Raw materials for our clay recipes are delivered to Boston Valley in dry powder form in supersacks weighing 2200 lbs. We source all the clays used from North America and blends 2–3 clays in each recipe. Each recipe is dry-batched according to the current formula by a computer-controlled cart that is calibrated quarterly. After the proper amount of each ingredient is weighed out, the batch is placed in a muller-mixer where it is first dry-blended, and then the proper amount of water is added and the batch is thoroughly blended. This mixture is dumped into a souring bin where it is allowed to age for at least 24 hours before use so that the water has time to hydrate the raw materials at a molecular level to increase the plasticity of the clay. Once aged, the clay is extruded into a slug for use in the hand pressing or RAM pressing operation or extruded through a steel die into a specific profile.
All masonry units, no matter their forming method, pass through the finishing department and the hands of a Boston Valley sculptor. Any textures are added in the finishing department so that the new piece replicates the historic sample or approved new construction unit in all aspects of shape and texture. Extremely detailed ornament is carved into or onto the units by our talented sculptors. The finishing department is also responsible for adding any slots for anchoring, identifying mortar indents, and making any final modifications before the piece is dried.
Drying & Glaze Application
Upon completion in the finishing department, units are loaded into the driers. The driers are temperature and humidity controlled to ensure a proper drying rate and thus prevent cracking. Those projects that have specified a glaze finish are then taken to the glaze application department. The glazing department utilizes various application techniques to achieve the range of finished appearances and topographies required to replicate historic glaze finishes or create new unique finishes.
Our facility houses six large gas-fired kilns. Because the product can vary in volume from piece to piece, Boston Valley has the capability to customize our hearth structure when needed. This allows us to provide such a wide range of custom shapes, with depths that vary from as thin as 5/8″ for Guastavino tiles, all the way up to 4-foot wide masonry units. As with any ceramic product, terra cotta units demonstrate a color range when fired because of the small differences in temperature from the top to bottom and left to right of the kiln. These variations are inherent to this natural material formed of substances mined from the earth, adding to the beauty of the finished product.
After firing, almost every unit manufactured at Boston Valley Terra Cotta spends time in our sizing department. Masonry units are typically made oversized on their non-finished face sides so that they can be cut to a clean and accurate dimension post-drying and firing. Boston Valley uses large format, laser-aligned, diamond-edged wet gantry saws to cut blocks to the width or height specified on the approved shop drawings. The scrap generated from this process is recycled with our own pulverizing equipment and put back into the manufacture of additional terra cotta units.
The RAM press is a hydraulic press originally used as a forming method in the porcelain industry. Pieces made with this forming method are not generally as sculptural as those made by hand pressing, however like hand pressing, this forming method does not allow for undercuts to be included in the die. The RAM die is a two part die made at Boston Valley to the approved shop drawing in the model and mold shop. A clay slug is placed on the die and hydraulically pressed into the form. Air forces water in the plaster die to the surface, releasing the piece from the mold.
This forming method is used for sculptural pieces, particularly ones that contain non-linear detailing or details with undercuts. The molds that are the negative of the finished piece are filled by hand with malleable clay. Rubber mallets are used to press the clay against all five faces of the mold to a set thickness. Internal webs are formed according to approved shop drawings and provide stability for the walls during drying and firing. Once pressed, the piece sits for a period. The plaster dehydrates the clay, allowing the piece to hold its form when released from the mold.
The extrusion forming method is most useful for the production of a linear directional profile that requires many units. This method of production utilizes a large extruder to force the clay through a steel die, forming a hollow cored unit. The profiles are extruded into lengths longer than the finished size required to account for the shrinkage during drying and firing. They are cut to a finished dimension in our sizing department after firing. Boston Valley has amassed a large catalogue of extrusion dies that can be used for new construction masonry projects.
Slip casting utilizes a special recipe that produces a liquid clay body. This clay body is poured into a plaster mold and allowed to sit for a specified period of time. Again as in hand pressing, the plaster begins the dehydration process, drawing the water out of the liquid clay suspension and causing a measurable quantity of the solids to build up on the sides of the mold. Once the desired wall thickness of the finished piece is achieved, the extra liquid clay body is evacuated from the mold, and the piece is allowed to dry further before being released from the mold. This forming method is particularly suited for pieces with fine ornamentation and units that need to be hollow for anchoring such as balusters.