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The Stone Corner

Studio Notes - Slate - May/June 2002

 

Slate is a rather mundane, common stone that we remember from our school days.  Recall those days when the teacher asked you to do your math problem on the blackboard (slate, in the olden days) and the dog had eaten your homework?  Perhaps you could have pulled out your carving tools, turning it into an art class, and rendered a spectacular art piece right there in front of the class for a bit of diversion. In the hands of an artist, this stone is indeed anything but mundane. Reliefs of beautiful and varied texture can be rendered from this stone.


Geology

Slate is a metamorphic rock, found in many places throughout the world.  Its main constituents are quartz, illite, sericite and calcite, but other minerals such as plagioclase feldspar, chlorite, dolomite, pyrite and graphite are also found.  Where the state is colored red, it probably contains a significant amount of hematite.  Its particles are very small, less than 0.001 millimeters in diameter.


The parent rock for slate is shale, which was formed from the deposition of mud (clay and silt). Slate is a very close cousin to argillite, also used for sculptural purposes, and associated with the Haida of the Northwest Coast. Slate is moderately hard, but it is very brittle.  Its durability is one of its chief attributes.  Its unconfined compressive strength ranges from 7,000 to 10,000 pounds per square inch.  Its very low porosity, less than 2 percent, is responsible for its impermeable nature and the reason that it has been used for roofing for centuries.  It is difficult to assign a hardness to this stone, because it depends on the direction in which it is worked; however, it normally falls between 1.5 and 2.5.


Slate is formed deep beneath the earth’s surface by the slow pressurizing of fine-grained sediments, such as shale and clystone. The pressure increases from the addition of overlaying sediment, slowly squeezing the water out of the pores. Then folding caused the individual mineral grains to realign and form parallel sheets. The resultant rock has a very pronounced directionality, known as cleavage; that is, the rock will split easily in one preferred plane.  The plane of cleavage is usually not the same as the plane of the bedding, and in fact, the original bedding of the stone may be very hard to discern.  In natural deposits of slate, veins of quartz and dikes of intrusive igneous rock are not uncommon, and the rock may be severely fractured along fault zones.


Although black is the most common color of slate, it is also gray, purple, green and red.  It is found in France, Finland, Scotland, Pennsylvania, Vermont and California. In Washington State, poor quality slate is present in the western part of the state, but commercial deposits are found in Stevens County in the northeastern corner.


The highest quality slate is used for blackboards and pool tables.  Because of its high electrical resistivity, it is used for switchboards and electrical panels. It is also used for mantles, flagstone (interior and exterior), baseboards and roofing. As there is very high wastage in the production of dimensional, the waste is crushed and used for roofing granules, insulation material known as rock wool, and for filler for paint, linoleum, acoustical tile and brick.


Quarrying

Because of the extreme directionality of the cleavage in slate, quarries sometimes continue at near-vertical angles to depths of 700 feet. Drilling and blasting are utilized to remove the weathered rock, but discontinued thereafter owing to the sensitivity of this brittle rock to shock.


Primary cuts are made by channelling or wire saw.  Individual blocks are then separated from the quarry floor by splitting parallel with the cleavage. Large blocks are then subdivided in a similar manner. Due to the brittle nature of the stone, the percentage of waste in a dimensional stone quarry may be 60 to 90 percent.


Working Slate

The most common manner of carving slate is in low relief;  however, it can be used to create a truly three-dimensional sculpture. Isamu Noguchi produced some large-scale pieces with thick slabs of slate, and Barbara Hepworth fashioned apiece called  “Two Figures” in which she pierced two large pieces of slate in a stunning slate sculpture.


Slate can be obtained from a salvage yard or thrift store where it may have been part of a blackboard or pool table. These are the highest quality slates available.  Alternately, flagstone slate can be bought from a stone yard or landscaping materials supplier.


