12.01 General Rules

Repair of rough material serves two purposes: to prevent cracks, fractures, vugs or any other type of defect from propagating (that is, growing) during rough grinding and to fill defects that detract from the appearance of the finished sphere. Both can be one and the same, but not always. If filling for the first type of problem with Hot Stuff ® is done ideally, further grinding will remove most of the plastic. Some may remain being nearly invisible. If repairing for the second type of problem is the object, many defects can be closed so that they become invisible on the finished piece.

Defects often propagate or grow as grinding progresses. This progressive damage is the result of what the worker alone is causing. Halting this damage seems a sensible step from the beginning or at whatever point it is perceived. The simplest and most effective way to accomplish this is to apply a plastic seal in the form of Hot Stuff ® to each defect. With further grinding, instead of making the damage worse, the repairs are largely removed and disappear completely while the rock remains intact.

12.03 Repair the Defect

The object is to repair the defect with a material that both cements the stone and fills the defect with a nearly invisible material. As noted above, shallow defects may eventually be reduced to a nearly invisible plastic filling or be ground away completely. In addition to the fact that defects can be of several types, the rock can be one of dozens of varieties and the repair process can be effectively performed with several forms of plastic. Only a few steps apply to repairs:

12.03.01.01 Cleaning

The stone and the defects must be both clean of grit and rock flour and especially clean of saw oil. It must not be forgotten that fingers are a serious problem in transporting of oils to the work piece. Saw oil is a major problem, however, our own body oils can be equally troublesome. People are constantly scratching, touching their faces an other activities where finger can pick up skin oils in abundance. Not to mention the fact that finger-prints are useful because they are made from skin oils secreted on our fingertips all the time. If cleaning the rock and hands with the usual means of soap and water is not effective in removing grit or other particles visible with a 10× lens, consider the following:

12.03.01.01.01 Acetone

Acetone is indispensable for removing saw oil. Immerse the piece in clean acetone for 24 to 48 hours followed by air drying until the odor of acetone is no longer detectable (about an hour for deep defects). Long or heat-assisted drying, is not necessary. As discussed in the next section, in certain circumstances, apart from the purists hang-up with repair, filling a space such as a small vug with Hot Stuff® is recommended. However, for authenticity, the plastic filling in the vug can be removed at the end. This can be easily accomplished by submersing the piece in acetone for as long as it takes. Generally 18 to 24 hours suffices.

12.03.01.01.02 Paint Thinner

Using paint thinner instead of acetone makes it possible to dissolve away oils when the piece has previously been repaired with Hot Stuff® or epoxy, both of which will be removed with acetone. Immerse the piece in paint thinner or a commercial deglossing agent such as is used to clean surfaces before painting, then soak the stone in this solvent overnight followed by air drying in a warm place for several hours. This solvent is especially useful when a piece requires secondary repair and already has pockets of Hot Stuff® somewhere.

12.03.01.01.03 Oxiclean®

Immersion in a warm (160˚ F, 71˚ C), strong solution of laundry detergent (OxiClean®, for example) will do an excellent job if the piece is soaked overnight to several days. Rinse thoroughly after, and air or oven dry (160˚ F, 71˚ C). An especially troublesome piece will benefit from soaking in this detergent and heated to below boiling for an hour or more and left to cool overnight.

12.03.01.01.04 Oxalic Acid

Water soluble oxalic acid is a must for cleaning rock with a rough surface and pits that have become filled with hard to remove grit, dirt and rock flour. A nearly saturated solution (1 tablespoon per quart), enough to cover the piece, for one to several days can be very effective. After each day’s soak, rinse the stone and clean with the Water Pik®, removing as much as possible. If dirt remains, immerse it for another day, etc, etc. A caution with oxalic acid (see footnote) is that it can damage certain stones. Oxalic acid will not remove saw oil. If the piece has been immersed in saw oil, it would be wise to soak the piece in a solvent such as mentioned above, first. When the oxalic acid solution becomes yellowed, discard and make up a new batch.

