated from within by coals. This
part of the core is called the oven.
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 Rotating
the template as a guide, layers of a clay mixture were applied to
the brick oven until it was smooth and formed the shape of the inside
of the bell. The clay mixture contained horse manure, horse hair,
and hemp to increase its strength. The core was dried by placing
hot coals in the oven.
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Make
a clay bell
After
lubricating the core with ashes or pig fat, a clay bell was built
on the core from the same material and by the same process used to
form the core. The "outer" template guided the workmen.
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Make
the cope
The clay
bell was lubricated and the outer mold ("cope") built upon
it. Again, using the same material and process as making the core.
When it was sufficiently thick, it was dried and lifted clear. The
clay bell was broken up and removed - its only purpose was to imprint
the outer shape of the bell onto the inside of the cope. |
Add
the inscriptions
There
were two popular techniques for inscribing bells. Pass and Stow cast
the inscriptions and decorative rings in wax and set them onto the
clay bell. In this way, the cope would automatically have the desired
inscriptions when lifted off of the cope.
The
other method, used by Whitechapel, involved cutting a strip out
of the inside of the cope and replacing it with a fine soft clay.
This was then impressed with the inscription.
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Assemble
the mold
The cope
was lowered over the core and a separate mold of the "cannon"
was fitted to the top of the assembly. The cannon is the set of protrusions
on top of the bell used to fasten it to its wooden yoke. These were
molded into the bell for strength. |
Pour
the mold
The pit
was filled with sand to keep the assembly from shifting and molten
metal was poured into the space between the core and the cope. The
metal is a special kind of bronze called "bell
bronze" - about 77% copper and 23% tin. After hours of cooling
the mold was broken open revealing the new bell. |
Tuning
After
all this work, what if the bell didn't sound good? Well, bell tuning
in the 1700s was an inexact science at best. If the bell had a disagreeable
tone, craftsmen could gouge (or "hand-chip") the inside
of the bell. These methods were crude and their efforts were usually
unsuccessful. The most common remedy for inferior tonal quality in
this period was to breakup the bell and recast it - and even this
was a hit-or-miss proposition. The Liberty Bell's second recasting
was for this reason. Today bells can be tuned with a precision that
was unheard of in the 1700s. |
