Additional notes I found on the web years ago. unfortunately I do not know of the source anymore.
Stone Preparation - To begin making a piece of tufa cast jewelry you start by cutting a piece of tufa stone to the desired size that works best for your design. After the stone has been cut to size you must rub two sides of the stone together to create two sides that are perfectly flat and fit flush together.
Create of a Design. After the stone is ready for use, a design is sketched directly onto it.
Carve the Tufa Stone. The design is carved into the stone with hand chisel tools, wood files, sewing needles, paperclips and other tools. A dremel can be used for removing larger amounts of material. The carving must be done with perfection. Once a part of the stone is removed, it cannot be added back. The areas of negative space become the positive space where the silver fills in and forms the piece. Any mistake can render the stone as useless. You must add a sprue hole at the top of the rocks to allow the silver to be poured into the mold. It is also important that you carve air vents that go from the design all the way off the edge of the tufa stone. This will allow for air to escape as the silver is poured in.
Carbonize the Tufa Stone and Melt the Silver. It is essential to have the tufa stone carbonized before pouring the silver. This can be done with using just the gas of a torch without the oxygen. With the two parts of the stone covered in soot and bound with a piece of rubber or clamps add the correct amount of silver into a crucible and melt it with a torch to a molten state. Pouring only takes a moment, and if done correctly should fill the entire mold.
Sand and Clean the Cast Silver. The silver piece is removed from the mold and the extra silver in the sprue hole, not a part of the design, is cut from the piece. Sanding cleans and smooths the design, but the texture that is distinctive to tufa cast work is left where desired. This is the beauty of this form of casting. There may be smoothed sections that are created to provide contrast with the texture left by the tufa stone. Links that have been soldered together should also be individually sanded to make for a seamless link of silver. The piece can also be cleaned in a citric acid pickling solution to remove any fire scale caused when casting or soldering.
Polish. Polishing can be done with water and a rubber wheel or it can be done using polishing compounds. There is pre polishing compounds. Cut and finish compounds and final stage compounds. They can range from abrasive to non abrasive.
Complete the Work. The final stages of creating a one of a kind piece may include various methods in order to set stones, create unique forms through the melding of multiple tufa cast silver pieces, adding links to connect pieces, or implementing whatever other inspiration comes to mind. Sanding and polishing a piece can be very time consuming. Depending on the piece of jewelry it can be up to forty or fifty percent of the work.