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 olde 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.
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 slate 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 claystone. The pressure increases from the addition of overlying sediment, slowly squeezing the water out of the pores. Then folding causes the individual mineral grains to realign and form parallel sheets. The resultant rock has 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 fuel, 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, Fiuland, Scotland, Peunsylvania, 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), haseboards and roofing. As there is very high wastage in the production of dimensional, the waste is crushed and used for roofing granules, insulating material known as rock wool, and for filler for paint, linoleum, acoustical tile and brick.
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.
The most common manner of carving slate is in low relief; however, it can be used to create a truly threedimensional sculpture. Isamu Nognchi prodnced some large scale pieces with thick slabs of slate, and Barbara Hepworth fashioned a piece 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 ofthe 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 softstone carving tools can be use for this stage, such as flat chisels, ronelles, and toothed chisels.
After completing the outline, the interior body of the sculpture can be modelled and curved with rasps and rifflers to create effective shadows and a sense of threedimensionality. Polishing is started with 220 grit and can proceed to 1200 grit to obtain a high sheen. If desired, the surface can be waxed andbuffed 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 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.
The chief flaw in slate is the separations 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 harurner or the butt-end of a chisel will tell a 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 very well may be some more on the interior.
Because no hammering and power tools are reqnired, no eye protection in required with slate; however, the stone is finegrained, so a mask would be prudent if you are making dust.
Thanks to artist Ward Lynch of Everson, Washington for sharing his slate carving experiences with me.
Sculpture, ahh, sculpture. Poetry in form and light. I was introduced to bronze and sculpture together in 1980. Coming from a 2-D background where I was employed as a scrimshander (engraving on mammoth ivory), I was initially attracted to sculpture as a way of stretching artistically and bronze as a method of preserving the hundreds of hours I have a tendency of dumping into my work.
The journey that began back then has become the core of my experience. It has led to the founding and operation of North West Artworks, an art casting foundry in Sultan, Washington, that I helped establish with my father and brother. Casting for others has introduced me to many different styles and how bronze relates to them. In my own work, bronze's nature and soul play an important roll in the statement I want to express. My intention for this article is to not ouly illustrate the process of lost wax casting, but also that bronze is more than just a material to reproduce sculpture.
Bronze is 50% to 93 % copper. There are many kinds of bronze and brass alloys, the differences lying in what's been added to the copper to change undesirable characteristics such as low fluidity, gassing and weak castings. Common additions include tin, zinc, lead and silicon.
Most art casting is done in a silicon bronze, a lead-free alloy.Evedur and Hurlaloy are the two types of silicon bronze available. The main difference is the amount of zinc present. Each foundry has its own preference (Evedur being ours), but the desired characteristics are basically the same: fluidity, attention to detail, reparability and finish.
Bronze is bearing metal, which means it is soft and slippery or resistant to surface friction. This gives the metal a unique tactile quality. It is very malleable and can endure pounding and bending (cold work) without tearing.
Bronze is permanent (our insurance company claims it has a life of. 100,000 years at the bottom of the ocean). Bronze is musical (325 Ib. ingot resonates when dropped).
There are many ways of finishing bronze using chemical patinas. Some can accent the form and texture of a piece; others can give a depth of color rivaled only by stone. A faux granite, marble, and sandstone can be achieved with patinas.
Bronze is a structural material making it possible for expanded delicate forms (fingers, etc.) and a great versatility in scale. These and other characteristics make bronze an ideal sculptural medium.
Bronze has been linked into the evolution of civilization since its development 5,000 years ago. In the Near East, it was discovered that the addition of tin to copper created an alloy that was extraordinarily versatile in the production of tools, weapons, housewares and art. The lack of tin in the Near East created an expanding trade network that eventually encompassed Europe and the Far East.
The technique of lost wax casting is almost as old as bronze and has remained relatively unchanged until the 1960' s, when high-tech, high-temperature ceramic investment was introduced by the aerospace industry for use in precision castings. This, coupled with the development of silicon and urethane rubber used in the molds, made high detailed casting more achievable. But even with the modern advancements, art castings remain a hands-on, labor intensive, highly skilled craft.
