Synthetic amber produced by consolidating amber fragments under pressure
with an oil binder. Syn:reconstructed amber; amberoid.
Similar to molded cameo, but pressed. CF:molded cameo
Synthetic copal produced by consolidating copal fragments under pressure
with an oil binder.
In ceramics, a worker who molds the handle, ears, and decorative reliefs
to be applied to a pottery vessel before firing. Fay
a. A machine that forms ceramic shapes by forcing plastic or semiplastic
raw materials into a die or mold.
b. A machine in which the whole forming operation is carried out by
pressing the plastic glass by a plunger forced into a die or mold. The
machine may be operated by hand or it may be fully automatic. CTD
a. The force exerted across a real or imaginary surface divided by the
area of that surface; the force per unit area exerted on a surface by the
medium in contact with it. AGI
b. A commonly used short form for geostatic pressure. AGI
c. As used in mine ventilation terminology, it is sometimes defined as the
available energy content of the air and as the pressure difference between
two points in a ventilation current as the energy lost due to friction
between two points. Roberts, 1
d. Force exerted by air per unit area, either gage or absolute.
Atmospheric pressure is measured by a barometer. Measured in pounds per
square inch, kilopascals, or inches of mercury. Hartman, 1
a. An instrument for measuring the velocity of ventilating air currents in
mines.
b. An anemometer indicating wind velocity by means of the velocity head
exerted. Standard, 2
The driving of a narrow roadway results in the development of a pressure
arch over the excavation. The strata within the arch bend slightly and
cease to support the overlying beds, and the load is transferred to the
solid or rock along the sides. The wider the roadway, the greater the span
of the arch and its height at the crown. A similar but larger pressure
arch is formed across a longwall face, with one leg resting on the solid
coal and the other on the solid pack behind the coal face.
See also:abutment; arch structure; load transfer. Nelson
The pressure arch theory in roof action suggests that, when a narrow
heading is advanced, the layers of rock in the immediate roof deflect
slightly and relieve themselves of the load of the overlying strata by
transferring it to the sides of the opening by means of a pressure arch.
The arch width just short of that which the higher strata cannot span and
transfer the load to the sides of the opening is called the
maximum-pressure arch. The depth mainly influences the minimum width of
the pressure arch, although the type of overburden also plays a part. The
following formula has been developed for approximating the minimum width
of the maximum-pressure arch (W = minimum width of arch, in feet; D =
depth of coal from surface, in feet): W = 3[(D / 20) + 20]. The equation
does not apply for overburden less than 400 ft (122 m) or more than 2,000
ft (610 m) thick. Coal Age, 3
When an area of a mine has been sealed off from the remainder of the
workings by barriers or stoppings inserted at suitable points, it is
important to prevent the circulation of air within the sealed area. This
means that external air pressures must be equalized on all the seals. The
object of equalizing the atmospheric pressures on the seals is attained by
inserting or removing doors or brattice cloths at appropriate places. It
is possible to make all the seals contiguous with a common airway by this
means, so that, if they are not widely separated, they will be subjected
to the same external atmospheric pressure. Roberts, 1
Pressure formed over the workings by masses of rock being severed from the
surrounding formations, creating pressure on the pillars, walls, or other
supports. Pressure blocks of large size may result from natural geological
phenomena, such as faults, or may occur as a result of mining operations.
Lewis
A machine or blower having either pistons, cams, or fans for furnishing an
airblast above atmospheric pressure. Standard, 2
An occurrence when a coal pillar suddenly fails on becoming overloaded by
the weight of the rocks above it. Generally, the coal is forced with some
violence into the roadways and other open spaces. See also:rock bump;
rock burst; shock bump; bumps. Nelson
a. An enclosed space arranged on the access side of a stopping, which
seals off an area and is furnished with means of raising or lowering the
air pressure within it. BS, 8
b. If the mine area to be sealed off is extensive, and the seals are
widely scattered, the fact that they are subject to different pressures
may be unavoidable. In this event, pressure chambers may be required on
the outby side of seals. Pressure chambers are also of value when the
seals cannot be made tight, because of broken or fissured ground. The
principle consists of building an outer chamber by erecting a second
stopping on the outby side of the seal. The air pressure in the
intervening space is then controlled to prevent movement of air across the
seal. Roberts, 1
c. A method of driving tunnels and sinking shafts through running sand by
holding back the loose material by compressed air. The technique is no
longer applied to any great extent in mining. See also:caisson sinking
Nelson
The most effective method of preserving timber by impregnation with
creosote under pressure in tanks. Hammond
See:hydrophone
The usual die casting process in which the molten metal is forced into
highly finished molds under heavy pressure by plungers, compressed air, or
combined methods. Hammond
a. See:air dome
b. The bonnet on a steam boiler. Long
A process of rotary drilling in which the drilling fluid is kept under
pressure in an enclosed system. Brantly, 1
The decrease in pressure at which a liquid or gas is made to move between
the intake and discharge of a pipeline or drill stem. Long
In drilling, a diaphragm connected to the fluid column by a series of
ports incorporated in the design of some core barrels and preventing the
entry of drilling fluids into the core-barrel-head bearings. Long