In relief, slate can be worked in a similar manner to wood.  A drawing is copied onto the flat slate surface either by freehand or with carbon paper.   The outline of the drawing is scored deeply with a very sharp instrument.  Although a diamond tool may be the most efficient, any sharp steel tool will do. It is best to pull the instrument toward you to maintain control of the tool and to avoid chipping. The subject is then highlighted in relief by carving from the outside into the incision. Great care must be taken to avoid advancing past the incision because the layered slate will chip easily.   Standard soft stone carving tools can be used for this stage, such as flat chisels, ronelles and toothed chisels.


After completing the outline, the interior body of the sculpture can be modeled and curved with rasps and rifflers to create effective shadows and a sense of three-dimensionality. Polishing is started with 220 grit and can proceed to 1200 grit to obtain a high sheen. If desired, the surface can be waxed and buffed. Outlines should be retraced with a sharp tool to remove the wax in the grooves.


Texturing can be very effective on slate. This can be accomplished with a rasp, a toothed chisel or even a frosting tool, provided that the sculptor is very light-handed. A wooden mallet would be advisable for such work. Slate can be cut with a hack or coping saw, but care should be taken that the brittle nature does not cause the stone to break in a place not desired.


Flaws

The chief flaw in slate is the separation between the individual layers along the cleavage.  Look very carefully at the slate to see if there are any weak planes. As with other stones, tapping the stone with a small hammer or the butt-end of a chisel will tell the tale, but tap gently with slate.


Pyrite and quartz crystals are much harder inclusions that will ruin the integrity of the carving surface, so if some are showing on the surface, there may very well be some more on the interior.


Safety

Because no hammering and power tools are required, no eye protection is needed (unless you should choose to work with a Foredom and carbide bits).  However, the stone is fine-grained, so a mask would be prudent if you are making dust.



 

Stone Corner - Firing Soapstone Mar/Apr 2002

 

Many years ago I wrote an article about firing soapstone to make the stone harder. It is possible to harden the stone to the point that you cannot scratch it with anything short of steel. This will also close the structure of the stone enough to allow it to be used in outdoor applications. I have had several people approach me about this technique so I thought that I would reprise it with some of the things I have learned since then.


I have been working soapstone for the last 12 years and have talked to many of you about your experiences and here are some of the things I have found out. First, soapstone is soft. Now, that may seem obvious but there is more to it than that. Soapstone is chemically close to clay and as such, can be handled in a similar way. It can be “fired” to artificially do what takes nature much longer. I have found out from Ron Gietgey that this technique of heating soapstone is not new or strange. Insulators on the electric poles were at one time made of fired soapstone.


This is not a technique to be used with any other type of stone, especially alabaster. I have not taken the opportunity to try this on other types of stone in the same family, such as talc, pipestone or chlorite. Many of these stone will get harder over time or as they are worked so I didn’t feel this extra step necessary.


Some problems with this method occur because soapstone can have organic intrusions that will burn out during firing affecting the outcome of the piece. Be careful picking the stone that you are considering firing.  Be aware that these intrusions may not be noticeable prior to firing. If you are feeling very attached to a piece, it may be better to simply accept the limitations of the stone. So, keep intrusions in mind when choosing and working soapstone you are planning to fire. In the worst of cases they can leave voids that may have devastating results, even causing the pieces to fall apart. In addition, soapstone, which is available in a wide variety of colors, may alter in color when exposed to high temperatures.


My first test of this technique was in a kiln. I created a number of small pieces of soapstone sculpture. They were then fired at different temperatures. Fortunately these pieces for the most part had very little intrusions of organic material. They became very hard and the color changed to a rich brown. The patterning remained but was much more subtle. However, later pieces I worked had intrusions that did leave voids.  In some cases I left these voids as they worked with the “look” I wanted. In other cases I filled the voids with clear or tinted epoxy. The different temperatures impacted the pieces in appearance and form. The higher the temperature, the harder and darker the stone becomes.


Getting the desired results may not require a high temperature firing. You can get reasonable results using your home oven, raising the temperature slowly over an extended period. However, if there are organic intrusions these will burn out even at the lower temperature.


