The Ehret Magnesia Manufacturing Corporation in Valley Forge, PA was a leading manufacturer of insulating blocks and fire protection for pipes from the late 1800s until the 1970s. The company produced specialty coverings for the military, construction firms and other segments of the manufacturing industry where heat protection was required.
Thermalite was, for a long time, the trade name given to the material developed by the Ehret Corporation, which was a combination of asbestos and magnesia. This substance was used much like a putty or drywall compound and was literally painted onto steam pipes in boiler rooms, or made into solid blocks that covered hot pipes or was used as a wall to act as a heat shield.
Magnesia is made from magnesium carbonate, and after separating the compound asbestos was added to reinforce the block and make it more durable. Both magnesia and asbestos are natural fire retardants and do not conduct heat. The resulting blocks were made either as a rigid structure or as a moist substance that was easily applied to pipes and other plumbing, boiler walls and ovens. Ehret sold a number of pre-molded products such as asbestos sponge felt, asbestos paper, corrugated paper and millboard.
The softer sponge-like product made of Thermalite was wrapped around pipes and used inside wall panels much like foam insulation is used today. The asbestos fibers that made up approximately 15 percent of Thermalite were invisible to the unaided eye; workers who installed this product did not know that some of the fibers broke loose and stuck to their skin and hair. One of the biggest customers of the Ehret Corporation was the shipyards along the United States seacoasts, as nearly every steam pipe on these vessels was covered either by asbestos pipe wrap or a molded combination of asbestos and other material. Thermalite blocks were built up around furnaces in smelting plants and were used as an outer layer of protection around commercial baking ovens.
Manufacturing of these open-cell blocks was itself dangerous because workers inhaled loose asbestos fibers during the manufacturing process. Wherever these blocks were installed, they sat for years, often breaking apart during demolition of older buildings. The tiny asbestos fibers became airborne and settled into the collected dust, only to be carried away by drafts and ventilation fans.
Persons inhaling asbestos fibers often show symptoms of mesothelioma as many as 50 years later. The fibers acted as a carcinogen and malignant growths appeared, usually not diagnosed until it was too late.