12.03.01.01.05 Water Pik®

As noted above, a Water Pik® can be extremely effective in clearing grit and rock flour from crevices and vugs in your work. This inexpensive item is available at most pharmacies, delivers a high pressure jet to a very small area and can flush out grit and mud especially when the tip is placed directly into the defect. Adding a drop of detergent to the reservoir makes the process more effective since spaces in the rock undoubtedly contain residual saw oil. This cleansing step should be repeated between every grit change and before filling with Hot Stuff®. This can prevent contamination as the result of coarse grit lodged in the defects, escaping and damaging the work at later steps. Some stones will be riddled with spaces that can trap grit, so a thorough examination with a good lens is advised. Continue this until all defects appear clean. Only then move on to the next step. Some rocks should have these spaces left intact and not filled, for authenticity. Pyrite and in agate with a crystal lined pocket, are examples. This will require that every step all the way to the final polish must be interrupted for WaterPik® treatment.

12.03.01.01.06 Dental Pick

A tool that is indispensable in cleaning rock is a metal dental pick. Its construction allows the worker to remove damaged and weak rock from crevasses. Your dentist has a drawer full of damaged picks and most likely will be happy to provide one for your use.

12.03.02 Drying

Dampness in fractures and vugs must be driven off before plastic repair. Keeping a piece warm (~170˚F, 77˚C) for an hour will do the trick. An alternative would be overnight on a radiator or a few hours in the sun. Do not use a microwave oven! Some stones will heat severely and may even shatter. If using the kitchen oven, set to the lowest temperature (~170˚F, 77˚C) for just long enough to heat the stone completely; 20 to 30 minutes. Cool the piece slowly to room temperature before trying to use the plastic, otherwise the cement will plasticize too rapidly.

For rapid drying, immersion in clean acetone then air drying is effective. When paint thinner has been used, evaporation will take longer. As a safe measure, since paint thinner is flammable, wait until the piece has lost any odor and place in a preheated oven with the gas off, for an hour or more. Again, if in a hurry, submersion in acetone will accelerate the removal of paint thinner.

Repair and filling is usually done after the work piece has been ground to a smooth sphere, but some of the most important repairs may have to be performed earlier or even at the very beginning. Once defects can be clearly identified and assessed as to how best to make the repairs, there are several approaches. When to use each requires a decision based upon the nature of the defect(s) and your preference.

The object is to stabilize and to support the edges of the fracture or defect such that future grinding action will not flake off additional pieces of rock (see Figure 18, below) Filling the fracture or pit with a hard plastic is an art in itself, but when and at what step is this practical? You can develop your own feeling for this; between carborundum grits #220 and #400 works well, but at times before #90 or even before doing anything to the block, as just noted. To make repairs after one has started to use diamond slurry is possible, but requires thorough cleaning and degreasing before the repair is started.

Figure 18. A diagrammatic view of a fracture that reaches the surface with the resulting edge that breaks away during grinding. The finer the grit the smaller the fragment.

At times a piece of material is found to have a crack that indicates the piece will crumble during working. The alternative to hoping for the best is to finish the job under controlled conditions, before starting. With a sharpened mason’s chisel and a good steel hammer, place the blade at the most obvious point and strike the chisel a sharp blow. Generally once will do the job and you can go to the repair process beginning with the immediate next section, after a good cleaning. An example of this dramatic solution is described below:

A 4 inch block of Charoite had an obvious crack around 90% of the equator. It was clear that this beautiful piece would not survive the rigors of rough grinding. The only way to proceed was to preemptively cleave the block under controlled conditions using a sharp mason’s chisel after marking the stone’s periphery at several points. Record landmarks by either a sketch or a digital image across the crack (felt pen marks will disappear with the needed acetone treatment to remove saw oil). The halves split apart easily with one firm tap of a metal hammer. By placing the two parts together carefully after cleaning, using the landmarks as a guide, it was clear that they could be reassembled. With good quality clear 2-part epoxy cement spread over most of the surface, the two halves were gently placed together, aligned to the landmarks, making sure that the original relationships, as denoted by the landmarks, were maintained. Once that was assured a wooden furniture clamp was applied across the poles of the reassembled block and tightened in steps over several minutes until pressure was very firm. It was left for 24 hours until fully set. It could then be worked safely as an intact block. Amazingly the original crack was essentially undetectable on the completed sphere.

