Abrasives

Finishing Marble

There were major improvements in the technology of abrasives in the Twentieth Century, which have significantly affected how marble sculpture is finished. The finishing process that is most often seen today, particularly on abstract sculptures, results from a process that resembles the finishing of commercial marble or wood. This process produces a flawless finish that may or may not be suitable for figure sculpture. Both processes and some variants are described below.

For both finishes, smoothing usually starts with either rasping or grinding with some other coarse agent, either of which leaves visible scratching. These scratches are smoothed away using a sequence of finer and finer abrasives.

The standard commercial process, which is used by the majority of modern sculptors follows:

• Each abrasive grit is used until the entire surface is uniformally finished to that grit level, i.e., no scratches coarser than the current grit level are visible.
• Then the next finer grit is applied until all scratches from the previous grit are removed, and the procedure reiterated with finer grits until the desired degree of polish is reached.
• The finer grits are usually applied with water, which lubricates and washes the waste away.
• If a truly glassy finish is desired, the final abrasive is a paste rouge, or a paste of tin dioxide.
• Either a petroleum based marble sealer or oxalic acid marble sealer is applied as the final stage. (More on oxalic acid below.)
• Some sculptors follow this with wax. This is not a good practice--the oxalic acid will leave it plenty shiny without polluting the surface with organic chemicals that may later discolor.

The finishes used on marble before the Modernist era was often more complex, particularly in the late Nineteenth Century. Many earlier sculptors did not step through this rigid polishing process, and the differences were essential to the vividly lifelike appearance of the work of many sculptors, particularly in the late Nineteenth Century. The importance of the traditional smoothing process is often missed by modern sculptors. Although the abrasives we use have changed tremendously from those used a century ago, but the same effect can be obtained with modern abrasives. The older smoothing process was roughly as follows, but there were many variations. Modern abrasives are not exactly analogous step for step, but approximations are given in modern terms.

• The surfaces were rasped or ground to a fairly coarse grit that leaves visible scratches--equivalent to perhaps a 60 grit, 40 grit, or even coarser level.
• After the coarse smoothing, a fine grit was used that to finish the surface without fully obliterating the network of scratches. In modern terms, this would be like jumping directly from 40 or 60, directly to #150 or #220 grit paper. Under magnification, you see that perhaps 95network of scratches remain, much like the minute cracks you see if you look at your skin under high magnification.
• Tin dioxide was often applied as a polish.
• The final step was rubbing with oxalic acid. The oxalic acid not only sealed the polished flat surface, but also the whiteness at the bottom of the fine scratches.

The result is a much richer surface, that imitates some of the more complex visual properties of skin. In particular:

• The scratches have a significant depth, and their rough unpolished inner surfaces throw off light in all directions, resulting in sub-surface light scattering that more accurately resembles that of live skin.
• Much as with real skin, the resulting surface varies in reflectivity, being a network of tiny, slightly lustrous patches separated but minute cracks. The scratches separate the relatively shiny areas not just by their minute internal shadows, but by the luminescence they produce around them below the surface.

Note how different both approaches are from the practice of the classical sculptors. We now know, as the Renaissance sculptors did not, that Classical sculpture and temple architecture was painted or othewise polychromed. To the ancient masters, marble sculpture of the Renaissance and later periods would look as unfinished as would a bare plaster wall. Thus, they did not attempt to achieve either the flawless surfaces of contemporary carving, or the visually richer surfaces typical of sculpture before the 20th C. Many ancient sculptures have either been so weathered or so roughly cleaned over the centuries that it is difficult to tell what their original unpainted surfaces were like, but there appears to have been very little attempt to achieve the lustrous appearance of more modern works. Instead, early Classical and Archaic period works are usually chalky looking, more like limestone in appearance, at least partially because of the degree to which they were bruise in carving.

Traditional Marble Finishing Sequence

The traditional materials for final smoothing and polishing are mostly obsolete. Actual chunks of stone were used, and the final polishing was usually with various powdered abrasives. Modern materials are much better for most stages of smoothing. The traditional polishing sequence used in the late 19th and early 20th C. is given by Malvina Hoffman. Note that many of these are not longer commercially available, having been replaced by modern abrasives.

• Fine emery stones: these are sold for sharpening tools.
• Coarse sandstones: not commercially available anymore.
• Fine sandstone: not commercially available anymore.
• Pumice stone: This is a foamy volcanic glass that can be composed of many kinds of minerals. The walls of the tiny air cells are rigid mineral, and the broken edges proved a continuously renewed abrasive as the surface wears away. It is no longer sold as an industrial abrasive in rock form, but can be had in powdered form at the hardware store. The rock form is often sold for cosmetic use for smoothing off calluses.
• English hone: a fine mineral powder.
• Putty powder: a putty composed primarily of chiefly stannic oxide. Also called also jewelers' putty. Stannic oxide is also known as tin dioxide. Older preparations often contained lead.
• Powdered oxalic acid: used as a paste or as a solution in water, this chemically seals the marble and makes it more lustrous. This chemical is still available in the hardware store. It is used to remove iron stains from stone, enamel fixtures and masonry, e.g., rust stains from water that has been in iron pipes. While oxalic acid occurs naturally in small amounts in some foods, it must be used with care, as it is both irritating and toxic in the more concentrated form used by artists.

