How to Make Cherry Wood Marble Molds
by Drew Fritts  

Many marble makers don’t use cherry wood molds because they are fairly expensive to purchase based on the number of marbles that each mold can be used to produce. Even if you are not trying to make all of your marbles exactly the same size, there are limits to the useful life of a wooden marble mold due to burn-out. Graphite molds, on the other hand, can be used almost indefinitely, so the cost per marble ends up being much less than that of wooden molds. However, the "finish" that a wooden mold imparts on a marble is far superior to that of a graphite mold, and with some basic woodworking tools and a bench grinder you can make your own cherry wood marble molds for a FRACTION of the cost of buying them.

When making wooden molds it is important to select a wood that has both very tight, straight grain and very little pitch, or sap. Tight, straight grain is important because it allows less warping or "movement" in the wood when the wood is wet. Very little pitch is important because when pitch burns (on contact with the hot glass) it can smoke or discolor the glass.

Many fruit woods have both tight, straight grain and little pitch, and are therefore good choices when making paddles or molds to be used with hot glass. Of the fruit woods available in the US, cherry has extremely tight grain, very little pitch, and happens to be readily available in the thickness necessary for marble molds. That, I believe, is why marble molds are made almost exclusively from cherry wood.

  TOOLS AND MATERIALS

- bench grinder with fine grinding wheel - preferably aluminum oxide
- wood saw
- drill press
- clamps
- rubber cement
- cherry wood
- 5/16" dowel
- permanent marker

  ACQUIRING WOOD

If you live in or near a town that has a hardwood dealer you should be able to purchase cherry wood. My local hardwood dealer saves me ends that have been cut off and split pieces that are at least 2 inches thick. Because he can’t sell them to most woodworkers, I usually get them very inexpensively. Other sources include custom furniture shops and sometimes custom door shops. All you really need is their scrap, and many places don’t mind if you look through their dumpster. In any case, the piece needs to be as thick as you can get it. I prefer to use pieces that I can cut into two inch cubes. However, if all you can get is one-inch stock, that will work too, you just won’t be able to use all sides of the block for mold holes.

  SELECTING CUTTERS

I use two different types of cutters when making my molds: carbide-tipped router bits, and spade-style drill bits that have been reshaped on a bench grinder. For my 1/2-inch, 5/8-inch and 3/4-inch molds, I use carbide-tipped "core box" router bits. They are reasonably priced and it saves me the time of having to make the cutters myself. They cost about $20 each (which is about the cost of two commercially made cherry wood marble molds) and the number of molds I can make is virtually unlimited. However, to make molds larger than 3/4-inch, or if you don’t want to spend that much on router bits, I use reshaped spade bits.

  MAKING YOUR OWN CUTTERS

Go to any hardware or home store and purchase spade bits for the sizes of mold holes you want to make (Figure 1). They will probably cost between $1.50 and $4.00 each depending on size and brand. I’ve even found them at garage sales and pawn shops almost free!

Using a compass, or a computer drawing program, draw a circle the size that is marked on the spade bit. For example, a 1-inch spade bit will need a 1-inch circle. Cut the circle out as close to the outside of the line as possible. Now coat the back of the circle with rubber cement and coat the front of the spade bit with rubber cement. Let them both dry and then stick the circle onto the spade bit so that it is centered from side to side and is not hanging off at any point along it’s arch. (Figure 2)

You will notice that the circle slightly overhangs the width of the bit on both sides. This is because the shaft of the bit is narrower than the cutting edge so the drill bit won’t bind when drilling a hole. That means that a 1-inch spade bit is going to produce a mold hole that is slightly under one inch in diameter. This actually works to our advantage because, before we can use the mold, we have to char the inside of it and in so doing will actually make it slightly larger.

With the tool rest on your bench grinder set perpendicular to the grinding wheel, place the spade bit flat on the tool rest and grind away everything up to the line on your circle. (Figure 3) If you have standard silicon carbide grinding wheels, you will have to take this step slowly, touching the wheel and then taking the bit away from the wheel repeatedly, to keep from overheating the steel. Too much overheating will untemper the steel and the cutter will dull quickly when used. If you have an aluminum oxide grinding wheel, overheating is still a consideration, but much less so. When you’re finished, remove the paper circle and rub off any rubber cement residue from the bit.