Now for the specifics of how this is done. I found that it is best to finish the piece to the final sanding. Sign the piece at this point. Then you simply place the piece in a kiln and fire to cone 01 using a slow, stepped temperature increase that will probably take as much as 24 hours to achieve. Afterwards the cooling should take another 8 hours. When this has been done, the piece can be treated as you would any other stone with a sealant and wax. Patience is the most important ingredient. Although I have taken pieces up to cone 4, I do not think that it is necessary to achieve the desired results.


If you are going to use your home oven, put the piece into a cold oven on the middle rack and start at the lowest temperature available, slowly increasing the temperature as the stone, not the oven, reaches that temperature. This may take up to an hour or more with each step depending on the size of the stone. Repeat this as you raise the temperature in 100-degree steps, to a high of no more than 500 degrees. Cool the stone with the door closed for several hours, until the stone is cool to the touch. The center of the stone will probably still be hot so leave it alone for a day or so.


I was interested in investigating this because I wanted to make small pieces that could be handled and used as worry stones. I have had very good results with this technique. To use this on larger work it is important to go slowly. The larger the piece the more problematic it will be to heat it up evenly.

 

Stone Queries - Sept/Oct 2002

Q: Where can I go to collect my own stone for carving or for bases?

A: First, a word of warning.  Every bit of stone out there belongs to someone; individuals - companies, state governments, federal governments.  I strongly advise determining ownership and getting permission before collecting any material.  This can be more difficult than finding the stone itself.  With individual landowners and companies it may be as simple as just asking.  Most state and federal agencies have permit systems and nominal fees for noncommercial removal of small amounts of stone, although some will allow collecting only from existing or abandoned quarries rather just anywhere on their land holdings.  Check with district offices of the U.S Forest Service and the Bureau of Land Management.  In the U.S., staking a mining claim for decorative stone or building stone is not valid.

Every state and province has a geological survey (although it may go by a different name) that publishes geologic maps identifying the rock types in given areas, often specific topographic quadrangles.  Many maintain lists of active quarries or can guide you the appropriate regulatory agency that does.  Most also have an industrial minerals specialist on staff who will be knowledgeable about decorative or building stone.  An index of state geological surveys can be found at www.stategeologists.org Look for geologic maps in areas of your interest and check for reports on building stone.  There are few such quarries in operation any longer in either Canada or the U.S., but a century-old report can be useful in locating old quarry sites.  It will be helpful to learn basic geological terminology, how to read geologic and topographic maps, and how to use the section, township, and range system of locating land parcels.  You won’t find GPS coordinates in a 1914 report.

If such a systematic approach is not to your liking there is always the option of simply exploring beaches, riverbeds, and road cuts.

And now a final word of warning.  Most stone quarries produce crushed aggregate for concrete and asphalt.  Those uses require hard, tough stone - perhaps not the best choice for carving.  The first stage of crushing is usually blasting with explosives to produce maximum shattering.  Similarly, road cuts are drilled and blasted.  Any pieces left over big enough for carving may contain incipient fractures that will break open during carving, often as you are working on that final delicate bit.

Feel free to contact me: Ron Geitgey, (503) 235-3474,This email address is being protected from spambots. You need JavaScript enabled to view it.

Stone Corner - Basalt - 2001 Nov/Dec

Basalt is the predominant igneous rock on the surface of the earth, the moon and probably other bodies of the solar system.  It is hard and dark and its presence evokes feelings of strength and power.  In nature, basalt’s high vertical cliffs stand tall for millennia; in sculpture, it is a symbol of eternity and the simplicity and fundamentalism of nature.  We are blessed in the Pacific Northwest to be in the midst of one of the larger outpourings of basalt in the world.

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Stone Corner - Limestone 1999

Bill LapradeWhite, creamy limestone has been a staple of Pacific Northwest sculptors since 1992 when 6 tons of Utah oolitic found its way to the Northwest Stone Sculptors 5th annual carving symposium.

It owes its continued popularity (10 tons at the 1993 symposium and many more tons subsequently) to its ease of carving, low cost and wonderful finished demeanor. It is indeed a “poor man’s marble,” both

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