An Economical Protective Coating for Stone Lab Benches David Shipley and Thomas Trumpinski School of Chemical Sciences, University of Illinois at Urbana-Champaign, Urbana, IL 61801
An ongoing pmblem in teaching laboratories is the protection of the working surface of the lab benches. Beginning with a coating formula supplied by a manufacturer, worn and damaged stone benchtops can be preserved and economically reconditioned The general chemistry teaching labs a t Illinois serve up to 4500 students per semester. Years of use had pitted the surfaces of the stone benches. In addition, the junctions of adjacent tops were sometimes mismatched in height by a quarter inch. To recondition the benches, we began with a procedure developed by Steven Hardy of the Alherene Stone Company of Schuyler, Virginia. (See original recipe in the table.) The kerosene is heated and small chunks of paraffi are added. (Caution: hishlvflammable. do not use oDen flarnen.,Atter the mixture i;cooled, the raw linseed oil snd activated carbon are added. (Note that m w linseed oil is necessary for hardening to proceed properly.) This mixture was applied with cotton cloths and allowed to dry. The recipe worked extremely well on those benches that were still smooth and in good condition. The coating takes three weeks to dry completely. Any time after the first two days, the surface can be sealed with Johnson's Wax "Complete for Floors." This is applied straight fmm its container and rubbed on using rough cloths. The sealant prevents the coating from rubbing off on clothing.
This combined coating resisted most lab chemicals used in freshman chemistry. However, it was removed by concentrated sulfuric acid, sodium hydroxide, or acetone. To fill pits or uneven joints, a thicker mixture must be used. (See repair mixture in the table.) This mixture is applied warm with a 12-in. paint scraper. A heat gun can be used to make the mixture flow into pits and cracks. The filler shrinks as it dries, so successive layers at two-week intervals are necessary. A thin layer of the original mixture can be applied to the filler and then waxed over normally Each batch of the original mixture wvered 75 mZ (800 feet2)of tabletop. For four labs that served 4500 students in the fall and 1700 in the spring, the total cost for eight batches (includingtwo repair batches) came to $145. Maintaining the labs for future years should cost less than $80 per year. ingredient
Original Mixture
Repair Mixture
Kerosene
1L
0.5 L
Paraffin
500 g
500 g
Raw linseed Oil
2L
1L
Activated charcoal
250 g
350 g
Volume 70 Number 11 November 1993
947
