Build a plywood form, following the instructions on page 157. Select stones for the top of the circle with sides that are squared off or slightly tapered. Dry-lay the stones around the outside of the form, spacing the stones with shims that are roughly Vi" thick at their narrow end.
Number each stone and a corresponding point on the form using chalk, and then set the stones aside. Turn the form around, and label a second set of stones for the bottom of the circle. Tip: To avoid confusion, use letters to label the bottom set of stones instead of numbers.
Prepare a stiff mix of Type M mortar and lay a 1A"-thick mortar bed on top of the wall for the base of the circle. Center the stone that will be at the base of the circle in the mortar.
Set the form on top of the stone and brace the form by constructing a sturdy 2×4 scaffold and secure it by constructing a bracing structure made from 4×4 posts and 2×4 lumber. We used pairs of 2 x 4s nailed together for lengthwise supports. Check the form for level in both directions and adjust the braces as required. Screw the braces to the form so the edges are at least Vi" in from the edges of the form.
Extend the mortar bed along the wall and add stones, buttering one end of each stone and tapping them into place with a trowel. Keep the joint width consistent with the existing wall, but set the depth of new joints at about 1" to allow for tuck-pointing.
Attach mason’s string at the center of the front and back of the form and use the strings to check the alignment of each stone.
Stagger the joints as you build upward and outward. Alternate large and small stones for maximum strength and a natural look. Stop occasionally to smooth joints that have hardened enough to resist minimal finger pressure.
Dress stones as necessary to follow large bulges or curves as you lay stones around the circle. The sides should be roughly squared off.
Once you’ve laid stones about Y" beyond the top edge of the form, disassemble the bracing.
Invert the form on top of the wall in preparation for laying the top half of the circle. The bottom edge of the form should be set roughly V" higher than the top of the lower half of the circle. Check the braces for level (both lengthwise and widthwise), and adjust them as necessary and reattach them to the posts.
Lay stones around the circle,
Working from the bottom up, so the top, or keystone, is laid last. If mortar oozes from the joints, insert temporary shims. Remove the shims after 2 hours and pack the voids with mortar.
Once the keystone is in place, smooth the remainingjoints. Remove the form. Let the wall set up overnight, and then mist it several times a day for a week.
Remove any excess mortar from the joints inside the circle. Mist lightly, and then tuck-point all joints with stiff mortar so they are of equal depth.
A putty-like consistency will develop in joints. At this point, use a joining tool to smooth the surface. Let the mortar harden overnight. Mist the wall for 5 more days.
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