The following paper
appeared in the 1999 National Conference on Cultural Property Protection
Proceedings: Cultural Property Protection from the Ground Up:
Locks & Keys: Technical Aspects of Locking Devices
By: Leon Dixon
Leon Dixon has worked for
the Smithsonian Institution since 1964, serving first in the Cabinet Shop. He
later joined the Locksmith Shop under the Office of Plant Services until its
transfer to the Office of Protection Services in 1973. Other than a short stint
with the Military Police, Leon has been the singular consistent force in
Locksmith Shop development at the Smithsonian.
A Washington, DC native, Dixon
graduated from the University of the District of Columbia with a Bachelor of
Arts degree in business administration.
Dixon regularly conducts lock
and alarm demonstrations for many organizations.
ABSTRACT
Locks are the most commonly
employed security devices in museums and cultural institutions today, and
can provide excellent security for the limited money invested. Their placement
ranges from perimeter doors to exhibit gallery doors, to collection storage
areas and individual storage cabinets, to those installed on windows and skylights.
Locks, however, are only as secure as the quality of the materials and the
building techniques used to construct the door, cabinet or window to which
they are attached. I explain this point throughout this review of lock types,
and how they can be used.
DISCUSSION
There are a number of different
locks on the market today appropriate for cultural institutions. Better quality
locks are more likely to withstand picking, manipulation, jimmying, cutting
and/or drilling.
The two major categories
of pin tumbler keyed locks are deadbolts and spring-latch locks. Deadbolts
require the use of a key or knob to move the locking bolt, while spring-loaded
locks employ a spring to assist the action. This allows the beveled latch
to retract as the door and then return to its original position when it locks.
This provides automatic locking of the door from one side. Deadbolt locks
are generally more secure than spring-loaded ones, which are easily jimmied.
The most common method of
mounting deadbolts and spring-latch locks is that of surface mounting. Here,
a lock is mounted with screws onto the surface of the door or window it is
protecting. Mortising comprises another method in which an opening or wide
slot is cut into the door, and the lock is installed within it, keeping most
of the lock from view.
TYPES OF LOCK CYLINDERS
Although some institutions
employ warded and other specialized styles of locks, the most common type
of lock in most facilities is the pin tumbler keyed cylinder. These represent
the most widely used lock keying system in the United States for exterior
and interior doors, and for collection storage areas as well.
Pin tumbler locks use a series
of pins, which normally keep the lock's cylindrical 'plug' from rotating to
unlock the locking mechanism. When the correct key is inserted into the cylindrical
plug, the pins are raised into proper alignment by the cuts of the key, allowing
it to rotate and activate the locking mechanism.
High quality pin cylinder
locks can be rekeyed by trained professionals to accept a variety of new keys
almost indefinitely, and allow an investment in lock cylinders to be used
continuously for years. Pin tumbler cylinders allow master keying. That is,
a supervisor can hold a key that opens all doors on a given floor, while an
occupant's key only opens his/her office.
One variety of pin tumbler
locks features an interchangeable core. The insertion of the proper core changing
key will allow the lock's core containing the pins and keyway to be removed
in seconds without having to dismantle the lock from the door. This allows
a qualified individual to make up a new key and cylinder in a shop as time
permits, and to use the core changing key to remove and reinstall the new
core on site in seconds, at little or no inconvenience to the user of the
area.
The locks described in the
following list may prove very useful in museum applications.
WARDED LOCKS
A kind of lock that may be
found in older structures is the warded lock. They use large keys of simple
design, often referred to as skeleton keys. These may be found today in older
buildings with the lock mortised into the door. These locks and keys can be
very effective, but are often weakened by the presence of a large gap between
the door and the frame. A major disadvantage is the ease with which the lock
can be picked due to the large keyhole and fairly simple locking mechanism
it uses. They have largely been replaced by the contemporary locks described
earlier.
MORITISED DOUBLE CYLINDER
DEADBOLT LOCKS
These locks are popular because
they can be operated from either side only with a key. While they do not totally
secure a door which contains glass panels (an intruder could enter the broken
window without opening the door) they are considerably more secure than a
locks with a turn-knob on the inside.
Because the door cannot be
opened without a key from the inside, the use of this type of lock may be
restricted (because of fire regulations) or be restricted for use on secondary
exits. These are also available deadlocks, which can be operated remotely
by electrical switches.
SPRING LOCK
Spring locks are used extensively
because of the ease with which they can be locked. Unfortunately, it is easy
to open a spring bolt from the outside with the use of a celluloid or thin
metal strip. Because the latch must be beveled to allow the door to shut and
lock on its own, it is possible to force the latch back into the case and
release the door by pressing a strip against the beveled side. Even a young
person can master this trick, and for this reason the lock is considered a
low security device offering little real protection. Protective plates can
be placed over the area, but only improve the security marginally.
SPRING LOCKS WITH LOCK-OUT
FEATURE
Some spring latches feature
a safety button, which can be engaged to block the return of the latch, leaving
the door unlocked. There is always the danger that someone will inadvertently
leave the latch retracted. Indeed, this may not be known until someone physically
pulls or pushes the door to test it.
SPRING LOCKS-KEY IN KNOB
TYPE OR CYLINDRICAL LOCKSETS
This lockset is mortised
into the edge of the door to provide an uncluttered appearance, but it is
vulnerable to entry by means of a celluloid strip, and can also be broken
when the external lock ring is subjected to a strong twisting force, such
as that applied by a strap wrench or pipe wrench. In some cases, this forcing
may not be evident after a premise has been entered, and the lock may continue
to function.
DROP BOLT/DEADLOCK
The deadbolt is by far the
most acceptable of all surface-mounted locks. This type of deadbolt features
a set of vertical locking pins, which engage horizontal, rings in a receiving
plate secured to the jamb. This is difficult to jimmy if properly installed
on the inside of a strong door.
This type of lock is almost
always of heavy construction, and while it has the broadest range of applications,
it should not be used on doors with weak frames, or on glass doors with lightweight
casings, or no casings at all. Strong screws and bolts security it to the
door are a must. Make sure that if bolts are used, they cannot be removed
from outside the door.
BUTTRRESS-TYPE OR POLICE
LOCKS
This type of lock is most
commonly found in older buildings on weak door frames. Instead of securing
the door to the door frame, it locks into a floor receptacle and acts as a
buttress. It is most suitable for resisting forcible entry. It is usually
key controlled from the outside.
If the bar is not properly
set in place, a flat metal shim, such as a bed spring, may be inserted under
the door can be used to slide the bar out of the lock casing. A metal plate
known as an astragal can be mounted to block the space between the door and
its frame.
DOUBLE BOLT LOCKS
A double bolt lock can be
used on doors where it is desirable to bolt both the opening side of the door
and the hinged side as well. This type of lock is extremely effective when
installed properly. It usually employs a locking mechanism fitted for key
operation on the outside of the door as well as the inside of the door.
CONCLUSION
A secure lock is only as
effective as the strike plate it engages in the door frame. Strike plates
vary in shape, size and thickness of the metal, and are made for mortised
or surfaced mounted locks. An important thing to look for is a close fit between
the lock and the strike plate, which ensures that there is little room for
door movement when it is closed. Security strike plates are available for
use in some applications.
It is extremely important
that the strike plate be affixed securely to a sturdy door frame to prevent
forcing by anyone intent on forcing the lock. Long screws that go into the
door frame perpendicular to the direction of force on the door provide the
best protection, but it is important that there be substantial material present
to grab and hold the screws!