Gypsum block

Gypsum block was made of calcined gypsum, or plaster of paris, with the addition of 3 to 5% of fiber (usually wood) that could be solid or cored. This material traditionally made up non-load-bearing walls but precast gypsum of the same formula were also made for roofs and floors or sheathing for steel columns and girders. The American society for Testing and Materials standards of 1925 required the manufacturer’s stamp on gypsum tiles they were shipping for resale.

History
In 1792, gypsum was discovered in New York. It had been used for decades as a fertilizer in fields and then deposits of gypsum were mined for agricultural use in Virginia in the 1830s and in Michigan and Ohio in the 1840s. By 1895, it was being mined in thirteen northern and western states. The commercial production of calcined gypsum began in the 1890s. The history of calcined gypsum is rumored to have originally started in Germany in the late 1890s. Calcined gypsum served as a component of precast tiles used for floors, partitions, roofs, and as a plaster for finishing interior and exterior walls. The precast gypsum tiles became popular for the fire retardant properties and in 1903, the United States Gypsum company’s plant began producing gypsum tiles. These tiles were marketed as being fireproof replacements for clay tile as long as the situation called for a non-load-bearing wall. By 1915, there were almost 70 mills in the United States that were producing gypsum blocks, the largest producer being the United States Gypsum Company of Chicago, the Acme Cement Plaster Company of St. Louis, and the American Cement Plaster Company of Chicago. With a record growth in the building industry in the 1920s, the gypsum product gained popularity at the expense of other building materials, being appreciated for their fireproofing qualities. Between 1900 and 1926, gypsum rock production in the United States increased, as did its value. By 1931, gypsum was being precast as form molds to reinforce concrete beams and columns. The process was used to pour walls, floors, and roofs. The next year, a flooring system used a similar concept that featured gypsum floor blocks laid on steel floor beams, and were then overpoured with concrete. This system, the Unitrave system, provided smooth surfaces for a plaster ceiling below and support for poured concrete above. Technological competition was slow compared the gypsum partition tile. In the 1920s, gypsum blocks were falling second favorite to gypsum wallboard. Gypsum wallboard was easier and quicker to install because gypsum blocks had to be set up and dried like traditional bricks with mortar. Also, gypsum wallboard was as fireproof as the gypsum partition board. Slowly, gypsum and the companies that manufactured the product fell out of favor.

Manufacturing Process
Gypsum tiles were originally made by hand until the 1920s. Framework as high as the thickness of the finished product was set on a rubber mat. Tapered wood was placed through the frame lengthwise to form the hollow core and then a thin mixture of calcined gypsum and wood fiber was poured into the frame. The mixture was given time to cure then the plugs were removed and the set tile was trucked to long shed for drying. By 1925, the more standard shapes were made on either a belt of circular machine. Custom shapes continued to be made by hand. Mills were traditionally built beside open pit mines. The gypsum was clacined, crushed, weighed, mix with water and fibers, and then poured into the molds and placed on a small car. After the molds were set, the car was moved forward, which caused the core-forming units to be released. The tiles were then unloaded and placed on another car which was bound for the dryer. Tiles traditionally took 12 hours to bake at 160° Fahrenheit.

Uses and Installation
Gypsum blocks were marketed as being fireproof in themselves or as protection for steel structural members. Gypsum was promoted as being sound with excellent thermal insulation properties, resistance to cracking from vibrations, workability, the ability to be sawed into proper size, and their low cost. Saving plaster or mortar was another benefit promoted by the company because the blocks were smooth. Also, the gypsum blocks were light weight and could be made larger than bricks or stone, causing construction time to decrease. Specially finished blocks could be developed for special purposes. United States Gypsum developed blocks that were smooth on one side for backing of finished plaster or final wall surface in warehouses. Gypsum was popular in dryer climates like Arizona because the exterior surfaces resembled that of a rough-dressed stone. The development of roofing systems using precast gypsum tiles with a waterproof coating grew in popularity. The tiles were advertised for their lightness and their ability to insulate and fireproof. The fact that they could cover large areas with fewer sheets was also an impressive characteristic. The H.H. Robertson Company in Pittsburgh made precast roofing slabs as wide as 8 ½ feet. The company described the tile installation as the slabs being molded in steel forms, with steel cable reinforcement to deflect tension. The cables are securely anchored to the molds at points beyond the end of the slab and the top of the surface. Floor installation using precast gypsum was first advertised in the 1920s by the Structural Gypsum Corporation of New York City. These floors had rolled steel channels that were spaced 30 inches on center. Precast gypsum floor slabs were laid on the steel and from which were suspended ceiling slabs of the same material. These slabs were reinforced and fireproofed the channels. Steel reinforcing bars ran through the slabs lengthwise. The floor slabs were reinforced with wire rods. In 1928, United States Gypsum said that its precast floors could be used with concrete joists for larger spans (up to 30 feet). The company was also advertising Pyrobar floor voids and roof tiles made of Structolite, a specially prepared gypsum.

Conservation
Gypsum is stable if it can be kept dry. Being used for inside walls and flooring or being protected by waterproof layers when being used externally should keep gypsum in a relatively good condition.

Deterioration
The main cause for gypsum’s deterioration is usually water. While gypsum does lose compressive strength when saturated, it will generally regain strength when given proper time to dry out. Moisture can cause corrosion of the steel rods found inside the gypsum. Calcined gypsum is chemically inert and will not cause the steel rods embedded in them to corrode. Gypsum blocks or tiles are usually never exposed so doing investigative testing and evaluation may require deconstructive work. Borescopes are helpful in examining partition blocks. The scope is inserted into a small hole in a wall to determine whether any units are cracked or have sustained water damage. Roof and floor tiles are usually evaluated only when roofing or flooring systems are being repaired or replaced. Corroded reinforced bars signify water penetration and the location and source should be identified to prevent further damage. It is most likely to occur in roofing systems where flashing has failed. Tiles and blocks that are not damaged severely can usually be retained but severely damaged pieces should be removed to ensure structural integrity. Fire code requirements should also be considered.

Conservation Techniques
With a mixture of one pint household vinegar and one gallon of water, a powdery gypsum block can be consolidated. A spray bottle is an ideal way to apply the solution and treatment should be continued until the surface is hard. Once the block has dried, it can be repaired with certain techniques. Gypsum plaster is the ideal way to repair gypsum blocks that are cracked or chipped. Portland cement and lime plaster will not usually bond with the gypsum, and repairs will be in vain. Gypsum partition tiles can be patched in a manner similar to most other plasters. Plaster of paris can be shoved into narrow cracks with a spackling tool. If the crack is large, loose particles should first be vacuumed out, the area wetted, and a thin layer of plaster joint compound should be spread over it. Cloth or paper joint tape should be placed over the crack and pressed down until the wall dries. The joints should be covered with a thin layer of joint compound, being sure that it is smoothed down to be less noticeable. After curing, wiping down the wall with a damp sponge removes loose debris. Another layer of joint compound can be applied, but the layer should be extremely thin so that it may blend with the surrounding areas. Larger holes are undercut on the edges with a knife and then vacuumed. This is followed by wetting the area and then filling the hole with plaster of paris or spackle. Filling in stages may be required when dealing with extremely deep holes, with no layer being more than 1/8 of an inch thick. Wadding newspaper into the hole can provide a temporary backing if the original backing is missing. Finish these holes with thin layers of plaster.

Replacement
Gypsum is no longer produced in block or tile form in the United States. If blocks have become so deteriorated that they need to be replaced, substitute materials may be the only answer. Block wall systems can be replaced by wood, concrete block, or metal studs and wallboard, all requiring a veneer of plaster.