The Basics of Dopping by Wayne S. Barnett
Dopping is the art of attaching the rough or cut stone to a dop stick so it can be oriented over a cutting or polishing surface. In many ways it is an art as the best orientation and position on the dop will help determine the final yield of the rough. Proper orientation, in some cases, determines the best color in the finished stone. The methods that are used to dop a stone vary as to the supplies available to a cutter and the methods that they feel comfortable using. Traditionally colored stones have been held on the dops with some sort of wax or resinous material. Modern dop wax is a mixture of shellac and clay fillers. The melting point of the waxes varies according to its composition. The most widely used dop waxes for faceting are the black and brown waxes. The melting points of these waxes are 155 and 160 degrees F respectively. Developments in the past several years in glue technology has made available several glues that are quite good for dopping stones. These are the epoxies and cyanoacrylates.
A set of brass dops that range in size from about 2 mm to about 25 mm. Dops may be made of brass, aluminum or steel.
Dops are the metal sticks that are placed in the quill of the facet machine after the stone is ‘glued’ to them. They are usually supplied with the machine when it is purchased. Depending on the machine the dops are 3/16, 1/4, or 5/16 inch in diameter.
There is a machine on the market that uses 1/2 inch dops native. Most machines have dops that are about two inches long but there are some that have dops that are longer and placed in the quill with a nut and washer. Most modern machines have quills that accept indexed dops so that they can be replaced in exactly the same position if they have to be removed before the stone is finished on one side. The end of the dop that holds the stone is 1/16 to one inch in diameter. Most stones are probably cut using the diameters in the range of 1/4 inch. A dop stick should be chosen that will be smaller than the finished diameter of the stone being cut. The dop stick should not be reshaped during the cutting of the stone.

The first step to dopping a stone is to determine the side that will be the table and grind a flat surface on the stone if a good surface is not available. The grinding can be done on a coarse lap or on a water cooled grinding wheel.

There are various ways to use wax to attach your stone to the dop. The most direct way is to use an alcohol lamp. The stone, dop and wax are heated over the flame to bring them up to the correct temperature, then pressed together. It is important not to over heat the wax in the process. If the wax is over heated or flames it may not hold the stone to the dop as it becomes too brittle to withstand the vibrations that are generated during the cutting process. When an alcohol lamp is used it is also important not to hold the stone in the flame in an attempt to heat it up too quickly.

First warm the dop stick and wax in the flame until both are quite warm and the wax will stick to the dop. Scoop enough wax onto the dop that will hold the stone to be cut. The right amount of wax will be a volume that will fill all of the cracks between the stone and the dop and give enough to form a cone around the dop to the back edges of the stone. The stone and the waxed dop then needs to be passed back and forth through the flame to warm each component gradually. In this manner the heat is passed through the stone and wax and allows them to become hot enough gradually, without causing thermal shock to the stone. When each are hot enough to have the stone stick to the wax they can be pressed together. The entire assembly should then be warmed by passing it back and forth through the flame until the wax fully adheres to the stone. This can be determined when the wax appears to flow out from the main body of wax and forms a slightly sloping platform on the stone and on the dop. Once the stone and wax have made a bond the stone is centered on the dop in the position that will give the best yield for the stone. Allow to cool to room temperature before trying to work with the stone.

A second method using wax is to use a wax pot or dop pot where the wax, dop and stone are heated over an indirect heat source then the three are brought together and then allowed to cool. This method may be better for larger stones as the heating is usually more gradual than in the direct flame of an alcohol lamp. If the wax is left to sit at an elevated temperature for long periods, it will become polymerized and brittle. When this happens it is difficult to use and may not stick to the stone. Orient the stone on the dop then let it set to cool in an orientation that will maintain the position of the stone on the dop. Once the stone is cool cutting can begin.