An alternative would have been to clean the entire block then drying it thoroughly. Place the Hot Stuff® to the widest part of the crack and allow the liquid to flow until it is obvious that the space has been well filled and do not use accelerant at any point. Stop application and wait several minutes to allow the plastic to set. Repeat the application in the same manner and if needed several more times until the crack is filled to the surface. This may require repeating the application at several areas around the piece until the fracture has been sealed. Wait an hour or more before continuing whichever step preceded the repair, sawing, grinding etc.

It is possible that your block will break into several pieces. This is trickier, but manageable. All pieces have to be reassembled at the same time, before the epoxy sets, then subjected to pressure during curing. Several clamps may be required. Results can be perfect and nearly invisible. Some surface defects at the fracture may remain and can be repaired with Hot Stuff® later. Experience is the best teacher for good results.

12.04.02 Broken Sphere

A broken sphere, blank, or rough starting block can be reassembled with commercially available 2-part epoxy. This extreme situation has only one reason for repair; to reassemble a piece of valuable material that has split early in the process. Some serious repairs such as broken or dangerously fractured material must be repaired before much work is performed. This type of repair may not end with a fully filled piece. Its purpose is to securely reassemble a broken block. More detailed repair will be required later.

12.04.03 Serious Crack

Serious cracks, fractures on any sections that cannot be sawn or ground off or loose or separated fragments can be repaired using Hot Stuff®.

12.04.04 Deep, Narrow Defect

Filling deep, narrow defects is best accomplished with Hot Stuff®. (Keep the bottles in the freezer to extend shelf life.) Superficial defects can be effectively repaired with this material even if there are many of them.

12.04.05 Fine Crack

A fine crack is best repaired with Hot Stuff®. This is especially important when the crack demarcates a cap on the pole of a stone. This piece will very likely fracture off during grinding and require long additional grinding to correct.

12.04.06 Superficial Defects

Superficial defects are often seen in jaspers, such as Pudding Stone, Crazy Lace, moss agates, and dendritic agates. Hot Stuff® is recommended. However, if there is a number of small defects repair can take quite a bit of time. In these circumstances repeated application of Hot Stuff® and sanding to smoothness may be required.

12.04.07 Fine Fracture

Fine almost invisible fractures such as are seen in rose quartz or fluorite causes concern because they are very visible as internal reflective surfaces. Opticon® or Bond-Optic© are the best choices for this circumstance although careful application of Hot Stuff® is effective if time consuming. These fractures in crystalline materials are of particular concern because they enlarge and extend as you grind. These must be stabilized early in the grinding process, sometimes even before beginning to work to a rough sphere.

12.04 08 Many Fine Defects

For myriad tiny fractures in multi-mineral stone such as granites, pegmatites or labradorite, etc., use Bond-Optic© or Opticon®. However, with experience you may prefer to spend the time to perform this process using Hot Stuff®. Several hours may be required, however, careful repair can give you a spectacular product not otherwise attainable.

12.04.09 Flat, Smooth Surface Defect

Flat smooth surface defects such as residual saw cut flats are difficult since there is only a smooth surface for attachment. Hot Stuff® is your only safe option. Hot Stuff® will adhere to a smooth surfaces only after thoroughly cleaning the surface of saw oil with fresh acetone. Soaking in acetone and using a toothbrush with acetone is suggested. Opticon® can be used however this material yellows with age and become obvious with stones that are pale in color. Bond-Optic© is reported not to yellow, but I have not confirmed this.

12.04.10 Vugs

Vugs of varying sizes and depths can be stabilized with Hot Stuff® to insure that rough grinding will not seriously damage the edges. The plastic can then be removed completely at the end by soaking in clean acetone for as long as it takes. Eighteen to 24 hours usually will suffice.

