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Although
the earliest known artillery pieces were made from bronze, it was
not until the early 16th century that casting
techniques had developed sufficiently to allow for the production
of large bronze guns. From
1550, cast-iron guns also became available, although these proved
harder to cast, and bronze remained the favored material for gun
production until the mid to late 17th century.
Whatever the material being used, the casting methods were
the same.
Making
the Model
The
process began with a long wooden spindle, its diameter smaller
than the intended bore size of the gun.
Rope was wrapped around this spindle as tightly as
possible, so that one turn closely touched the next.
Layers of clay were then built up around the rope.
A template known as a “stickle board” was used to gauge
when the clay had been built up to the size of the gun which was
being made. All the
reinforcing bands which formed part of the pattern of the gun were
also included at this stage, creating a full-sized clay model of
the gun. The clay was
then dried over a fire, creating a tough surface.
The
whole clay model was then coated in melted wax, which could be
shaped even more accurately than the clay.
Any decorations planned for the finished piece were also
included at this stage, such as “dolphins” and coats of arms
were pinned onto the gun in the form of wax sculptures or plates,
so that the pins could later be removed.
Wooden patterns for the trunnions were also fixed to the
model at this stage.
Making
the Mold
The
model was coated in more wax, then an outer mold was built up
around it. The
founders used clay mixed with sand (a mixture known as
“loam”), and the first few coats were very watery, and were
brushed on, creating an accurate copy of the model underneath.
Each coat of clay was allowed to dry before the next one
was applied. After a few coats, thicker clay was used to speed up the
process, and fire was used to help dry the clay.
The final thickness of the mold depended on the size of the
gun being cast; 1 ½ inches for a 6-pounder, 3 inches for a
24-pounder.
The
next stage was to reinforce the mold.
Iron staves were fitted to the outside of the loam mould,
and held into position using iron bands.
This was needed to protect the mould from rough handling in
the foundry. It was
then lifted from the “turning frame” which had held the
original wooden spindle and was lowered onto a temporary timber
cradle on the foundry floor.
The
next process was very delicate, involving the removal of the
original model. The
spindle which formed the base of the model was tapered, and gentle
blows with a hammer at one end allowed it to be slid out from the
surrounding layers of rope and clay.
It was extracted very carefully, so as not to damage the
mold itself. The rope fell away from the rest of the model as this
was happening, and it too was carefully removed.
The
rest of the model was then taken away, along with any of the
wooden trunnion formers or pins holding on the dolphins or other
moldings. Foundry
workers plugged any holes made by the extraction of the trunnions
or pins, and ensured the inside of the mold was smooth.
A small fire was then lit inside the mold, to bake dry any
of these repairs to the mold itself, and to melt the wax which
formed the patterns for the dolphins or inscriptions.
The inside of the mold was then brushed with a grease
solution (called “lye”) to prevent the molten bronze or
cast-iron of the final gun sticking to the clay mold.
One
end of the mold was then closed by a second smaller mold.
This was one modeled into the shape of the cascable, or
breech (back) end of the gun.
The mold of the cascable was much thicker than the main
mold, as the mold was held upright when the metal was poured into
it, and the cascable end would have to take most of the weight of
the molten metal pored into the mold.
Once the molds were fitted together, more iron straps held
the two pieces together. From
the late 18th century, metal cascable molds were
introduced, and the two pieces joined in the casting pit, held
together by gravity.
Making
the Gun Core
The
interior of the mold was build to the shape of the exterior of the
gun. If filled with
metal at this stage it would create a gun without a bore.
The solution was to add another mold for the bore itself,
known as the “core”. An
iron bar about 2/3 the size of the final gun bore was wrapped in
rope and clay, just like the model for the main part of the gun.
The end of the core was modeled to represent the desired
shape of the chamber at the end of the barrel.
In
many guns, the core was held in place using a support near the
base of the mold known as a “core-piece” (also “cuxetta”
or “chaplet”) held in the mold itself, which supported the
core in place running straight down its center.
A clay disk held the core in place at the muzzle end of the
mold. Some other
foundries simply suspended the core over the main mold using a
winch built over the casting pit.
With
improved gun-boring machines in the mid 18th century,
it became increasingly common for guns to be cast without a gun
core, and the bore was drilled out during the cleaning process
after the gun was cast. This
was known as “solid-bore” casting.
The
Casting Pit
Bronze
used in gunfounding was approximately a
mixture of 90% copper and 10% tin, although exact
percentages varied, and some founders added other metals, such as
lead or zinc. The gun
mold was placed upright in a casting pit, right in front of the
“tapping hole” where the molten metal would be poured.
A funnel-shaped “feeding head” was added to the mold to
ensure the metal would flow smoothly into the mold, and great care
was taken to ensure that the mold sat exactly upright and level.