Stones that are heat sensitive should be dopped with glue or great care should be taken to warm and cool them gradually so thermal shock will not crack them. There are several types of glue that can be used. One of the more popular is 330 two-part epoxy. The super glues (cyanoacrylates) can also be used. If you use glue to mount the stone on the dop then make sure that it is given plenty of time for the glue to cure. Usually overnight. The biggest drawbacks to using glue to dop with is the inability to change the position of the stone once the glue has set and the difficulty in removing the stone from the dop once the stone has been cut. If care is taken the first problem usually is not significant. Removing the stone from the dop can be more of a challenge. One method is to heat the dop then giving it a sharp tap to break the bond. This technique has been used successfully by a number of cutters to remove the dop from the stone during the transfer. Once the stone is finished the final dop can be removed by soaking the stone and dop in acetone or Attack©. Attack© is a chemical (methylene chloride, it is toxic) that will dissolve epoxy. Acetone will soften epoxy and will dissolve the super glues.

One of the methods that have made the process of removing a glue dopped stone from the dop easier is to add some sort of filler to the glue. This may be cornstarch or some other additive that will allow the solvent to penetrate the glue faster. Normally the finished stone and dop would be put in a covered jar of solvent overnight to remove the stone from the dop.

Once cutting has begun a stone may become detached from the dop. In most cases the stone can be returned to the dop in exactly the same position by using one of the cyanoacrylates glues to re-glue it back to the dop. Check the orientation of the stone and make sure that the stone is oriented to the same position as it was held with wax. This is done by matching the wax remaining on the stone to the wax remaining on the dop. Once the position is verified use a small amount of glue to attach the stone to the dop. Enough glue should be used to have a little bit extrude from around the stone when a little pressure is applied. Hold the stone to the dop until the glue holds. Let the assembly set overnight to become fully cured. Finish cutting the stone as normal. When the stone is finished or to be transferred, use a flame to remove the stone from the dop as there will still be wax to melt in the assembly. Place the stone in acetone to remove any glue that may be stuck to the stone.

A combination of techniques can also be used to dop a stone. The first dopping can be done with wax and then the second can be done with glue. If a combination is to be used heat a dop stick and wax. Apply enough wax to the dop to make a good impression in the wax as the stone is pressed into it. Heat the wax and the dop until the wax is well bonded to the dop. Place the stone in the transfer block on a temporary dop and lock down the dop. Move the warm wax mounted on a dop onto the cold stone to create an impression in the wax of the stone to be mounted and allow to cool. Once the wax is cool, apply some super glue or epoxy to the wax cavity then press the stone into the cavity in the same position as when the impression was made. Allow the assembly to set overnight then cut as normal. When the first side of the stone is finished just repeat the process for transferring the stone. When the stone is finished warm the wax enough to remove the stone then place it in solvent to remove the glue.

Another combination method of dopping the stone is to dip the stone in a solution of alcohol and dop wax and then let it dry. Once it is thoroughly dry the stone can be glued to the dop. When the stone is finished it can be soaked in a solvent until the wax is loosened. If the stone will take a little heat, gently warm it from the dop side until the stone comes free. A little solvent on a paper towel should remove any residual wax from the finished stone.

Orienting Rough on a Dop

The orientation of the rough material on a dop will depend on several factors, which may include size, shape, color, or cleavage in the material being cut. If possible, it is best to have a rough stone pre-shaped before being dopped. If the stone is not preformed, the shape of the stone to be cut needs to be visualized so that the rough can be put on the dop in an orientation that will yield the best stone for the beginning rough. If the rough needs to be trimmed it can be put on the dop and oriented so that it can be trimmed of the excess efficiently. Once mounted on the dop the rough can be trimmed or preformed on a saw blade mounted on the spindle of the faceter.

The color of a stone may determine how it will be cut and therefore, dopped. In some stones, such as some tourmalines, the light is blocked down the c axis. In this case the stone needs to be oriented so that the stone is cut with the c axis parallel to the table. If the stone is cut with the table perpendicular to the c axis the stone will be very dark or completely opaque. Many stones show a distinctive color in one direction but little or no color in another. In these stones the orientation of the rough will determine the color of the finished gem.

