A: Hot-melt glue can be pretty good stuff: it’s strong, it fills gaps well, and it’s often flexible enough to hold even if the piece is stressed or metal parts flexed. Its big disadvantage is that you must work very quickly, and it’s not always easy to clean up the excess.

Another glue that you may like, which seems to perform rather similarly to hot—melt, is E-6000 adhesive. This comes in a tube, and is a solvent-based cement, drying in a period of hours. Near as I can tell, it’s about the same as hot melt for strength and flexibility. You need ventilation when using it, for the solvent fumes, and you’ll have to set up your glue joints so that things can sit for a while while the glue dries, without things moving around. The advantage of hot-melt is that you only need to hold it for a moment. But with this you’ve got more time to be neat, and arrange things the way you like. It fills gaps just as well as hot-melt, and is also flexible.

Some people also like RTV silicone caulking, for instance the GE pure silicone sealer/caulk that’s often used to assemble glass aquariums and the like. It comes in clear and white and black, and is often sold in auto supply stores as well as hardware and paint stores. It’s very water resistant, and quite strong. Spanning a large gap, it’s probably not quite as strong as the previous two, but it’s a pretty tenacious glue just the same. With this stuff, you have to be a little more careful to be sure the objects are clean before gluing, but that’s a good idea anyway. With the E-6000 and other solvent-based glues, the solvents seem to provide some slight cleaning action as the glue is applied. Not much, but it can sometimes compensate for a little film of whatever and still hold. Hot melt and silicone sealer aren’t as good at that.

Neither are the epoxies, which definitely require parts to be very clean. This family of two-part glues is the most often-used lapidary cement, for the last few decades, anyway. The Hughes Associates brands, epoxy 220 and epoxy 330 are very popular, and are formulated especially for lapidary work. The 220 is a slightly amber color; it sets slower and is slightly more waterproof than the 330, which is clear in color and sets slightly faster. It takes about an hour at room temperature, but you can cure it in about 10 minutes under a heat lamp. Its working time is around 5 minutes from the time you mix it to when it starts to get a bit too sludgy to work well. While the 330 is fairly waterproof, it seems to degrade slightly faster over time than the 220, which is quite resistant to water. All epoxies can cause a sensitization reaction, which can give various allergic symptoms. Use rubber gloves and ventilation, and keep it off your skin.

Many of the readily available brands of epoxy, are rather less serviceable with regard to water resistance. This is an important factor to consider when making jewelry, which can be exposed to perspiration and precipitation. Almost all the “five minute” quick epoxies are terrible in this regard, although they may be strong otherwise. While epoxies can fill gaps, they work best in moderately thin films, and tend to be harder and less flexible than the previously discussed glues. If the objects are rigid, you will get a stronger joint. If some parts are flexible, then it’s a weakness.

Properly used, epoxies will give you among the strongest glue joints you can get. But you do have to pay attention to details. First, parts must be truly clean. Epoxies don’t like grease and oil and fingerprints. Second, you must take care to measure out precisely a 1:1 ratio of the two parts, the resin and the hardener. (Only rarely is the ratio different, but read the instructions.) Dispensers for some store brands use a twin tube affair which can help, but be careful to avoid bubbles of air in one half of the syringes. The 220 and 330 brands come in tubes, and you need to be quite careful to measure evenly. The best way is to use a small scale for weighing the two parts, although volumetric measurement works if you’re careful. If improper amounts are used, epoxies can appear to be working well, yet be yielding a much reduced bond strength. A difference from the desired ratio of as little as 10% in either of the two components can cut bond strength almost in half. This depends on the particular brand of epoxy; some are more forgiving in this respect than others.

Epoxies do also have one other distinct advantage, in that they can be disassembled with relative ease. Use “Attack” solvent, also made by Hughes. Watch out for the fumes though—it’s volatile stuff, and extremely toxic. With any gluing assembly, you can help your product by being sure the parts to be joined are clean. Roughing up smooth metal parts with some emery paper or a file or the like will also greatly increase strength, as well as helping with cleaning. Also, providing a physical attachment point, such as a prong that penetrates the object being glued, will make the joint much stronger than one that depends solely on the glue.