The inside of the pit was then filled with earth to hold
the mold in place, and the soil was tamped down, holding the mold
securely inside the pit. It
was common to cast several guns at the same time in the pit, to
save time and resources.
Everything was now ready for the metal pouring.
The
Casting Process
A
brick or clay conduit was laid from the tap-hole to the
feeding-head at the top of the mold, set into the earth fill of
the pit, and foundry workers stood by with plates to stop or
divert the flow along the conduits.
Once the metal scraps thrown into the furnace area had
melted and the molten metal was ready, the order was given to open
the tap-hole, releasing the liquid. Once the molten metal started flowing, the tap-hole opening
could be opened or closed to regulate the speed of the flow.
This
was a spectacular part of the process, and spectators often came
to watch the casting. Once
all the molds were filled with molten metal, the tap-hole was
closed, and any excess diverted into an overflow reservoir, which
would be returned to the furnace for the next casting batch.
The
molds filled with molten metal were then allowed to cool for 24
hours or more, then the inner cores would be removed.
The hot earth surrounding the molds was then removed (an
extremely unpleasant, dusty, steamy task), and the molds removed
from the casting pit. The
molds were laid on their sides and foundry workers would remove
the metal bands surrounding it, then chip off the clay mold using
hammers and chisels. If
all went well, what lay beneath the clay mold was a perfectly cast
gun, completely with all its moldings and decoration.
The
feeding head formed a kind of sprue which then had to be sawn off
from the gun itself, a process which could take two or three days
for large guns. The
gun would now be ready for the finishing process, including boring
out the barrel if required.
The
Boring Process
If
the gun was cast using a core, this was simply a cleaning task,
ensuring that the inside of the bore was free form imperfections,
so any shot would shoot straight.
For guns cast from the solid, this was a whole process in
its own right. For
the former, boring machines were simply animal or water-powered
lathes, where any burr or imperfection could be drilled out of the
bore. Many of these
boring machines were used vertically, so tall wooden frames were
constructed, and the cleaning up of the bore could last several
days.
From
the mid 18th century, guns were increasingly cast
“from the solid”, meaning no core was used.
The entire bore of the gun had to be drilled out.
Guns were fitted into vertical boring lathes, and drilled
out over a period of days or even weeks.
A gearing system linked the lathe to a
horse mill, which powered the machine.
The
Cleaning and Finishing Process
Once
everything else was done, the finished gun was checked for
impurities, and any casting blemishes was removed.
One of the inspection methods was to mound a candle and a
mirror on a stick, and to slide it down the barrel looking for
flaws. Any holes were
plugged using special tools, and then the vent
(or touch-hole) was drilled; usually a vertical channel,
but sometimes the vent sloped backwards at an angle of up to 70%
or so.
For
cast-iron guns and some bronze ones the gun was weighed at this
stage, and the weight marking inscribed into the barrel of the
gun, along with any other inscribed marks, such as a maker’s
mark or stock number (see Cannon
Identification for more details). The final stage was
to test fire the gun; a process known as “Proofing”.
The
gun was taken to a
firing range and loaded with a “proofing charge”, usually with
one charge equal to
the weight of the ball, then followed by two
more at 2/3 of this weight of powder.
Exact quantities varied, and from the late 17th
century the norm was to simply use the standard service charge for
a gun of the appropriate size.
After the gun was fired, the gun was deeped as having
passed the proofing test, and English guns were then stamped with
the letter “P”. The
gun was now finished, and ready for sale or delivery.
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Where
were guns produced?
Until
1760, all gun foundries were located in either Europe or
Asia (for Oriental guns). Asian guns were used mainly in
their local areas of production, usually China, India or the
Spice Islands (Indonesia), the most common for of gun being
small bronze swivel guns known as “lantakas”.
Although the Chinese were the first to invent
gunpowder, they never developed a large gunfounding
industry, and Chinese guns were rarely seen outside their
country.
In
Europe, bronze guns were produced all over the continent,
but the largest centers were in England, Spain, Flanders,
Northern France and Southern Germany.
Following the introduction of safe cast-iron
gunfounding techniques in the mid to late 16th
century, England and later Holland became recognized centers
for cast-iron gun production.
By the mid 17th century Sweden rose to
prominence as a gunfounding region, but Swedish guns were
rarely seen on board ships outside Northern European waters.
While
some gunfounders (particularly those specializing in bronze
guns) cast guns for use by their government’s armed
forces, others cast exclusively for the civilian market,
arming the merchant and cargo ships which needed some form
of protection.
Many
guns retained certain regional characteristics which can be
used to help identify where the gun came from (see
Gun
Identification
for more details).
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