VINTAGE 1970’S SAND CAST / TUFA CAST STYLE CONCHO BELT
SAND CAST/TUFA CAST JEWELRY
The Sandcast Jewelry Method: Sandcast Native American is a style of Navajo jewelry was first created by silversmiths sometime between 1840 and 1860. The basis of this process is a mold made by carving the desired design into a stone. Using this mold, a piece of jewelry is made (belt buckle, bracelet, ring, etc.) and becomes the “master sample” from which the subsequent sandcast pieces are made.
Sandcasting is a labor-intensive, hand process not to be mistaken with centrifugal and vacuum casting processes. Artisans using this method estimate that it takes approximately three days from start to finish to make a medium sized bracelet.
Sandcasting evolved from casting using sandstone or tufa stone, a relatively soft stone created from compacted volcanic ash. Sandstone held up better, but the tufa was easier to carve.
The process involves many steps, starting by cutting a stone in half and grinding both halves until smooth on the cut side. The tufa stone is sometimes soaked in water to keep dust down and to make it carved in reverse on one of the smoothed halves. Then a cone shaped channel is carved from one edge of the design to the outer edge of the stone. A corresponding channel is carved on the other half. When the two halves are fitted together, this channel becomes the passage way by which the molten silver is poured into the mold. Superheated air trapped in the mold can ruin a design. To prevent this, vents are carved from the extremities of the design out toward the edge of the stone, allowing the air to escape. The stone mold is heated with a torch until it is carbon smoked on the inside of both halves. This prevents the silver from sticking to the mold. The mold is now ready for casting.
The two pieces of the tufa stone are placed together with the pour channels aligned. Originally the pieces were held together by placing strips of wet leather around the stone. As the leather dried, though, it would shrink slightly and bind the two pieces of stone tightly together. Today, artisans use a number of methods, such as wrapping with wire, plywood board and clamps, or rubber straps.
Following the completion of the stone mold, the silver is heated in a crucible with a torch until it is the right temperature, approximately 1850 degrees. Historically, temperature was judged by the color of the molten silver. That is still the most common method used even though many artisans now have access to crucibles with digitally controlled heat. Such fine control increases the chance for a successful pour. The temperature of the outside air must also be taken into consideration. If it is very cold, the silver can become too cool before it runs through the mold. When the silver is judged to be hot enough, it is poured into the mold through the previously carved channel.
The force of gravity pulls the molten silver down through the channel and into the carved mold. Thus the piece is said to have been gravity cast. The tufa stone mold is allowed to cool and the silver piece is removed. Extra silver from the pour channel and the air vents are cut off (debarring) and placed back into the crucible to be reclaimed for future use.
A tufa mold may allow several pours if the design is small and relatively simple. With larger, more complex designs, one or two pours is usually all that can be made from a tufa mold before the stone breaks or the design is ruined by the heat of the silver.
The piece is finished by buffing, filing, soldering on findings, setting stones, etc. and then the piece is ready for the consumer; however, if the piece obtained from the tufa casting is good enough, the artisan may decide to use it as a “master sample” (a model) in the sandcasting process. The process of sandcasting is very similar to that of the tufa casting just described, with the exception of the necessity of carving a new design.
Is an alloy of silver containing 92.5% by weight of silver and 7.5% by weight of other metals, usually copper. The sterling silver standard has a minimum millesimal fineness of 925.
SILVER, for example 99.9% pure silver, is relatively soft, so silver is usually alloyed with copper to increase its hardness and strength. Sterling silver is prone to tarnishing, and elements other than copper can be used in alloys to reduce tarnishing, as well as casting porosity and fire scale. Such elements include germanium, zinc, platinum, silicon, and boron. Recent examples of alloys using these metals include argentium, sterlium, sterilite and silvadium.