I've glued a lot of insulators, so I can offer some insights on the subject. The most important features of glass glue is for it to be absolutely colorless and transparent. It also needs to be strong, and should be easy to work with.
This is the best glue I have found for repairing glass. One product comes in a small red syringe, and is sold under at least two brand names, Loctite and "Super Glue". I've seen it for sale at hardware stores, and even in some grocery stores. The best thing about it (other than being truly clear and non-yellowing) is that you have as much time as you need to get things positioned right, then when you are happy with it, simply take the insulator out into direct sunlight and it cures in about 30 seconds. You must do the initial adjustments in the house, away from sunlight. The biggest detriment is that you do need a sunny day. There are also "professional" versions of this type of glue that harden under a UV lamp. This is what I use now, obtained from Kemxert Corporation. The product I have used is their KOA 300-1. It does have the disadvantage of curing a little slower, in the neighborhood of 2 or 3 minutes. Pretty quick, unless you have to hold it absolutely still that long! Cleaning the surfaces well is extremely important, including all your fingerprints. I use acetone as a last step (after normal cleaning of dirt, deposits, etc.). Brush off the final lint and dust using a new clean paintbrush, or compressed air. Apply the glue to one surface, and squeeze it together tightly. Be sure to give it time to fully fill in the entire seam, and check the positioning carefully. When everything looks good, simply expose it to sunlight or UV light as needed. Using this method, even several pieces can be put on one by one so each one is exactly positioned correctly. Don't wipe off any excess until after it has cured and sat for a day or so (I learned this the hard way... for some reason small bubbles of air can be "drawn in" from the edges of the crack). Excess glue can be easily removed after hardening with a razor blade or exacto knife, and final cleanup with acetone. This is often needed in a multi-piece project to remove excess glue from a surface that needs additional pieces glued to it. Check the fit carefully before applying the glue! Getting it right is important, as the stuff is very difficult to remove once hardened.
First, and very important is to clean the surfaces very well! If very dirty, cleaning with a stiff brush and water is a good start. Rust can be dissolved with acid, and organic material with bleach. Brushing with a toothbrush and water is also good for finer particles. Then check the pieces for a clean tight fit... if they don't fit right, look for small bits of debris that might be stuck to the broken surfaces. Picking them off with a small knife works well. Also look for small splinters of porcelain that might be slightly out of place... sometimes removing a small internal splinter does the trick and does not harm the external appearance. If the pieces have been soaking in acid or bleach, soak them in water for a few hours before drying and gluing.
As for glue, I have found that the 2-part "5 minute" epoxy (the clear stuff) works best. The best stuff is nearly clear in both parts, or even slightly blue. "Super Glue" brand Quick Set 5-minute epoxy works very well... (not the "super glue" that comes in a single small tube). It yellows with time (so it is not good for gluing glass) but this does not affect porcelain repair. Many other products I have tried will work, but are either a little too thick or cure too quickly or too slowly. Don't use JB Weld, as it leaves an unsightly dark line at the glue joint.
When getting ready to glue, check each piece for a clean tight fit. Brushing (dry) with an old toothbrush will remove any small particles just before gluing. Then mix up the epoxy, apply carefully and sparingly and place the pieces together, positioning the pieces carefully and tightly until you get a good fit. Hold in place until it hardens (about 5 minutes). Don't worry about cleaning up excess glue before it hardens... it's easy to remove with a razor blade, and far too easy to disturb the positioning if you do it before it hardens. Don't do more pieces than you can handle at once... and also make sure the next piece will still fit in after the current is fully bonded. You should plan the order of how you assemble things, otherwise the last piece might not fit in at all! If there are a lot of small pieces, I have found it helps to assemble them into several larger pieces before finally fitting those together.
Occasionally, I have found that a piece just does not want to fit in quite right. In that case, grinding away a small bit might help. Look for the area that seems to be blocking a good fit. I have a diamond grinding wheel that works very well, and a small diamond wheel for a Dremel tool (not very expensive). Another thing that happens is that glue squeezes out of the crack leaving a small bead, and this blocks further gluing if that seam is along an edge that needs further gluing. Scraping it off with a razor blade or small knife clears the problem. In the end, if you are careful, you can barely feel the crack seams. After a day or two, it will even ring when you tap it just like an unbroken one. I've done quite a few now, some (if they were cleanly broken) you can barely tell. After finishing, I do not recommend scrubbing with steel wool as it can stain the seams dark. If there is excess hardened glue on the surface, it can be scraped off with a blade. Thinner glue haze and fingerprints are easily removed with acetone and a clean rag.
(Not the brand name in general): This is the quick setting cyanoacrylate stuff (also called "Crazy glue"). Fast setting, but doesn't hold for the long term. In fact it is too fast, making it easy to glue things wrong. Eventually falls apart, and flakes off. Also does not match glass well, making the seam more visible than the UV stuff.
Fast and strong, but every type I have seen will turn yellow in time, especially if exposed to sunlight. This makes it unsuitable for glass, but it works excellently for repairing porcelain as explained above. Soaking in acetone or lacquer thinner will remove it, but sometimes it takes a very long time. Hydrochloric acid will also sometimes remove it.
Most types are not truly clear, they are slightly milky. Very hard to get good results, as it is thick viscous stuff. Virtually impossible to remove, as it is impervious to all solvents I have ever tried. Don't use it! I've seen some otherwise nice repairable insulators "ruined" by this stuff. This also includes "shoe goo" and related products.
Not truly clear, takes forever to dry. Does come off if soaked in water, but in one case it took several months to do so.
Haven't tried these myself, but I'd experiment on something worthless first. If it yellows, it probably wouldn't matter for many automotive uses, so I'd be concerned about that for repairing glass. Also, anything that sets very quickly without giving you adequate working time is a recipe for disaster (especially if difficult to remove with solvents).
This is the stuff that is sometimes used to repair or rebuild insulators. This stuff can work well sometimes if used properly, but it does cure slowly (over several hours). This gives good working time, but the pieces have to "sit" in place all by themselves, as you don't want to have to hold them that long. Be careful, a little slippage before it hardens will lead to a misaligned repair. If that happens on the first piece of a multi-piece repair, the rest of the project is doomed. You'll have to de-glue it. The main advantage is that if you are repairing an insulator that also needs some rebuilding of missing parts, there is no "compatibility" problems. I have found that the UV curing adhesive reacts with the epoxy used for rebuilding missing sections, leading to a yellow color at the junction. Suitable solvents for de-gluing include acetone or lacquer thinner.
An excellent solvent to use with epoxies is Jasco epoxy remover, but I haven't tried it with glass yet. It sure works great on poorly glued porcelain though! Be careful with it and use it outside, it seems to be fairly nasty stuff. Finally, a last-ditch strategy for stubbornly glued items is the careful application of heat. Most epoxies break down above four or five hundred degrees. Getting them even hotter turns the glue to a fine grey ash, making for a very easy cleanup job. But the risk to glass is very real... you really need a very carefully controlled oven or kiln, that will apply heat evenly and slowly, and control the cooling rate to be very slow also. You must also be careful not to get the glass too hot, or some colors may be altered! If you don't have a computer controlled kiln, I would not recommend it. Make sure the area is well vented as the heat will release some nasty gasses.
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Written Saturday, February 9, 2008; update Friday May 2, 2008