Billiard Slates
The playing surfaces of pool tables were originally made of wood, but because of warping, billiard table surfaces were eventually changed to slate. Some billiard tables come with one piece slate beds while others are made of three pieces of billiard slate. Billiard slate is usually between ¾ and 1-inch thickness.
Billiard slate comes from sedimentary rock beneath the earth's surface. Slate is an organic stone with a high content of carbon, calcite and fine grained mica (quartz). Because it is organic, slate moves and shifts with temperature and humidity changes as it emits moisture. In this respect, slate is similar to wood and forms the perfect adjunct to wood in the making of a pool table. These characteristics of billiard slate supply the elasticity needed for the proper installation and playability of a pool table.
Various forms of billiard slate are found throughout the world, but its mineral properties vary greatly from place to place making suitable slate hard to find for billiard playing surfaces. For billiards, the slate must be fine grained, elastic, and have a low propensity to absorb moisture. It's rare to find slate with the precise mineral properties required for billiards. Traditionally, Italy has been the main source for billiard slate but large deposits of high quality billiard slate have been developed in Brazil. Recently Brazil has surpassed Italy as the chief source of billiard slate and now exports raw slate to Italy for processing. Billiard slate is also being imported from China, but the quality at this time is suspect.
Billiard slate comes in various thicknesses. The Billiard Congress of America specifies that three-piece slate, with a thickness of 1-inch, be used for tournament play. One piece billiard slate works fine on small bar tables but presents problems when using it on bigger tables. This is why tournament players prefer three piece slate tables over one piece. Three separate pieces of billiard slate allow for more accurate leveling, easier handling and less breakage. Three piece billiard slate also has its advantages when you consider that neither the table nor the floor will is absolutely level. In addition all three components, floor, table and slate, will flex to some extent. One piece slate is also more difficult and dangerous to move because of its weight.
Most pool tables are made of "oversized slate" which means the billiard slate extends underneath the rails. If the billiard slate isn't oversized the rails may sag and become misaligned. If this happens it may be unfixable and your table will be useless. Pool table rails sad and become misaligned when they are sat upon. Sitting on a pool table can also cause its cushion to separate from the rails.
Billiard slate backing or framing is essential for accurate leveling and covering the pool table. Some cheaper pool tables don't have slate backing. On quality pool tables, the cloth is attached to the slate backing with staples. If there isn't any backing the cloth will be glued on and won't evenly stretch over the slate. Particleboard or other materials used for backing often crumble and erode when staples are removed with each recovering. On the best pool tables, billiard slate is backed with hardwood. The backing should be wide enough to provide a firm platform of about 4 to 6-inches and extend down the middle of the slate to make contact with the frame's center beam (if it has one).
Billiard slate backing is an important part of leveling the table. When a table is installed the tightness of the slate screws will affect the leveling. The wood backing provides some give, so that by tightening the screws one at a time and inserting wooden shims underneath the backing, you can level the playing surface more accurately than in a table with unbacked slate.
When purchasing a pool table, please be aware that in addition to ¾ - inch slate, some manufacturers are cutting costs by supplying 7/8-inch slate to uninformed buyers instead of the standard1-inch slate. Thicker slate adds about 50 to 150-pounds to the weight of a pool table, making it more stable. If you even slightly bump a ¾-inch table the balls will shift. For a solid feel and quiet, true roll of the balls you should have a table with 1-inch billiard slate.