about your job.....

["Simon Brown" <moonsong@zzzzzzzzzzzzzzz>]

George,

Apologies in advance for the length of this message- hope you can fight your
way through it!

Was this the museum you visited? http://www.egaonline.com/museum.htm

Could the granite balls you saw have been a product of a lapidary process
using sphere-shaping pipes? http://www.pearsons-uslj.com/spheres.htm

I have a book from my local library at the moment: Gem Cutting: A Lapidary's
Manual by John Sinkankas, published by Van Nostrand Reinhold. There is a
section in it that explains the process of making semi-precious balls very
well. This method is OK for shop production, and is similar to what I do,
however the reason for my students' ball exercise is skills training, and
the ball they take home is a by-product of the training, as well as a
motivation to finish it.

I aim for a 300mm (1 foot) diameter ball, any smaller and the object bounces
around as it is being worked. We also make a profiled circular sandstone
base shaped to sit the ball on as part of another exercise. The balls weigh
about 40 kg finished.

The material is Samford Granodiorite, a local intrusive igneous rock
commonly called granite which varies from pink to grey in colour
http://www.em.gov.bc.ca/Mining/Geolsurv/Publications/InfoCirc/Ic1987-5/rocks/grndior.htm
I select boulders up to 2 tonnes from the local quarry, and have them
delivered. It is a very hard stone with a large grain size, bland in colour,
commonly used for boulder retaining walls.

There are a few skills to learn in doing this project (hope the detail won't
bore you), they include splitting stone (pneumatic rock drill, plugs and
feathers), shaping solid stone (hammer, pitching tool, punch, air bottle &
4-point) as well as setting-out skills. The shaping work is all done with
hand tools, I don't let the students use diamond saws for this one even
though it would make things a lot quicker. We also calculate the volume,
mass and surface area of the ball and investigate the exponential increase
as the diameter is doubled and trebled. We also visit a few websites
featuring famous balls.

I explained the squaring process in response to a previous posting
http://aboutstone.org/conversa/msg00550.html This is a really
important skill for stonemasons and is the starting point for stereotomy
(love that word!) making a knobbly chunk of rock into a useful building
stone. Apart from learning the disciplined setting out procedure, the
students also learn to use the heavy hand tools as well as overcoming the
fear of hitting their hand (by hitting their hands quite a lot until it
stops hurting).

Once the block is cubed and the faces are roughly flat, the students make a
reverse template (a semicircle removed from a rectangle of sheet steel) to
the size of the ball diameter. The base of the template is formed by a
diameter of the circle, with a perpendicular semi-diameter marked.

The set-out square on the floor is quartered (with the quartering lines
projecting beyond the outline), the block placed on top and the quartering
lines transferred vertically using a roofing square and felt pen (I use a
whiteboard marker so it doesn't stain permanently). The vertical lines are
joined across the top of the block. These represent 2 vertical planes
intersecting at the centre of the ball- the great circles, or lines of
longitude. The equator of the ball is marked by halving the block
horizontally, measuring half-way up the block from the floor at the vertical
edges, and joining across the vertical faces. The block is turned over, and
the lines joined across the bottom bed just like across the top of the
block.

So now each face is quartered, with the centrelines of each face crossing at
right angles. These crossing points form 6 points that locate the
extremities of the ball within the cube.

The reverse template is laid along each centreline in turn and marked until
a circle is traced on each face of the cube. These form the outlines of the
ball, just to help visualise the shape within as a globe of the Earth. One
centreline is punched roughly to shape and finished to the reverse template
shape with a 4-point. This like the flight path of a plane flying from zero
degrees at the equator over the North Pole down to 180 degrees at the
equator again. This is repeated for 90 degrees to 270 degrees, and then the
four points at the equator are joined by forming a horizontal draft around
the equator to the shape of the reverse template. The process is repeated
for the South Pole.

Now the ball is contained within 2 great circles along the original vertical
planes, as well as a horizontal circle around the middle of the ball. Next,
2 more great circles are added between the first two. The bossage is
gradually lessened during all this circling until about 12mm remains, which
is then easily buzzed off between the circles to the template shape.

The ball is now completed (with a 4-point finish) for the required training
purposes, but some students have taken extra trouble to polish them using
wet stones.

This is a difficult project, and demands knowledge of the material and the
use of the tools. It is rewarding for me to watch the apprentices develop as
if through a rite of passage, with a big smile on their faces when it is
finally finished.

Simon