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:

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

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 Note that many of these are not longer commercially available, having been replaced by modern abrasives.

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

Numberous abrasives have been and are used for sandpaper:

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.