Stone Polishing

Stone polishing is an ancient lapidary process whereby rough stone is polished and smoothed by hand or using simple machines to produce attractive stones.


 

A polishing treatment to sky blue topaz
A polishing treatment to sky blue topaz

image source: http://www.gia.edu/gem-treatment

What is stone polishing?

The most common means is tumble polishing, but there is also vibratory finishing. To effectively polish rock in a tumbler the stones must all be of the same approximate hardness. Rocks are placed into a tumbling barrel with varying degrees of abrasive grit and or water. Depending on the hardness of the rocks each step in the tumbling process can take place over a number of days.


info sources: https://en.wikipedia.org/wiki/Rock_polishing

 

 sculpture

Sculptures can be carved away either the direct or the indirect method carving. Indirect carving is a way of carving by using an accurate clay, wax or plaster model, which is then copied with the use of a compass or proportional dividers  or a pointing machine. The direct carving method is a way of carving in them to more intuitive way, without first making an elaborate model. Sometimes a sketch on paper or in clay rough draft is made.

The final stage of the carving process is polishing. Sandpaper can be used as a first step in the polishing process, or sand cloth. Emery, a stone that is harder and rougher than the sculpture media, is also used in the finishing process. This abrading, or wearing away, brings out the color of the stone, reveals patterns in the surface and adds a sheen. Tin and iron oxides are often used to give the stone a highly reflective exterior.

info sources: https://en.wikipedia.org/wiki/Stone_carving#Stone_sculpture

A worker shining a sacred sculpture
A worker shining a sacred sculpture

image source: http://canningliturgicalarts.com/stone-restoration/

polishING 

The process for polishing the edges of granite, marble, travertine, and limestone is indispensable when considering finished products like counter-tops, kitchen tables, and washstands. Since ancient times, the edges or corners of the stones have been rounded beautifully by expert workers and artisans whose craftsmanship ensured that the stones lasted well over 1,000 years. The custom of producing round edges has continued to the present today. We seldom see countertops or washstands with sharp edges.

The process of rounding off edges is not only for aesthetic purposes but also for functionality. The sharp edges of the table may possibly lead to injury if someone bumps against it, or the edge may crack or chip easily on impact with something hard. Even though stones like granite, marble, travertine, limestone, etc., are hard, they still have a brittle quality as well.

Edge polishing, represented by the “bullnose” trim, can be achieved manually by hand polishers or automatic machines, which can be applied to a large scale of material. The usual problem faced when polishing the edges of the stones is that, sometimes, polishing does not work evenly.

There is not much difficulty in polishing even, flat surfaces, but unevenness might pose a problem for the edges. During flat surface polishing, the tool makes contact with the stone surface by a close aspect, but for the edges, it’s relatively closer to the line or the point of contact. The grain of the edge tools will cut more deeply into the work material, and by varying the angle or pressure, the grinding ratio can be changed easily. To achieve a smooth, even shine, considerable skill is required besides ensuring that the abrasive part of the edge tool is evenly sintered.

info sources: http://www.granitepolishing.in/polishing/edge-tools.html

 

polished-travertine-floors-lake-forest-ca
Polished travertine floors

image source: http://carpet-tile-cleaning-lake-forest.com/2015/07/marbletravertine-floor-polishing-service-in-lake-forest/

 

How does a rock become a stone?

Rocks start out as something called a “tumbling rough”, an untreated stone. A tumbling rough is placed into a tumbling barrel with abrasive grit and water. This initial procedure is called the coarse grind. The purpose of the coarse grind is to wear down any sharp or uneven edges on the rock’s surface, eventually producing smooth round stones.

The second stage of rock polishing is the fine grind. This is the same as the coarse grind differing only in the use of a finer grit and that the stones are no longer shaped but are beginning to be polished. At the end of this step the rocks should appear shiny when wet, but matte when dry.

The Pre-polish step uses a mixture of very fine grit and plastic tumbling pellets. At the end of the process the stones have a slight luster when dry.

The final two steps are the polish and in some cases the burnishing.

In the polishing step, rock polish is added in place of grit as well as the plastic tumbling pellets. After further tumbling the rocks should now have a shiny look when dry. If this is not the case and the rocks appear to have a film on them, a burnishing may be necessary.

In burnishing the rocks are tumbled with only the plastic pellets and the addition of an oil free non abrasive soap.

info sources: https://en.wikipedia.org/wiki/Rock_polishing

what is rock tumbling ?

 

A typical rock tumbler
A typical rock tumbler

image source: https://en.wikipedia.org/wiki/Tumble_finishing#/media/File:RockTumbler1.JPG

There are two main types of rock tumbling: barrel (rotary) tumbling, and vibratory tumbling.

Rotary tumbling is more common, simpler, quieter and less expensive than vibratory tumblers. There are two differentiating factors, however, that may lead one to use a vibratory tumbler. First, vibratory tumbler retain the overall shape of the rough rock, whereas rotary tumblers tend to make rocks round. Thus, it is important to use vibratory tumbler to make faceted shapes and tear drop forms. Second, vibratory tumblers tend to process much faster than rotary tumblers, generally reducing the process time to half.

Sometimes, stone “preforms” are used. This refers to cutting shapes from the rough rock, before tumbling. This gives more control over the final piece, so shapes such as a tear drop can be produced. The technique is still limited to rounded shapes. Preforms may use less time with the coarsest step, or skip it altogether.

info sources: https://en.wikipedia.org/wiki/Tumble_finishing#Stone

Amethyst Sage Agate Preform Cabochon
Amethyst Sage Agate Preform Cabochon

image source: https://www.etsy.com/ie/listing/265801462/custom-order-for-terri-3-stone-preforms

During the 1970s, small rock tumblers were a common hobby item, and jewelry decorated with tumbled semi-precious stones was very much in fashion. Likewise, dishes and decorative glass jars filled with tumbled stones (often including common rocks not suitable even for costume jewelry) were frequently used as household ornaments.

info sources: https://en.wikipedia.org/wiki/Tumble_finishing#Stone

Purple glass display dish with 8 tumbled stones
Purple glass display dish with 8 tumbled stones

image source: http://picclick.ca/Purple-glass-display-dish-with-8-tumbled-stones-252565628619.html

 

info sources:https://en.wikipedia.org/wiki/Stone_carving#Stone_sculpture

http://www.granitepolishing.in/polishing/edge-tools.html

https://en.wikipedia.org/wiki/Rock_polishing

https://en.wikipedia.org/wiki/Tumble_finishing#Stone

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