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The Use of Pneumatic Tools in Repointing Historic Masonry
APT Communique, Vol XV (1), Technical Note 8, 1985
This Technical Note has been developed as a result of
restoration work undertaken on the Ross House, a Federal style
commercial structure remodeled to a residence in the Greek Revival
period. The structure is located on the Green in the Woodstock
(Vermont) Historic District, listed in the National Register.
Restoration specifications read: Power saws, power chisels, other
power and airóabrasive tools shell not be used to remove
old sortar from joints without the written approval of the Architect.
The mason had employed the pneumatic tools described herein on
restoration projects for seven years. He presented his qualifications,
product literature on the tool, and a successful demonstration
of its application to the required work. Subsequently, the tools
and techniques specified below were approved for masonry restoration
or the Ross House and several other historic structures.
The specification, together vith its bracketed comments, repre-sents
a collaborative contribution by the architect (Stephen Smith,
AlA, Partner. Northern Architects. Inc., Burlington, Vermont),
mason (Michael J. Watson. President. Green Mountain Restoration
Co., Shaftsbury, Vermont), tool manufacturer (Norman Akley, Manager,
Trow & Holden Company, Barre, Vermont) and archi-tectural
conservator (the author). The author accepts responsibility for
any faults and omissions in this Technical Note.
Research on this subject was furthered in 1985 in preparation
for the University of Vermont Historic Preservation Summer InstItute's
Masonry Conservation Workshop. Initial development of this Workshop
was funded in part by the Faulkner Trust and the New York Community
Trust.
MASONRY REPOINTING AND RESTORATION - SECTION 04100
Repointing: Joint Preparation and Unit Removal
- Only hand tools, used in conjunction with the pneumatic tool
specified below, shall be employed to remove deteriorated and/or
inappropriate mortar and masonry units.
- Other power chisels, power saws and air-abrasive tools shall
be strictly forbidden without written approval by the Architect.
- The permitted tool shall be a Barre Short Stroke Pneumatic
Carving Tool: Type B or D (Dallett) with a Splitter or
Cape Chisel as manufactured by Trow and Holden Co., Inc., 45
South Main Street. Barre, Vermont 05641 (Vermont: 802-476-7121/out-of-state:800-451-4349),
under the following conditions:
- The chisel shall have a round shank and be hand held in
place in the carving tool with no retainer. [ A round shank
permits the chisel blade to be oriented independent of the
tool, an essential feature that is impossible with square-shank
tools. The absence of a retainer, or any mechanical connection,
enables the user to defeat power of the tool immediately
by pulling the chisel away from the piston, without any
other action. Precision and control is affected by the tool
design enabling one hand to operate the tool, while the
other manipulates the chisel. Importantly, this tool was
developed as a finishing instrument to sculpt stone for
hours at a time.]
- The width of the cutting edge and the diameter of any
portion of the chisel blade which enters the masonry joint
shall not exceed three-quarters of the width (face thickness)
of the mortar joint. [Tempered steel chisels are available
with carbide tips appropriate for raking out portland mortar
and removing stone units. Lime-rich mortar can be removed
using a tempered blade without a carbide tip. Blades are
produced in varying widths. The chisel body can be manufactured
to any length.]
- The compressor activating the carving tool shall have
a variable pressure control and be regulated to provide
air pressure consistent with effective cutting of the mortar.
[Air consumption of the tool varies from 3 to 8 c.f.m. depending
on diameter of the piston which ranges from 1/2" to
1-1/4".] Air is controlled three ways:
- the compressor (requiring approximately 3/8"
p.s.i. for the tool to be run full throttle).
- an air stopcock located on the 3/8" pneumatic
tubing about two feet from the tool
[enabling the tool to be operated with as little as
ten p.s.i.] and
- by finger control of the exhaust port [effecting
a subtle control with back pressure]. Employment of
a quick-connect coupling will dampen air delivery to
the piston. A 25 c.f.m. compressor (with condensation
separator) will operate two or three tools concurrently.]
- In areas located by the Architect provide samples uf
mortar and masonry unit removal. Samples will be reviewed
by the Architect for uniformity and conformance to the Specifications.
Samples will serve as the basis for acceptance or rejection
of the use of equipment and personnel as determined, in
writing, by the Architect.
- Approved samples shall be marked for identification, retained
and protected. Samples shall constitute a standard for acceptance
or rejection of completed work. Samples shall not be altered
during subsequent work without the Architect's written approval.
- The approved demonstrator(s) shall be the sole operator(s)
of this tool. Employment of any other personnel for this
purpose must be approved in writing by the Architect.
- Protect all masonry to remain as part of final work.
An exception to this shall be masonry units, described on
the Drawings, that require removal and replacement. This
tool may be employed to remove such units.
- Protect workers and pedestrians from debris and the inadvertent
rejection of the chisel from the tool. [To leash the chisel,
drill 1/8" hole through square section above the round
shank, secure one end of a tempered wire to the chisel,
and loosely wrap the other around wrist
of hand operating the chisel. Such a modification may be
undertaken at the factory.]
- Inspect masonry surface to determine nature and extent of
repointing and unit replacement once chemical cleaning has been
completed and approved in accordance with Section 04500. [A
base bid can be requested based on areas delineated in the Drawings
and called for in the Specifications. Thereafter any changes,
estimated by unit or s.f., can be added or deducted to the based
bid.]
- Repoint only joints exhibiting erosion and failure of lime-rich
mortar. Do not indiscriminately repoint joints in good condition.
Areas requiring repointing include, but are not limited to,
those shown on the Drawings.
- Remove all portland-cement-rich mortar.
- Remove mortar to a depth 2ó1/2 time the thickness of
the joint. Remove all loose mortar even if it is deeper than
the depth indicated.
- Remove mortar from both surfaces of adjacent masonry and cut
square at the back of the joint.
- Employ a regulated and light application of compressed air
to clean masonry of dust and debris. [Compressor air, regulated
using the air stopcock and/or an air valve with trigger, is
available by uncoupling the tool.]
Captions:
Employment of tool to remove mortar along a bed joint. The
tool is particularly effective for removing mortar in irregular
joints (such as random ashlar) and portland cement-rich mortar.
Dallett-style pneumatic carving fool with exhaust port, located
above index finger of the right hand. The left hand is introducing
the round shank of a special-order cape chisel into the tool.
Tool fitted with 3/8" pneumatic hose and quick-connect
couplings. Note two special-order cape chisels and flat ("clean
up ") blade.
Illustration from Trow & Holden Stone Cutting Tools. Catalog
13, Barre. Vermont. n.d.
Note: This is number 8 in a series of Technical Notes,
with which we hope, in drawing upon contributions by APT members,
to encourage exchange in a variety of technical areas. Subjects
contemplated for this series include extant recording, building
inspection, material conservation, structural repair, building
systems conservation, and energy conservation.
Please write to Communique if you would like to make
a technical notes contribution.
Communique: Box 2165. Albuquerque, NM 87103, USA; (505-265-3838)
Technical Notes represent on-going research. The information
given and conclusions reached are those of the author. Comments
are invited.
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