Patina in Viewing Stone Appreciation
Examination of natural and induced patina in viewing stone appreciation
By Darrell Whitley and Thomas S. Elias, November, 2020
The appearance of "oldness" is highly valued in classical Chinese and Japanese rocks utilized in stone appreciation. This evaluation factor complements the traditional criteria of shape, color, composition, and texture (including patterns). Informed stone collectors often associated the concept of oldness with the notion of "ancient." These attributes contribute to the idea that a stone can appear to be elegant. In China, the term jiu ( 久 ) is used for rocks that appear as "old" and the word gu (古 ) for ancient. The word gu is also used to refer to specific elegant stones that have been held in collections. While in Japan, koshoku ( 古色 ) or patina is used for an impression of antiquity or an antique look. Knowledgeable Japanese collectors also use the word mochikomi (持ち込み ), meaning held for a long time, for stones that have been in collections for a long time and cultivated to develop a surface appearance that conveys the feeling of "oldness."
There is a significant difference between a viewing stone that has been cherished for many years in contrast to a freshly harvested rock. This aged look is due to the surface appearance of stone or its patina, a thin layer that forms on the surface of a rock that results from natural chemical and physical processes such as oxidation and polishing or direct manipulation by people.
Below are two stones that have been held in collections for many years and have developed aged patinas. The low arch-shaped stone is a Chinese Baowen; while the black mountain-shaped stone came from the Kamo River near Kyoto, Japan.
Outside of the viewing stone community, patina is frequently used in reference to metals that describe the chemical reaction of oxygen, rain, carbon dioxide, and sulfur-bearing compounds with certain metal surfaces to form a thin layer of oxides, carbonates, sulfides, or sulfates, depending on the metal and the substance reacting with it. Certain metals used in construction, such as copper-clad roofs and drains, will oxidize to a dull greenish color. Corten steel, a steel alloy developed to be corrosion-resistant for outdoor use, will develop a rust-colored outer surface through oxidation in the first several months of exposure. Silver-plated cutlery or a silver coffee service will naturally oxidize to form a blackish silver oxide colored patina in household goods. Newly minted silver and copper coins are bright and shiny but develop a darker patina over time due to use and oxidation processes.
Patination also applies to stones, particularly those composed of certain minerals that react to water, air, and other substances that may be present. Marble sculptures develop a patina; an ancient Roman sculpture will have a very different surface than a modern sculpture.
In archaeology, stone tools, particularly those made of flint or chert, virtually always have a patina. The surface color and texture can be very different in an old stone artifact than a new fractured "worked" surface. If an ancient stone tool, perhaps manufactured thousands of years ago, has been recently "chipped" in modern times, the stone's new fractures are glaringly obvious. In the following photograph, the newly broken surface looks very different from the stone's highly patinated surface. This flintstone is an entirely different color on the inside and the outside. In this case, the patina likely developed over thousands of years. Archaeologists also differentiate between different types of patina. Patina can be produced by mechanical processes (for example, polishing) and also by chemical methods. Archaeologists refer to "gloss patina" on cherts as a kind of polish due to the tool's use and wear pattern. The formation of patina due to chemical weathering can destroy or diminish the "gloss patina." Not all forms of patina are the same.
Patination may be a natural process or an induced process by human intervention. The latter includes the application of oils, waxes, stains, and various antiquing methods combined with rubbing or buffing. The Japanese technique of cultivating stones, yoseki, combines both natural and induced processes. Murata Keiji, one of the leading stone connoisseurs during the peak of stone appreciation in Japan, described this process in his book Suiseki (1965); he recommended watering stones or rubbing them with a dry cloth. Martin Pauli, a European stone collector, referred to watering stones out of doors as "garden yoseki" and rubbing stones indoors as "in house yoseki." Rubbing stones with hands or cloth to modify their appearance has become a common practice worldwide.
The following photograph is an example of yoseki, the Japanese practice of cultivating stones to develop a suitable patina.
Observations:
We have made a series of observations based upon several years of experience in handling stones and information gathered from other sources. Scientific evidence concerning patina formation on stones is mostly based on rocks used in architectural construction and different air pollutants' effect on their surfaces. The development of patinas is more complicated than simply placing stones out of doors and watering them for many years, and expecting them to develop an aged appearance. We are attempting to look at this process from a more realistic view to help viewing stone enthusiasts to understand patinas better. Furthermore, this article attempts to open an ongoing dialogue about patina and stimulate serious studies of this patination process. The majority of information about the development of patina on viewing stones comes from experiences in temperate climates. Sound data from tropical and subtropical climates is needed but is presently lacking.