12.04.11 Weak or Porous Rock

Weak or porous rock such as poorly consolidated rhyolite or welded tuff, sandstone and other sedimentary rocks that will absorb large quantities of liquid, saw oil, or water can be nicely repaired using Bond-Optic© or Opticon®. Great care must be devoted to ridding the rock of residual oil and water. Fluid plastics will enter the spaces easily and produce an excellent repair which can be worked easily and polished well with tin or cerium oxide or diamond if desired.

All repairs are basically for the same purpose; that is to fill an unwanted space with some hard material that will not yellow with age, dry and be lost or be visible. Plastics are the most suitable for this task and all products to date are variations of a plastic known as epoxy or polyepoxide. Generally speaking these plastics set with the application of heat and/or a second plastic that assists the cross-linking of the monomer. For lapidary work there are three forms for consideration to be discussed below. Cyanoacrylate or Hot Stuff® and Superglue, resists water and oil, polish well with cerium or tin oxide. It does have it issues, however, as noted in the boxes below.

12.05.01 Two Part Epoxy

Using this type of repair is generally considered when a block has broken or been broken intentionally. If the block has never been exposed to saw oil, reassemble the pieces carefully and make a series of marks across the crack with a felt-tip pen or nail polish that can be easily seen so that proper alignment for reassembly is possible. Marking the block before intentional cleaving is advised, however, it can be performed satisfactorily after, as well. If it has been sawn with oil, natural landmarks must be used and recorded either by a sketch or a scanned image of the piece since subsequent exposure to acetone will remove the markings. Clean out the small fragments and attempt to place the pieces together so that they are in their original relationship. After cleaning, make up a reasonable amount of epoxy cement, spread a sizable amount over the surfaces and replace them in their original position and slowly press them together forcing the epoxy to spread into the crevices. This spreading can take several minutes and it should not be forced. Wiggle and tap the pieces to aid in their sliding together. When you are certain they are in the proper position, apply pressure with a C-clamp large enough to span the poles of the block or an old fashioned, 2-jaw, wooden clamp. The latter is recommended. Epoxy should begin to ooze from all aspects of the joint. Increase the pressure slowly wiping away the excess. When the pieces are properly reassembled, allow the plastic to cure overnight. Once cured you can treat the block as though it was undamaged.

12.05.02 Hot Stuff®

While it is called an adhesive, Hot Stuff® is selective. Yes, your fingers are at risk, porous materials are as well, but glass and smooth rock must be extremely clean. A very satisfactory way to clean the work area and tools of Hot Stuff® is a soak in acetone.

Some sort of stand such as a medium to large sized, flat-topped plastic bottle cap is suggested. Cover the work area with a disposable pad, newspaper, magazine, etc. The plastic lids from kitty litter containers are perfect. If the monomer runs through onto a table top, removing it will be extremely difficult, even for granite or tile surfaces whereas the plastic cover can be separated from the glue easily. Use the solvent mentioned above (Super Solvent™, Golden West) or acetone to clean up or if your hands are used to heavy work, try white vinegar.

A word of caution for the uninitiated. The adhesive releases fumes which are very irritating to your eyes and mouth, and it is a menace to one’s fingers. Never apply Hot Stuff® while holding the stone! You may find yourself spending some time attached to your work; amazingly attached! The shelf-life of the bottles is less than 2 years. Towards the end of the bottle’s life, small cauliflower-like bulges appear on the sides. These places will eventually perforate and you can easily imagine what horrors can follow. The manufacturer recommends that new bottles be stored in the freezer to extend their life. Those of us who use the material for lapidary purposes should only purchase the 2 ounce size or smaller.

The application requires dexterity. Keep in mind that the fluid monomer has very low surface tension and will flow rapidly down a surface—to your fingers, for example! A smooth stone will allow glue to run rapidly downward while a rough surface will significantly retard the movement of the material from the application point. When you are ready to apply the glue, position the stone so that the area to be filled is at the apex or top of the stone.

