In-depth – How adhesives affect form and function in footwear

Most are aware of the rather extensive use of adhesives when making footwear, also for traditionally constructed shoes. What one don’t discuss as much is how this use affect form and function of the shoes, which it indeed can do in many ways. Here’s an hopefully enlightening article on the topic.

I’ve written about the different types of adhesives used in footwear before in this article, which sort of summarises the properties for paste, rubber solution and contact adhesives. As I touch upon in that article, it’s a bit of a mess with what is called what, and to add to that properties within various types of adhesives can vary greatly. So take all in this article as generalisations, and add to that the fact that some of the topics here are debated also within the shoe industry, since in many cases it’s hard to really test the reality and experiences may differ.

Many different types of adhesives are used in footwear manufacturing. One of the most natural one is starch paste, like this from Hirschkleber, which is used for leather stiffeners and some heel building.

Paste on leather toe puff.

To start, it’s worth noting the properties of leather that are so highly sought after for footwear. Apart from things like its durability, pliability and ability to be shaped and keep its form, the fact that it both has breathability and absorbs moisture are positive things when it comes to creating a favourable environment for feet. It’s common that one discuss how corrected grain leather that is covered with a thin layer of plastic doesn’t breathe as well or even worse so called synthetic “leathers” , or that rubber soles would feel more warm than leather soles, etc.

However, how the use of adhesives affects these natural properties of leather is rarely talked about. There’s several areas though that surely are to be impacted by the use of certain types of glue, especially in factory-made footwear. For example, most factories use fabric backers on the uppers, these consist of a canvas with a sticky glue on it, which would create a barrier between the lining and the upper limiting breathability. However, the backings have other properties that are positive, it makes the shoes retain their shape better, and reduce creasing. Bespoke makers also use backers, but of leather and over smaller areas, though the adhesives used here would also affect breathability in the specific area.

How the uppers of most RTW shoes are made, with fabric backers glued on the inside of the upper leather.

For the sole area, it’s not as much about breathability as it is about absorbing moisture. This is since there’s such a big area between the foot and the outside here. The opinion that leather soles are less warm than rubber soles due to breathability would only the thickness of materials here contradict. But then add the fact that you have a thick layer of cork paste (cork and glue) and then in most cases a neoprene cement which is strong as hell but also creates a barrier for any moist, then at least in theory a factory-made leather soled shoe and rubber soled shoe shouldn’t differ much in this regard.

For bespoke, it’s more common that the strongest neoprene cement is only used along the edges and a less strong rubber solution on the middle part, though this would also have similar prevention of moist. The fact that these adhesives are strong and waterproof is what makes them good when it comes to keeping soles in place, but it do means that not much moist is coming through here. For the bottom of the shoes, it’s therefore the absorption properties that are of value, not breathability.

When using the stronger adhesives, it’s good to have good ventilation and / or masks.

Neopren cement at the edges, rubber solution in the middle, for a bespoke shoe outsole.

Worth noting though, is that the same adhesives that prevent moist from going out of the shoe, also prevents moist coming in. Leather soled shoes are indeed more water resistant thanks to the cement used. And another very important reason for using good adhesives is to prevent squeaking and similar sounds from appearing, especially if one has leather bottom filling it’s sensitive, but also leather shank, shank cover in leather or leather welt towards leather insole and / or outsole are potential noise creators if one doesn’t have good adhesives that keeps them fully connected also over time.

Now, there’s places already higher up towards the feet that has adhesives. We have the sock lining. And to those who have wondered why the half sock lining is so common on welted footwear (and think it’s bad since it’s common it rolls up at the edge), here’s your answer. One want to have the thicker leather insole towards your feet to absorb the moisture, especially since the bottom of the feet is the area with most sweat glands of the entire body. The sock lining is there to cover nail holes and protect against potential pegs, and nowadays also often covers a foam pad placed here for comfort. This is cemented on, and both the cement and the foam pad means that it’s mainly the thin sock lining that absorbs moisture here. But as mentioned, the insole is thicker, so one rather have this for the main part.

Here the half sock lining of a RTW shoe has been removed, one can see the cemented area, and the foam pad is visible.

The importance of absorbing moisture for the insole and lining leather is also why one ideally should choose a product with little or preferably no wax when conditioning it, like Saphir Medaille d’Or Nappa conditioner or similar. Cause condition also the inside, you definitely should do, as I’ve written about before here on the blog, since it’s a leather that is put through stress with all the moist and rubbing from the foot, which also is why one often see the lining crack first as well since people miss caring for this.

So, moving on from moist absorption and breathability, on to some other parts that can be worth discussing on adhesives. How about one of the most fiercely debated topics in shoe circles in recent years – gemming failure? Gemming is the canvas rib attached to the insole on Goodyear welted shoes (unlike hand welted shoes where it’s carved out of a thicker leather insole), which the welt seam is stitched to. Gemming failure refers to the issue with this rib coming off, and how that affects the shoes and their repairability.

The canvas rib is attached to the leather insole.

Not going in to that discussion more in detail, what I want to refer to here is the fact what fails when gemming fails is the adhesive. And in this area, development has been made in recent decades. The neoprene cement used to attach the gemming rib nowadays is stronger than ever, and although it’s not something that is fully clear, what I gathered from more than just one cobbler that has been around for many decades, is that they’ve noticed a trend on gemming sitting better than before. For many reasons this is hard to validate, both since shoes coming in for resoles vary in which brands they come from and how old they are, but it’s an interesting observation worth following, and especially when factories that have updated to newer cement for the gemming can see these pairs coming in and compare to older we can get real data on this.

For the channel in leather soles, it’s the strongest neoprene cement that is used.

Another thing worth going into when it comes to adhesives and shoes is relating to the increase of use of rubber soles. Leather to leather has a tendency to be easier to cement, they bond more naturally together, while especially certain rubber types are more prone to coming loose, both outsole towards welt and the heel block towards the rubber outsole. Note that it’s not that they are really coming loose, there’s stitching and nails preventing this, but you get these gaps between the layers that customers dislike. This has meant that many makers have gone for the absolute strongest cements to prevent this, and the con with this is that in cases this cement is more prone to sip out and create this white, uneven streak.

The cement on leather soles also tend to be pressed out a bit (which is why bespoke makers often build heels with paste which bonds even better with the layers and makes layers more clean, and less change of the look with time) usually blends in better with the rougher structure that the edges of welts, soles and heels get when becoming wet and put through harsh conditions. Remember that these edges consists of the corium, the loose fibre structure of the leather, and even with the best edge finishing this will change with time.

There’s actually plenty of more on adhesives and shoes one can talk about, but for now, it’s a wrap.

Factory-made heels use glue that set hard, but will leave a visible lighter line between the layers.

Some bespoke makers build the heels with paste, which is a bit more tedious since it takes a lot longer to dry.

Pro is that you can get this lovely looking heels, with very clean transitions between the layers, and no adhesive will be pressed out here after some time of wear.