The next step is to add relief behind the cutting surfaces so that they will actually be able to cut rather than just slide across the surface of the wood. To do this you must grind away material from behind the cutting edges. If you have a tool rest on your grinder that will tilt, set it at about 30 degrees. I don’t have that luxury, so I simply brace the bit against my stationary tool rest, raise the blade up approximately 30 degrees and freehand the relief. (Figure 4)

Start at the LEFT side of the arch (Figure 5) and grind away the relief, slowly rotating the bit around the arch until you get near the "point" of the bit (Figure 6). Then flip the bit over and do the same thing on the other side, starting on the LEFT again. (IMPORTANT: Due to the direction the bit will turn while in use, you must only grind relief on the back of the LEFT side each time you flip the bit over.) Keep checking to see what you’re grinding away. You will see flat spots where you have been grinding. The object is to get all of the flat spots to grow together to form one arched surface that comes just up to the back of the cutting edge, thus making it sharp! If at any time while you’re grinding, you see sparks coming over the front side of the bit, you’re grinding away the cutting edge! Pull back and adjust!

Once you get the majority of the relief cut away, it’s time to work on the "point". Don’t let this part scare you. The difference is that, at the tip, the cutting edge crosses over from the front of the bit to the back, and therefore you have to go a little slower so that you don’t over grind. The cross-over will actually take care of itself because of the way the relief is ground - always starting from the left and moving toward the point. Now it’s just a matter of cleaning it up.

Look at the bit and make sure that the relief surfaces, and therefore the cutting edges, are coming together in a sharp ridge across the "point" of the bit. (Figure 7) If you have a flat area rather than a ridge, work on getting the relief closer to the point starting from the LEFT side - flip the bit over and do the same from the LEFT again.

To check the roundness of your work, don’t look directly at the bit. Since it is ground on only one edge on each side, it will look uneven to you. Instead, hold the bit up to a light or window and look at the silhouette with only one eye. By observing the bit this way you will be able to see high and low points on the cutting edge. If you got fairly close to your initial circle and you didn’t grind any big chunks out of the cutting edge while adding the relief, the end product should be round enough.

Grinding your own cutter is actually much easier than it sounds. The thing to remember is that this is a very forgiving process. The relief angle doesn’t have to be exact nor consistent. It just has to make a sharp cutting edge. The bit doesn’t even have to be perfectly round as long as it’s close! Remember, the bit will be spinning and therefore needs to be balanced. However, because it is spinning, the edge that protrudes the furthest will cut all the way around! Therefore, even if it is a little uneven, it will cut fairly round.

  MAKING A MOLD

Cut a piece of cherry wood to size, using whatever saw you have. I use a bandsaw and I cut the wood to maximize the number of faces. For example, a two inch cube will allow five faces for molds and one face to attach the handle. Ultimately, the shape of the wood does not matter. What matters is that there is enough wood to leave at least 3/8-inch space on all sides of each mold hole.

Sand the block of wood so that each face is flat. They don’t have to be smooth and they don’t have to be perpendicular – just flat.

Mount a 5/16" drill bit in your drill press. Clamp the block of wood to the fence on your drill press so that one face is perpendicular to the drill bit. Drill a 1/2-inch deep hole in the center of the face. This is the handle hole.

Now mount the mold cutter in the drill press. Set a square flat on the table of the drill press and slide it up next to the side of the cutter. With a permanent marker, make a mark on the face of the cutter where the arch starts. (Figure 8) Move the square and do the same on the other edge. Then rotate the cutter by hand and make the same marks on the other side of the cutter.

Remove the cutter from the drill press. Lay it on a flat surface and, with a straight edge, use the marks to draw a line across the front and back of the cutter. Believe it or not, these lines will be visible while the cutter is spinning and will be used to manage the depth of cut. I use these same marks on my router bits - it’s just a little trickier drawing the lines. NOTE: I use my router bits in a drill press for making marble molds and they work great. You do not need a router for this process! Although carbide tipped bits are designed to work best at high speeds and running them at low speeds like this will dull them - eventually!

