Techniques

Make it Flat

To laminate properly the surfaces must be very flat to avoid gaps. For large stones, we used power equipment, in particular, a Blackcat which is a diamond grinding wheel on an angle grinder. It still required laying a steel flat edge over the surface repeatedly to find the high spots, mark them, and grind again.  Eventually, the surfaces matched and we were able to drill holes for stainless pins and epoxy them together. The pins are necessary as stones can separate, not on the epoxy line but on the next layer of crystals.  

On a smaller scale, you can use a concrete block with some sand on it and rub the stone on it. This goes fast with softer stones but does take patience with marble. All the high spots are ground down for a very flat surface before gluing.

Lastly, if the carving is substantially done you might consider slightly “hollowing out” the middle of the stone. Then you will only have to flatten the outside “rim” of the stone. For a base to be flat I might remove just ¼” of the center of the bottom.

Chemistry of Finishing

Sand, sand, sand. The more scratches removed the deeper the color and the more gloss. If you “finish” sanding and then wet the stone it might look much better. You can continue sanding to get more scratches out or apply a coat of color enhancer. The silicon in the enhancer seals those micro-scratches. Wax is a nice top coat for protection and a smooth feel. Buff the wax by hand or with a tool to heat the wax into the stone. Black wax enhances very dark stones. Outdoor sculptures can be left alone or protected with a breathable sealer like Limestone & Marble Protector. Water must be able to leave the surface.

Belgium Black Marble by Caro Pemberton

Sanding and Finishing

There is no “magic wand” to finishing a stone sculpture.  It is just plain patience and work.  The critical work takes place in the beginning – getting the worst scratches out.  The steps are: flat chisel, rasps, sandpaper, rouge, or polishing powder or cream. Details:

1 – A sharp chisel starts getting out the chisel marks. Old school teachings are that the flat chisel, used gingerly, should provide almost the final surface.

2 – The larger the rasp the coarser the teeth. Pick the shapes that get in most of the spaces on your piece.  Most commonly we use point, spoon, and knife shapes but there are many options. Rasps need to be used in one direction; forward. Diamond rasps come in different grits and can be used in both directions. A good rasping job makes the next job much easier.

3 – Sandpaper must be made of silicon carbide (not aluminum oxide). It can be used wet or dry. Water increases speed and sandpaper longevity. You might start with 36 grit for marble or 60 grit for alabaster or limestone. A normal progression for sanding is something like 60, 80, 120, 220, 400, 600, 800, 1200, and 2000, if necessary.  Of course, not all stones need to be sanded this high. Colorado marble might only be sanded to 220 grit; limestone to 600 grit; black marble to 2000 grit.  Wash the piece between grits and then let it dry to see what you missed.

As a general practice, the first grit will take half the total sanding time. The next grit might take ¼ the total time.  So by the time we get to the higher grits the time required is minimal. Note the inevitable frustration: as you go up the grits you will find deeper scratches that you missed earlier. You will need to go back to some steps. Expect this.

(Advanced: Diamond sponges and sanding rotary tools speed up this process. I use all these to get the job done.)

4 – Higher abrasives are in the form of powders, bars or creams.  These are most commonly tin oxides. There are rouge bars that are used with a buffing wheel. (Make sure to use rouge for stone, not the metal stuff.) Powders are rubbed on the stone as are the creams.  These options are basically 5000 to 8000-grit abrasives 

5 – Chemicals are the last step – possibly.  I think I am done but test it by wetting it down. Did it make a big difference?  If yes maybe I should sand further with a higher grit. Another option would be to use a color enhancer. This is a diluted silicon solution that fills the microscopic scratches and keeps the surface looking wet.

6 – Wax protects the sculpture from fingerprints and other contaminants. I would not wax outdoor pieces as the wax will just burn off.  There are other outdoor sealants that allow the stone to breathe.