The type of stone critically matters. Different mineral compositions found in rocks will develop different patina types. A rock is composed of various minerals with lesser amounts of other minerals, often referred to as impurities in the rock. The method in which a rock was initially formed and then later modified, sometimes by metamorphic processes, results in a hugely diverse array of rock types. Some of the minerals present in stones will react to oxygen and water more readily than rocks composed of non-reactive substances. It is an over-simplistic view to assume that all viewing stones will respond in the same manner to an extended yoseki treatment. Only certain rocks will develop a patina that gives an aged appearance.
The rate of the development of a patina varies widely. The time required for patination can be very different for different types of stones, for example, in carbonate, silicate, and iron-based stones. Wax stones from China, Malaysia, and other countries contain high levels of silica dioxide. Many of these stones are found in gravel and river beds, where they have been naturally polished by sand and gravel-laded waters. These stones are harder than calcium carbonate (limestones) stones. Wax stones appear to resist forming a patina when subjected to the Japanese yoseki treatments. These stones may develop a patina on a geological time scale compared to the brief time scale of an individual's life.
Similarly, we can look to the scientific literature on the formation of "desert varnish." This is typically found on stones in arid environments. One theory suggests that bacteria extract manganese from the environment and redeposits it on the stone's surface and that desert varnish may take thousands of years to form. Generally, basaltic rocks composed of magnesium and iron, are typically black and can respond to the yoseki treatment by developing a patina over time.
Chinese Ying and Lingbi stones, both limestone (calcium carbonate) with impurities, are typically light gray when removed from the soil. They will develop a somewhat darker gray color that suggests an aged appearance within the limits of an individual's life span. Natural black Lingbi and Ying result from the presence of organic matter (plants) in the stone.
We can see a rapid change in the surface of certain meteorites. Meteorites from the Campo del Cielo in Santiago del Estero in Argentina contain high iron levels, which largely accounts for these heavyweight rocks. The surface of a meteorite from this location purchased at the Tucson Gem and Mineral show seven years ago began to change within months. A significant amount of the rock's surface developed rust-colored patches that started to fall away in thin flakes or dust-sized particles. Over seven years, this rock has steadily become smaller and smaller. At this rate, the stone will no longer exist as a rock but will become a small pile of particles and dust within one or two decades. A second meteorite from the Campo del Cielo location displays the same reactions on the surface but at a much slower pace. Chemical changes of a stone's surface can be beneficial in viewing stone appreciation or detrimental as in some meteorites. We view rust as one type of patina.
An examination of two Argentine meteorites from Campo del Cielo reveals that both are showing surface oxidation. The rock on the left is showing rapid surface oxidation after ten months, while the meteorite on the right has a much slower oxidation rate over the same period.
Water quality and acidity will affect the patina. Acid rains, typically dilute sulphuric acid (H2SO4) and nitric acid (HNO3) resulting from fossils fuels, have a more significant effect on certain stone types. Limestone rocks are especially subject to erosion and etching by acid rain. This type of stone composed mainly of calcium carbonate will readily dissolve in dilute sulphuric and hydrochloric acids. This may help to explain the forms found in the Ying stone of southeastern China. Ying stones often have sharp edges due to removing the softer portion of the limestone rocks leaving behind the harder parts of individual limestone rocks. The etching of these rocks from acid rain causes a different surface appearance from the rock surface's slow oxidation processes when exposed to air.
Acid rains can also "clean" certain types of stones during yoseki. Some otherwise stubborn organic stains will disappear from stones after a few years of rain and regular watering.
Polished surfaces on stones can be natural or induced. Viewing stones originating from desert environments are often shaped and polished by sand and wind. The stones with higher quartz and silica levels are more likely to develop a smooth polished surface. Quartz and silica-based stone found in faster-flowing rivers and streams will also have a naturally polished appearance.
Furthermore, in arid climates, wind along river banks will transport fine sand and silt that can polish stones at the water's edge. In the following photograph, two sides of the same stone are shown. The highly polished side, as bright as glossy lacquer, was exposed to the wind on a riverbank, while the dull unpolished side was partially buried. Seven years of yoseki has failed to yield any additional change in this stone.
Stone can also be polished by collectors. Most Japanese chrysanthemum flower stones are enhanced by removing a portion of the matrix stone to expose the flower-like mineral formations and then mechanically polishing the surface to highlight the stone's patterns. The act of polishing a stone by natural environmental actions is creating a type of patina. In polished chrysanthemum flower stones and other pattern stones, the act of polishing eliminates any naturally occurring patina. It replaces it with another form of patina, one that does not necessarily evoke a feeling of oldness. The quality of the patterns in these stones are more important than the patina. Compare a natural stream-polished Chinese wax stone with a Japanese chrysanthemum stone that has been shaped and polished using power equipment.