Turn the block of wood so that a different face is perpendicular to the cutter and re-clamp the block. Cut a mold hole into the center of the face. NOTE: When making smaller mold holes, such as 1/2-inch holes, I frequently put more than one hole on each face. I place them so that I keep as much wood around each hole as possible. That way, as each hole burns out larger, I get more usable life out of the mold.

Proceed slowly as you are cutting, giving the cutter time to shave the wood out of the hole. If you press too hard you will get "tear out" which appears as scars on the inside surface of the mold. A little "tear out" on the inside of the mold is not a problem since the glass is being spun when it is in the mold, but it is best to go slowly and avoid the scars in the first place. Bring the cutter slowly down until the line that you drew showing the desired depth of cut is level with the surface of the wood. If there is any question, it is better to under cut the mold holes than make them too deep. Holes that are too deep have straight sides at the top which limit the angle at which the glass may be spun in the hole.

After cutting mold holes in all of the remaining faces, tap a six inch piece of 5/16" dowel into the handle hole. There is no need to glue the handle. When the mold gets wet it will swell enough that the handle will never come out!

  SEASONING THE MOLD

Before the mold can be used it must be "seasoned." This consists of first making the mold waterlogged, then reaming, and finally charring the inside surface of each mold hole.

To waterlog the mold, submerse it in a tub of clean water so that the entire block of cherry wood is completely covered. I use a butter tub with holes in the lid that allow me to leave about three quarters of the handle sticking out. This allows the handle float so it is easier to grab while working at the torch. Leave the mold completely submersed for at least two weeks. When it is ready it will stay resting on the bottom of the tub when the lid is removed.

Reaming the holes is necessary because of the slight warping or "movement" in the wood while it is becoming waterlogged. It is necessary to re-round the holes. I used to skip this step and use the charring process to round the holes, but found that reaming is much faster and more reliable.

First I remove the chuck from the drill press and mount the proper cutter in it. I then hold the chuck in one hand with it acting as a handle for the cutter. I hold the mold in the other hand and simply rotate the cutter in the hole. This will typically shave small amounts of wet wood from two sides of the hole. I continue to ream the hole until the cutter rotates smoothly. I then do the same to each of the other holes in the mold.

The charring process is necessary because there is a small amount of pitch or sap in the wood that will smoke when it is burned. If you don’t char the holes, the first marble or two you make in each hole will run the risk of having smoke discoloration. This is particularly bad with clear glass.

To char the holes, I start at the torch with a gather of scrap glass the size that I will need to make a marble in that particular mold hole. I first round the gather in my graphite mold since I am not concerned with the "finish" and I can get the exact size sphere I want. I then use each of the holes on the mold that I am seasoning as if I were finishing a marble. I spin the hot glass in the mold hole and then dip the mold in water while I re-heat the glass. I continue spinning, dipping and re-heating until the entire inside surface of the mold hole is charred. I then repeat the process on each of the remaining holes. When all of the holes are charred the mold is then ready for use (Figure 9).

Always store the mold submersed in water. If it ever dries out, you will have to repeat the seasoning steps. The mold will continue to leach small amounts of color and pitch into the bucket in which it is stored. Change the water once a week or so and periodically wash the molds off under running water or they will acquire a layer of slime.

Once you have made your cutters it will take very little effort to keep yourself stocked up with as many cherry wood marble molds as you need! Best of all, your cost per mold will be almost insignificant and the finish on your marbles will be superb!

  ABOUT THE AUTHOR

Drew Fritts has been woodworking for over twenty-five years and has been working with hot glass since 1994. His web site has a constantly changing selection of his contemporary art glass marbles for sale and will soon have his fused works as well.

Email: drew@frittsartglass.com
Web Site: http://frittsartglass.com
Drew Fritts Marbles, 3875 E. Kingsbury, Springfield, MO 65809. 417-886-0325
Copyright © 1998 by Drew Fritts - All Rights Reserved

This article was originally published in GLASS LINE Magazine, December / January 1998/99, Volume 12, Number 4