It’s easy to think that you use more or less the same type of adhesive for all types of shoes and purposes, but this is not the case. There are three main types of glues or adhesives, and it differs a lot between them in terms of both the use and the results with different advantages and disadvantages. Shogeazing goes through it all.
To begin with, you should be aware that one uses a lot of binders when putting together a shoe, and that also applies to shoes with stitched constructions. It is not unusual for people to think that the sole of a welted shoe is just sewn, so it’s something that bears clarification. But you use adhesive for the whole shoe more or less, for the upper between the lining and upper leather and for reinforcement elements, for heel and toe stiffeners, for attaching shanks (reinforcement in the waist made of metal, leather or wood) and parts of the shoe interior, between the different soles, between the layers in the heel, and so on. I’ts possible to make shoes without glue, but it’s very complicated and the shoes will be quite lousy, so there is no one who does it.
Below I go through the three most common variants of the bonding agent used in traditional shoemaking:
There is some confusion in this area in the differences between glues, at least in the “rest of society” so to speak, even if it is relatively clear what the differences are in footwear manufacturing. Paste was initially water and starch-based, although today there are products with different content, but it is always vegetable. The adhesive dries slowly and have to be fixed during the drying process, and when it dries, it forms a very hard surface. The advantage, or in some cases disadvantage, with paste is that it can be dissolved again with water (not that some rain will cause it to dissolve, however, it takes more than that), and then it can be dried again. Therefore it is very common to use paste for toe and heel counters in leather since it makes them hard and hold their shape well, but also can be reshaped if necessary. Shoes with leather stiffeners can thus be lasted out or redesigned when necessary, because leather is a living material and the paste can be “revived”.
For bespokes shoemakers or a some few RTW/MTO manufacturers, it is not unusual to use paste when building up the heel. It requires more work than with contact adhesive that attaches fast and hard, while paste needs to stay fixed when the drying. There is also a disadvantage in the fact that paste does not hold quite as good, therefore it’s even more important to do a proper job with brass or wood pegs. Why won’t all use conact adhesive then, one might ask? Well the great advantage of paste is that it “hardens with the leather”, and becomes one with it, so to speak, with the same hardness. So when you sand and do the finish of the heel it becomes almost seamless transitions between the different layers, instead of the thin bright lines which is often formed when using glue. Pasted heels for example also looks nicer over time since glue tends to seep out between the heel parts and also between the welt and the midsole.
There are many different types of paste used in shoemaking, some of the most common ones are Metrotex and Hirschkleber. It is also possible to mix your own paste, with for example water and potato flour, however, one should use some kind of preservatives when doing that, since otherwise it will rot with time.
The “in-between” variant of binders, and as the name tells it from the beginning consisted of raw rubber, but is now mostly synthetic. If you ever repaired a flat tire and glued on a small repair patch it’s rubber solution that you used. Rubber Solution is more durable than paste, but don’t bond as hard as contact adhesive. It is therefore used for surfaces where you don’t want things to be solid like stone where everything follows when you pull something away, that’s why it’s sometimes used for example to attach the sole so that it can be repaired without the shank and cork filling risking to be pulled off togehter with the outsole, or for the sock liner inside the shoe.
A big advantage over paste is that it’s much easier to work with, you can brush it on the two surfaces you want to attach together, then allow it to dry for some 10-20 minutes to get really sticky and react with the material, then press the parts together and voila, they are stuck. It’s less good from a health perspective than the paste, but better than the contact adhesive. So as mentioned, in many ways an intermediate between the other two categories of binders.
Contact adhesives are the strongest binder used in shoemaking. It differ from paste by containing solvents and were from the beginning animal based (bone, horn, skin and stuff like that), now however basically all are synthetic. The method to apply contact adhesive is the same as with rubber solution, where one add a layer on each surface one wants to put together and let it dry before being pressed. Contact adhesive bond directly, so you have to do things correct straight away. This type of glue is also almost always toxic, and you should be in a well ventilated space when using it. There are lots of different options of contact adhesive that works well for shoemaking on the market.
The advantage of contact adhesive is as mentioned that it’s extremely strong, and it’s easy to work with. Therefore it’s used by more or less all factory-made RTW manufacturers for both soles and heels. The heels are usually built separately, either the manufacturer make them themselves in the factory, or buy them ready-built, but there is almost no RTW manufacturers who build the heel directly on the shoe since it’s too time consuming. It is also the most common bonding agent for closing the channel if you do a closed channel sole. The disadvantages of contact adhesives are mentioned above, among other things of course that it’s health hazardous, and when used it can form bright line between the layers of leather (or rubber when such insoles are used, or leather board or fiber board which are used as lifts in cheaper shoes) and seep out after a period of use, and creating a bit ugly, rough surface in the gap. It is, of good and bad, not able to revive, so when it’s bonded it’s bonded.