What Is Glue Made Of?

Public Domain. Boston Public Library

Glue is a type of adhesive made from a variety of substances, with the humble aim of binding two items together.

Glue, it's a sticky topic. But we're here to pry fact from fiction and tell you everything you never knew you needed to know, from what it's made of (horses? what?) to what's in Elmer's and how to make your own.

As Encyclopedia Britannica defines it, an adhesive is "any substance that is capable of holding materials together in a functional manner by surface attachment that resists separation." The first known adhesive was comprised of tar from birch bark, which early humans used to bind tools to wooden handles some 200,000 years ago. Nowadays, adhesive materials run the gamut from simple natural adhesives to high-tech synthetic substances.

And speaking of simple natural adhesives ...

Is glue made from horses?

Big, muscular animals – like horses – have a lot of collagen, the principal protein of skin, bone, and muscle. "It’s also the key ingredient in most animal glues, as it can be made into a gelatin that’s sticky when wet but hardens when it dries," writes Forrest Wickman for Slate. We have been using animals, including horses, to make glue for thousands of years; in the 18th century, the first commercial glue factory started doing business in Holland using animal hides. Animal glues were traditionally used for wood joining, book binding, crafting musical instruments, producing heavy gummed tapes, and other specific applications.

But despite its good performance for stickiness, most animal glue has been modified or entirely replaced by synthetic adhesives. Synthetic adhesives are significantly more versatile, excel in performance, and can be manufactured with more consistency.

So what happens to all the old and/or unwanted horses these days? Thankfully, they are not sent to the glue factory, but that doesn't mean their fate is necessarily much better. While there are numerous horse rescue facilities in the United States, they don't have the capacity or resources to accommodate all the unwanted horses. Many are sent to Mexico and Canada and slaughtered for meat intended for human consumption. "Other horses are rendered into meat for greyhounds and food for large cats at zoos," writes Wickman.

But unless you are using specialty animal glue, chances are that you aren't using rendered horse parts for your adhesive purposes.

Glue ingredients

Glues fall into two main camps: Natural and synthetic. Humans have been using natural adhesives for millennia, but in the 20th century synthetic glues were developed and over time have largely replaced natural adhesives. Much of this was thanks to the aircraft and aerospace industries, which required adhesives with high structural strength and resistance to fatigue and extreme conditions. These high-tech, synthetic adhesives eventually trickled down into more mundane industrial and domestic applications.

Ingredients in natural glues

Natural glues are mostly of animal or vegetable origin. Though they are used much less frequently nowadays, they are still preferred for some applications, like for making corrugated board, envelopes, bottle labels, book bindings, laminated film, and foils.

Natural glues are made of everything from animal parts, as in rabbit-skin glue and horse glue, to milk proteins, serum albumin from animal blood, vegetable starch, natural gums like agar and gum arabic, and natural rubber latex.

Ingredients in synthetic glues

OK, time to put your chemistry hats on – but we'll try to keep it brief. Synthetic polymers are used to make synthetic adhesives, like Gorilla Glue and Elmer's, and they belong to two categories: Thermoplastics and thermosets. The resins used in thermoplastic adhesives include nitrocellulose, polyvinyl acetate, vinyl acetate-ethylene copolymer, polyethylene, polypropylene, polyamides, polyesters, acrylics, and cyanoacrylics. Resins used in thermosets include phenol formaldehyde, urea formaldehyde, unsaturated polyesters, epoxies, and polyurethanes.

Now on to the important stuff ...

What’s in Elmer’s Glue-All?

Did you ever notice that the logo of Elmer's glue is ... a cow? Elmer's was a spin-off from the Borden Condensed Milk Company; Elmer the bull was the husband of the Elsie the cow, Borden's uber-popular spokesperson spokesbovine of lore. But don't worry, this glue-and-cow connection is not about sending old cows to the glue factory.

In the late 1920s, Borden acquired the Casein Company of America, the leading manufacturer of casein glue, an adhesive made with milk byproducts (not cow parts, per se). In need of a marketing boost, they gave Elmer the job of representing the newly dubbed Elmer's Glue, and the rest is history.

In 1968, the company created Elmer's iconic School Glue, and then Elmer's Glue-All – both of which actually have the same ingredients. These days the Elmer's site is almost entirely a celebration of all things DIY slime. But a trip to the Wayback Machine answers the question of what's within. Well, kind of:

Elmer's Glues are chemical based. They are made or formulated from chemicals which are synthesized (created by Man). These chemicals were originally obtained or manufactured from petroleum, natural gas and other raw materials found in Nature. The exact formula and specific ingredients used in making Elmer's products are considered proprietary information, therefore, we cannot share those with you.

Interestingly, in 2013 the brand launched Elmer's School Glue Naturals. The pourable version is comprised of 99 percent natural ingredients, with the primary ingredient being plant-based, specifically American-grown corn. The glue stick formula is made up of more than 88 percent natural ingredients. No cows required.

How to make your own glue

The easiest homemade glue is a simple flour and water paste. It doesn't have the most amazing adhesive quality, but it's perfect for things like simple crafts and papier-mâché. Start with a half cup of flour and add a little water at a time, stirring until you have a paste consistency. That's it.

There are a lot of DIY glue formulas out there that use milk, but if you want a vegan option, here's a good one. It uses sugar, flour, antiseptic mouthwash, vinegar, baking soda, and water.

For more DIY glue recipes, visit ThoughtCo.