Color can also be spotty in a piece of rough. In these cases it is best to put the color in the base of the pavilion. When the light is reflected back, it will reflect the color at the base of the pavilion. If the color were placed on one side of the stone the color of the gem would be uneven. Colors in a piece of rough may vary. In specimens of ametrine, the color goes from yellow to purple. Orientation of the rough so that both colors will be a part of the final gem is more desirable.

Many gemstone materials have cleavage. In minerals, cleavage is the tendency of the material to break along distinct planes. The chemical composition and arrangement of the molecules in the mineral determine these planes. Topaz, for example, has perfect cleavage perpendicular to the c axis of the crystal. It is important to orient a piece of topaz so that the cleavage plane does not coincide with the plane of a facet, and especially the table. Topaz is very difficult to polish on the cleavage plane. Stones with cleavage must be handled with care. One of the major problems with stones that have good cleavage is, that if hit right, they will break along the lines of cleavage. Stones that have poor or no cleavage tend to be better suited for some gemstone applications where impacts might take place (e.g. rings and bracelets).

Transferring the Stone

Once the first side of the stone is cut, usually the pavilion, it needs to be transferred to another dop that will hold it so the opposite side can be cut. This is usually done with the assistance of a transfer block. This is a block that has a "V" cut into it that is lined up to another block that has a matching "V". The stone and dop are put into one side of the transfer block and the new dop is set in the opposite side.

These transfer blocks represent two styles. The one on the left is made of a single piece of metal and the dops are moved together under the clips that hold them in alignment in the matching groves. The block on the left holds the dops in place and brings them together as one block is moved to the other on a rod. If wax is being used to hold the stone then the dop that now holds the stone must be kept cool enough so the stone will not move before the transfer is made.

To assist keeping the stone cool, wrap the dop and wax closest to the stone in a wet strip of paper towel. When this is done heat the new dop and apply wax to the end to be attached to the stone. If the pavilion was cut first, the dop will probably be a "V" or Cone shape to better accommodate the shape of the pavilion. If the crown was cut first then a flat dop will probably be used. Once the dop is coated with hot wax place it in the empty grove and press it into the stone. Heat the assembly form the dop that the stone is being transferred to so the original dop can be kept cool and the original position on the dop be retained. After the stone is in position, it may need to be reheated to get it fully set into place and bonded to the stone. Warm the stone and dop until the wax adheres to the stone. Once the set is made let the assembly cool to room temperature. When cool remove the stone and dops from the block, remove the wet paper towel from the uncut side and then heat the first dop until it is just hot enough to be removed from the stone. Pull the dop from the stone then use a fingernail to remove as much of the wax from the stone as possible. The stone is now ready to be cut on the unfinished side.

If glue is being used to dop the stone, then the stone and dop are placed in the transfer block. Thoroughly clean the stone with alcohol or acetone to remove any dirt or grease. A bit of glue is mixed then put on the end of the new dop. The new dop is then put in the empty side of the transfer block then moved into the stone. This setup is allowed to set overnight to let the glue fully set, then removed from the block and the original dop removed from the stone. Using glue on the second side of the stone eliminates much of the chance that the stone will be moved on the original dop during the transfer because the stone will not have to be heated to make the transfer.

On the transfer, the dopping can be done using just glue, wax or wax and glue. If just glue is used that case has been covered above. Another technique is to coat the stone with a thin layer of wax that has been dissolved in alcohol or acetone. When the first side is finished dip the stone in the solution and then let it dry. Make sure that it is thoroughly dry then glue it to the transfer dop as above. When the stone is finished it can be removed by heating until the wax melts. A quick cleanup with wax or alcohol will finish the process. An alternative can be to warm a ball of wax on the transfer dop so that it can be pressed into the stone to create a pocket that can be used to seat the stone and use some superglue to finish the transfer. Once the stone is finished it can be warmed to remove it form the dop and then placed in some acetone to dissolve the glue form the stone.