The bottle applicator tip will often become plugged with plastic and must be cleared carefully. The manufacturers of Hot Stuff® recommend that you have a spare set of tips. Another suggestion; have a spare set of tips, but in addition, after each job drop the tip and black cap into a small sealed container of acetone. The same one you use to clear stone of saw oil is fine. Keep a fresh uncut tip on the bottle between uses and store in the refrigerator.

A simple method to restrict the amount to be delivered and direct the application safely and more importantly prevent the bottle applicator from coming into contact with the accelerant, if it has been used, is to use a sharp pointed toothpick as the vehicle. Let a small amount of plastic transfer to the toothpick and then transfer it to the defect. In this way you can avoid applying too much and having it run down the sphere.

A word about the accelerant (NCF Quick™, Satellite City, Cat. # QA6). A new can is under considerable pressure and when the valve button is depressed a large amount of material will be released. Press the button gently and only very briefly. The button has an arrow embossed into the top, but it is essentially invisible and you really need to see where it points! To help, use a fine felt tip pen and fill the arrow with black ink. The reason for this care is that the jet of spray is very directional so any bad aim will spray other than where you want to. Furthermore the manufacturer instructs the user to spray from 8 to 10 inches away. Believe them! You need very little spray to do the job, not enough to wet the surface. Obvious as it may seem but the manner in which you press the valve button is important. Note that the button can be moved from side to side. The result of this movement is that the spray can be directed away from center significantly. Learn to carefully press the button on it's center.

At this point you have three choices, none, accelerant before or after. Some cautions here. The fumes are very irritating to the eyes! The polymerization of Hot Stuff® releases large amounts of heat. So much actually, that you will see fumes rise from the area. The release of heat is so great that just before the Hot Stuff® polymerizes it can boil, producing unsightly bubbles in the clear plastic. The greater the amount of monomer the greater the chance for bubbles developing at the last moment.

A/ For deep defects, that is more than ¼ inch, (5 mm) it is wise to allow the plastic to polymerize on its own. It will take as much as 30 minutes to set well. At that point, additional applications of monomer can be applied more safely. The main object is to avoid producing bubbles in the plastic filling the defect.

B/ If your decision is to apply the accelerant after:

There is logic to applying accelerant last. A solid rule of physics is that two objects cannot occupy the same space at the same time! If the liquid accelerant is used first and the monomer applied soon after it will fail to enter spaces already occupied by accelerant and not fully evaporated. When the two make contact, deeper spaces will not be well filled with solid plastic. If one allows monomer to rest in a defect it will start to polymerize on its own as noted in A/.

As a general rule, deep defects had best be filled without accelerant and allowed to polymerize on their own over some time, generally 15 minutes to an hour. Deep defects will usually require several applications using accelerant late in the process.

With surface defects that are porous over a limited area, it is recommended that accelerant be applied last. Porous surfaces are especially troubled with this step if accelerant is applied first. Subsequent applications to the same area will be routine, that is, adding accelerant after.

Adding accelerant after adding a sizable amount of plastic will run the risk of having the plastic form a skim, shrink from beneath as it polymerizes, and suck in air through tiny holes. When that happens you have a serious problem in finishing the repair properly. Better to avoid this if possible. Allow the plastic to rest for 15 to 30 minutes to begin polymerization on its own. When the defect has been well filled with solid plastic the accelerant can be safely sprayed before the next application.

C/ If you decide to use the accelerant before, spray once at the area to be filled from ~8 to 10 inches away just before applying the plastic. You have 1 to 3 minutes before the liquid has evaporated. Polymerization begins from the bottom if accelerant is applied first and if a large volume of monomer has been added it will become superheated, boil and produce bubbles. As the plastic polymerizes, it shrinks. Refrain from adding too much monomer.

Once there is a significant amount of hardened plastic in the defect, from several applications, accelerant can be sprayed first and small amounts of monomer added. Continue this until the defect is completely filled to overflowing. The whole process could easily take 30 minutes to fill large defects properly. But once you mess up you have essentially eliminated the opportunity to do the job properly, unless you dissolve it all and start again.