There are two styles of lost wax casting. The difference is in the type of investment in which the wax is encased: traditional plaster investtnent and ceramic shell. Since we use the ceramic shell, the outline below addresses that method:
1. The original artwork is sculpted by an artist in wax, clay. plaster, wood, or stone.
2. From the original artwork (or from a found object), a reusable master mold is made. This master mold consists of a flexible inner mold and a rigid exterior mold or mother mold. The rigid mother mold is designed to hold the flexible inner mold in place and retain its shape. A single master mold can take from three days to several weeks to complete. The flexible inner mold is usually made from polyurethane or silicone rubber and the rigid outer mold from fiberglass or plaster.
3. From the master mold a wax pattern is made using a slush technique (i. e., pouring molten wax into the mold, allowing it to cool slightly and then pouring out the excess wax). This process is repeated several times to achieve the proper thickness (1/8" to 3/16"). Wax patterns for small pieces are usually cast solid. After the wax pattern is removed from the mold, it is chased (correcting in'Iperfections in the wax form) and dissected into pieces to aid in the casting process. This can take a week or more to complete.
4. Wax sprues, gates and risers are added to the wax pattern. They direct the way in which the wax evacuates or leaves the invested pattern and metal enters or fills the ceramic shell, and are crucial in controlling shrinkage of the sculpture as the metal cools.
5. After the wax pattern has been sprued, it is then chemically cleaned and invested (invested means creating a secondary waste mold around the wax). The waste mold consists of a "dip and stucco" technique using a silica slurry with stuccoed layers of imported sand. The result is a fireproof ceramic shell surrounding the pattern and the sprues, gates and risers. One layer of slurry is applied each day for at least eight days.
6. Burnout involves removing the wax pattern from the ceramic shell investment by using heat and pressure. The wax is evacuated when the cerantic shell is flash fired (plunged into an 1800 degree F furnace for 1-112 hours). Hence, the term, "'Lost Wax" .
7. Inspection of the evacuated shell takes place after it has cooled. The shell is vacuumed to remove carbon ash and patched if any cracks were created during burnout.
8. The pour involves melting the bronze in a silicon carbide crucible or cup. The molten bronze is then poured at 1950 to 2150 degrees F into the shells, which have been preheated to approximately 500 degrees F (preheating the shells reduces the chance of flashing and metal shrinkage).
9 The finish work involves:
(a) Knockout, or removal of investtnent;
(b) Degating, or removal of the sprues and gates;
(c) Welding and refabrication of the sculpture;
(d) Chasing the sculpture or fixing any casting flaws;
(e) Cleaning or sandblasting the sculpture;
(I) Patina work or aging, coloring and sealing the metal;
(g) Mounting the sculpture or making the base and fastening the sculpture to it.
Smaller sculptures can be cast in one piece, whereas larger or complex sculptures must be cast in many sections and refabricated. "Temple," an 80% life sculpture (pictured), was cast in seven pieces.
Turn around time is usually four to eight weeks (without mold work), depending on the scope of the project and the work load of the shop.
Costs are hard to generalize, but a 9" solid standing figures runs around $120, whereas a life-sized fignre is in the range of $4,000 to $6,000 without mold work. Costs can be cut dramatically with the artists participating in the labor.
Bronze offers an artist an expanded arena to express hislber vision. I am conducting a continuing series of workshops (see the paid advertisement in this newsletter) to introduce the fundamentals of reproducing sculpture. The artist can enter at any phase of the workshops to gain experience in a particular area of interest. The workshops are intensive, but give the artist a practical fundamental base on which to build.
If you have any questions concerning the current workshops or future workshops you would like to see, please contact me directly at: PO Box 777, Sultan, WA 98294; (360) 793-0783. If you need information about having your pieces cast, please contact: Todd Pettelle, clo NW Artworks, PO Box 658, Sultan, WA 98294; (360) 793-2412.
I would like to thank Bill Laprade and Sculpture Northwest for giving me the opportunity to contribute my perspective on this wonderful, though often misunderstood, medium. 1'd also like to express the privilege I feel to be a part of this exceptional organization. Welcome back, Bill.