Agriculture Reference
In-Depth Information
out and our voussoirs were then dry bonded face-up and the centre line trans-
ferred through either the key brick or key stone.
With these positioned directly over the centre line on the marked turning
piece, we used dummy spacers, 3 mm in size to represent the designed joints.
Once all the bricks were spaced accurately in position over the turning piece,
we positioned a horizontal timber profile called a 'tree'. This was placed at a
predetermined distance above the arch bricks, determined by the gauge of
the standard brickwork of the jambs. We transferred the centre line running
through the turning piece up on to the tree. This is most important as these
centre lines are crucial when setting out, both for preparation and for when
the construction starts above the openings.
With the tree in position we used a line to radiate the leading edge of each
brick, this line being long enough to reach both the tree and turning piece. As
each line was radiated we made an accurate pencil line on both the tree and
turning piece, positioning small nails to each pencil line; enabling us to wrap
string around them for the construction phase.
The skewbacks were easily scribed to the correct angle using the lines
ran-ging from the tree to the turning piece. After cutting and laying the
skewbacks the first voussoirs, or 'springers', on either side of the arch were
laid to the line and position. The laying of the arch to such a fine tolerance
meant that unless everything was prepared absolutely perfectly there could be
problems.
The delivered super-fine lime putty, which we mixed with a little amount of
silver sand, was placed into a dipping box. Before laying each voussoir, it was
sufficiently soaked to reduce absorption. The next process was to lower the
frogged face of the voussoir into the putty mortar, lowering it brick bed face
down at a slight angle, so the back-edge made contact first. If the brick was
soaked for the correct amount of time the frogged face would be emerge with
an even coat of putty mortar adhered to it.
We were aware from dry-bonding that some of the bricks, due to disc-
cutting, were slightly out of square from face to bed, being larger at the back
than the front, so slight adjustments in the bedding had to be made as we went
along. As some of the soffit cuts were also very slightly out of square to the
face it meant that some voussoirs could tilt forward; and this could be exac-
erbated with the weight of the keystone. To overcome this, once the arch was
turned we immediately clamped a prepared length of straight timber across
the face of the newly turned arch holding it to the standard face brickwork
on both sides of the opening with large 'G' cramps until it had hardened
(Fig. 173).
Once all the voussoirs were laid the putty joints were left to stiffen, or as we
term it 'hazel off', and then trimmed flush. After trimming they were jointed