Oxalic Acid and Other Chemical Sealers

Used either as a water-paste or as a dilute wash, oxalic acid was used as described above on marble sculpture, and as the final finish on commercially cut marble slabs. This chemical soaks into the stone and links the crystals together, forming a dense relatively waterproof layer. Note that the modification to the structure of the stone usually changes the light-transmitting properties of the stone, making it more transparent and reflective. Oxalic acid is sometimes applied mixed with tin dioxide, in either paste form or as a dilute solution, to combine a final polish with the sealing. It can also be applied after the tin dioxide (or other final abrasive) as a thin solution. Using using oxalic acide for the final step is a form of patina, analogous to a bronze patina, in that it permanently chemically modifies the stone.

There are also other chemical finishes that pentrate marble with mineral solutions, that work in a similar way, inter-linking with the calcium carbonate crystals to form a dense shell. These chemicals are used in commercial sealers for high wear surfaces, and tend to leave a hard finish that can be mirror shiny.

Modern Sandpaper

Sandpaper is a 19th C. invention, although it was previously invented in China in the 13th C. Many kinds of abrasives, papers and other backings, glues, etc., have been used, an the technology is still evolving. The most important attributes of a sandpaper are

• The type of abrasive, which determines what substances it can cut.
• Whether it is open or closed coat, i.e., whether there is space between the grains, or they are jammed tightly together on the paper. Closed coat papers have more cutting edges per square inch, but tend to pack with waste material, and work best wet. Open coat papers let the waste fall away more easily, but have more space and fewer edges per square inch, so they cut more slowly.
• The backing, and whether the abrasive is intended to be used dry or wet.
  • Dry papers are usually for coarser sanding on softer stones. These kind of papers made for woodworkers work well on marble, but usually won’t work for hard stones. The abrasive is often either garnet or aluminum oxide.
  • Wet-and-dry papers are mostly for finer sanding and polishing. These are usually charcoal gray and as the name implies, can be used either with or without water. To use wet, just dip frequently in water. They can be used dry, but rarely are, because they pack with waste quickly. They are usually silicon carbide, and so will cut almost anything.
  • Cloth backing (emery cloth) is usually used with silicon-carbide or emery for machining applications. They are not very good for softer stones, but may be useful on hard stone.
  • Numerous other backing materials are available. Sponges impregnated with grit can be used to polish larger surfaces if detail is not an issue. Sanding belts and sanding wheels can be cut up to get relatively stiff pieces that can be used flat or rolled into stiff tubes that can be used like rifflers.
• Grit Number: grits numbered 60, 80, 100, 120, 150 on dry-type papers are most useful for softer stone. On marble, grit below 100 tends to give visible scratching, above 100 leaves a more or less matte finish similar to typing paper, that hides a lot of sub-surface detail. Grits numbered 220, 240, 320, 360, 400, 500, 600, 800, 1000 usually come on wet-and-dry papers. On marble, 220 grit paper gives a silky surface that is almost transparent, and starts to show sub-surface detail. Higher numbers give more and more polish up to an almost glassy finish with 100. Beyond 1000 grit, paste rouge is used to get a fully lusterous surface.

Numberous abrasives have been and are used for sandpaper:

• Sea-sand and crushed shells were used in some of the earliest papers. These are no longer used.
• Crushed flint, a very hard metamorphic form of quartz, was once common, but is now rare.
• Garnet is a natural abrasive that is still common. It can mean any of a diverse group of silicate minerals, and is a distinctive reddish or orange crystal. It is somewhat less common in sandpapers than formerly, having been largely replaced by aluminum oxide. It is good for sanding marble to a moderate fineness.
• Aluminium oxide is the most common sandpaper for woodworkers, having mostly displaced garnet. It is abrasive used in the pale orange sandpaper with the kraft-paper like backing. It is an excellent all-around paper for coarser sanding on marble, and other softer stones, but is too soft for the hard stones. Use this paper to get a matte, white, non-transparent finish.
• Emery cloth is mostly used by machinists, but is hard enough to abrade hard stones.
• Silicon carbide papers are available in all grits, but are most familiar as wet-and-dry papers. These are excellent for finer sanding and polishing of marble, and also work on hard stones.
• Exotics: there are numerous other abrasives for special purposes. Most will work if you happen to have them, but they are expensive and don't add anything that cheaper papers don't have for ordinary applications. Alumina-zirconia — (an aluminium oxide - zirconium oxide alloy) is used mostly for metals. Chromium oxide — used in extremely fine micron grit (micrometre level) papers. Ceramic aluminum oxide — used in high pressure applications for both coated abrasives, as well as in bonded abrasives (e.g. wheels). Cubic boron nitride is a substitue for diamond.

Rotary Stones and Wheels

This is a placeholder for a section of rotary wheels and stones, fiber disks, and exotic abrasives.

Finishing Hard Stones

This is a placeholder for a section on how to polish granite and similar stones.