There is a fail-safe if things go seriously wrong, such as a deep area which has developed bubbles before setting. Submerge the entire work piece in acetone overnight, removing all the plastic and start again.

Whenever you are about to spray the work with accelerant, place the black cap on the Hot Stuff® bottle and move it to the side or hold it in your non-dominant hand, out of the way. (A very small amount of spray can clog the applicator).

The temptation is to invert the bottle and place the tip at the defect. This will flood the area uncontrollably with rapidly hardening plastic. Instead hold the bottle horizontally, but be cautious. A safe way to direct the monomer to the work is to hold a toothpick at the tip. Allow a small drop from the end of the bottle tip, attach to the toothpick and then transfer it quickly to the stone. In this way you can control the location of the application and also prevent residual accelerant touching the bottle-tip. Sometimes all one needs do is to tilt the bottle slightly, tap it to the toothpick, and a drop of glue will run out. Using the toothpick as a swab, direct the drop of glue along the crack or defect. When material begins to run down, swab it back toward the defect. When the area has been filled or at least covered, watch carefully for signs of polymerization. If the fill is not deep and is not setting, wait before applying more. Accelerant will remain active for about 3 minutes. Move the stone so the next area is well situated on top. If the stone has become attached to the stand, as it may, break it free as quickly as convenient. If, during application, monomer runs down the work, have a small piece of paper towel handy and wipe this material away quickly. This will save a great deal of work later in removing unneeded plastic. As I said, this all requires some dexterity; a third hand would help!

If the defect is relatively large or deep, ¹∕16" (1 to 2 mm) or more, it is very important to coat the entire surface of the defect and the adjacent cracks with the low viscosity form of the glue, the “Original” and allow air bubbles to escape. Accomplish this with a coat or two of the standard glue without accelerant. If a defect is very deep and narrow, great care is needed to prevent incorporating bubbles into the plastic. Adding monomer without accelerant allows the liquid to enter all the tiny spaces in the defect before setting. Without accelerant, the plastic will set with time, but this is far better than having a repair with light-reflecting bubbles. Once the depths of a defect have been filled with hardened plastic a small spritz of accelerant will allow the next coat to be applied without incorporating bubbles. Never fill a sizable defect with monomer at one go! Filing and sanding away the excess is recommended. When sanding the plastic there are a few steps that will make this easier. Use a firm, but flexible surface such as a patch of canvas or the equivalent beneath the sandpaper. Holding the sphere with the area to be worked down, rub the sphere in a circular pattern. Of course back and forth works, but the chance of sanding a flat is greater. The circular motion lessens this. It is important to remove as much of the excess plastic as possible since layers above the sphere’s surface can slow the grinding process remarkably even with #220 grit and may tear away. If the repair has been done between #220 and #400 grit it is very important to carefully sand the surface to remove all the excess plastic. Smoothing the sphere with #400 grit can take forever.

Hand or machine sanding or filing rock that is softer than Mohs 5 must be done very carefully; sanding flats can be easily accomplished.

The machine grinding process can resume with low pressure at first, 15 minutes or so to remove the highest points gently. After, treat as normal.

Repeated cleaning and repair may be required before a smooth orb will be ready for fine grinding and polishing. Return the stone to the machine and grind for a while, than examine for areas which may need a second or even a third repair. If the filling has been properly applied, the result will be a highly polished and often invisible plastic plug. Mixed mineral stone repairs are likely to be less visible than on these single-colored stones.

From Section 17.0, page 133

Q I have a beautiful piece of blast rock. Can I use it?.

A You can, but it will be a challenge. Before you do anything wash it thoroughly in strong detergent and dry it completely for a day or two if need be. Then apply Hot Stuff® to each crack. The liquid plastic should sink into the crack. Add more until it is full. Be careful from then on. As you cut away rock deeper cracks may appear which present a challenge since you have already used Hot Stuff®. Soaking in acetone will require that you refill whatever crack you have already prepared. But this can work very well, if laborious. It would be wiser to soak the piece in paint thinner for a day or more before thorough drying.