Typically Calcium Carbonate is the main filler used in RoyalCast™ columns. Calcium carbonate is essentially fine particles of crushed rock that when mixed with polyester resin, industrial glue essentially, solidifies or cures into a hard stone-like substrate. Essentially these hollow columns, a result of the rotational manufacturing process, will sound like you are knocking on thin stone. When projects call out for Class A fire rated columns, Alumina Trihydrate or ATH is mixed in with the Calcium Carbonate to form filler that meets the ASTM E 84 Class A test method for surface flame spread and smoke density. Without ATH, the columns will still pass the flame spread Class A test but fail the smoke test, which typically isn’t of concern in the exterior use of columns. Even though our RoyalCast™ fiberglass columns are robust, they are still considered a lightweight column (ranging from 46 lbs. to 4000 lbs. depending on column size). In instances where someone wants these composite columns even lighter, we substitute lightweight filler which decreases the column weight by approximately 50%, while maintaining the same material thickness. So a 10” x 10’ Round Tapered Column with a Tuscan Cap and Base would go from weighing roughly 128 lbs. to 64 lbs. The final ingredient is fiberglass strand chopped or cut to ¼” or ½” strands. The strand size used will depend on the diameter or width of the column and is used to reinforce the column and provide it with added strength.