Stone surfaces are modified due to staining from forest fires, pollutants (oil releases in streams and rivers). Streams rivers in mountainous areas subject to forest fires are filled with soot and burned particles immediately following a conflagration. In particular, airborne burning pine needles can deposit flecks of pitch on stones. The presence of these materials can stain stones. Likewise, releases of oil and other chemicals in these waters can also stain the surface of stones. The staining from these types of incidents results in an artificial patina that should not be valued in stone appreciation considerations.
Not all surface layers are patina. Some stones dug from the soil and used as viewing stones are coated with fine deposits that adhere to the stones' surface. These deposits need to be cleaned by mechanically removing the deposits to expose the stone's true surface. Once this is completed, the patination process can begin as the stone is held in a collection. The Chinese Lingbi stones, Italian Ligurian stones, and the Japanese Furuya stones are examples that need to be cleaned once they are removed from the soil. Cleaning organic and inorganic material from a stone's surface is an important initial step in preparing a viewing stone. This can be readily seen when comparing two Italian Ligurian stones. The stone on the right is a recently dug stone with a coating of fine light-colored particles adhering to the surface. Compare this to a similar-sized, gray colored Ligurian stone (left) that has been cleaned to remove the coating.
A final note: Collectors are encouraged to collect and study stones that they do not consider to be viewing stones to further the scientific study of patina and yoseki. This makes it possible to break the stone if necessary, to understand better how the stone's patina is different from the stone's interior. One could even cut a stone in half to create a controlled experiment, for example, comparing "garden yoseki" and "indoor yoseki" on two halves of the same stone. This would be most beneficial, of course, for those types of rock materials that potentially can be good viewing stones.
Artificially induced patinas are common. The demand for viewing stones with the most desirable forms, color, and patinas have exceeded the supply for hundreds of years. Dark-colored stones with an aged patina that evokes an aged appearance are held in high esteem. As a result, they have increased monetary value. This led entrepreneurs to engage in enhancing stones to obtain the desired features. Examples of this are described in Tu Wan's Stone Catalogue of Cloudy Forest published between 1126 and 1130 CE. The application of soot, oils, and waxes to stones to change the surface color and create the illusion of aged stone in a collection is ancient and has continued to the present. Elias and Nakaoji documented this practice in Japan in a two-part article in Bonsai and Stone Appreciation magazine (2016). The pair also noted this in Chinese stones in the May 2020 featured article on the website www.vsana.org.
Induced patinas can be created in a short time using different methods. The color and patina on this newly harvested Ying stone were made over three days of heating and cooling treatments in an antiquing solution.
Skilled stone carvers produce high-quality viewing stones for collectors by using different antiquing methods to create a feeling of antiquity. The different manufacturers closely guard these processes and their antiquing formulas. Some include repeated heating and cooling in concentrated black tea with lime and soluble black ink. Only a few people may know the proprietary formulas of some of the best antiquing methods.
Summary: A patina that evokes an aged appearance in viewing stones is a highly desirable feature among viewing stone connoisseurs. Patina is a subjective feature that elicits certain feelings of oldness and antiquity in viewing stone appreciation practices. Asian aesthetics value vagueness as opposed to western analytical approaches to understanding the nature of something.
Patina is a natural or induced chemical reaction on the surface of a rock. This reaction, often an oxidation process, occurs when air and water interact with stones' mineral contents to form a discrete thin surface layer. There are different types of patina depending upon the elements present in the rocks.
The development of a patina depends upon a stone’s composition and surrounding environmental factors. It is not a simple process, nor is it well understood. The patina on metals and stone used in construction and artworks have been better studied than patina in stone appreciation practices. But naturally occurring patinas, such as desert varnish, has been studied for decades by scientists but is still not fully understood.
References:
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Elias, T. and H. Nakaoji. 2016. Enhancing the Stone. Part One: The Japanese Reality of Modifying Suiseki. Bonsai and Stone Appreciation Magazine. 55(3): 26-31. Part Two: Sakurai Toshio, Japan’s Leading Professional Stone Carver. 55(4): 30-37.
Howard, C. D. 1999. River Patina on Flint Artifacts. Plains Anthropologist, 44(169): 293-295.
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Liu T. and W. Broecker. 2000. How fast does rock varnish grow? Geology 28(2): 183-186.
Murata, Keiji. 1965. Suiseki: all about viewing stones. Hoiku-sha, Osaka. (in Japanese).
