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Why liquid leather glue should be your first choice as an adhesive.
Sfrom the beginning of civilization until the Second World War, most of the furniture was assembled using animal-based glues. This fact changed almost overnight in the 20th century, however, with the introduction of polyvinyl acetate (PVA), what we call yellow or white glue.
In a sense, these modern glues are fantastic. They are inexpensive, easy to use and available everywhere – even the grocery store. But they may not be the best choice for the individual carpenter who makes furniture.
Modern glues are not reversible, so repairing furniture or clearing your mistakes is distressing. They install quickly, sometimes too quickly when you work alone. And PVAs are prone to appear as white spots if coated with a finish if you are not demanding on the cleanliness of the squeeze.
Frustrated by these downsides to yellow and white PVAs, I started looking for an alternative. As usually happens in woodworking, the problem could be solved by investigating traditional shop practices.
More than a decade ago, I started using liquid glue. And now it’s rare for me to use anything else. I don’t use leather glue because I make historical reproductions, I wear shirts with frills or I work in a museum of living history. I use it because it is simply the best glue for me: a guy who makes furniture one at a time in a small shop. The guy who occasionally inserts slot A into tab B by accident and has to correct the mistake.
Patrick Edwards, who makes the glue to hide the liquid called Old Brown Glue, summarizes it best for me: “I think protein glue is …” he said, a little exasperated, “it’s not even a choice. You should use the protein glue if you’re a carpenter. “
If you use yellow glue, I suspect that you are writhing in the chair right now or that you are going to call it pucky bull. Most carpenters object to all forms of leather glue. Even if you’ve never used it, I’m sure you’ve heard that it stinks of rotting meat, has a short shelf life, does not bind to PVA and is generally difficult to use.
I won’t lie to you: both leather glue and PVA have advantages and disadvantages. But in the past 50 years, the reputation of leather glue has suffered from many misinformations while the downsides of PVAs have been ignored.
Leather glue has been used to assemble furniture for thousands of years. After trying the glue for hidden liquids, you will find that it is as simple to use as PVA.
This article will expose the advantages and disadvantages of liquid leather glue and PVA glue from the perspective of a single furniture manufacturer, not a glue manufacturer, furniture factory, luthier or museum conservator. I chose to focus on liquid leather glue instead of hot leather glue in this article because liquid leather is cheaper, more similar to PVA and more readily available than hot leather glue.
Let’s examine the main differences starting from where you live.
It’s not just the heat …
In my opinion, the biggest advantage of PVA is that they are more consistent than leather glue. Whether you work in a swamp, in the desert or on the prairie, the yellow and white glues will tie the joints with few complications. A dry or cold day may speed up or slow down vulcanization, but you will almost always end up with a hard and stable joint.
Leather glue, on the other hand, is more sensitive to heat and humidity. In semi-tropical areas, such as Deep South, it can be difficult to get the glue to hide to completely cure on damp summer days, especially in a shop without climate control. The glue can remain sticky for weeks or months. (If this happens, take the piece to an air-conditioned room to allow the glue to cure. If this continues to happen, you’re probably better off with a PVA.)
Open time and assembly time
PVAs are set up quickly. With a typical yellow glue, you have 4-6 minutes to put the joints together before the glue starts to form (this is called “open time”). And you have 10-15 minutes in total to put on the cable ties and leave (this is called “total assembly time”).
Generally with a PVA it is possible to remove the terminals after 30 minutes. So if you have to work quickly (think: production work), PVA is your friend.
The liquid leather glue is inserted more slowly. The open time is generally 10 minutes, with a total assembly time of 20-30 minutes or even more in a cold room. You also need to let things get stuck for a longer time. While Titebond’s liquid leather glue says you can release the clamps in 30 minutes, I have found the best solution to leave things in the clamps for as long as possible. A blocking hour is better; the overnight stay is the best.
As a carpenter who builds pieces yourself and one at a time, the longer open time is useful for complex assemblies, and I don’t mind letting my assemblies sit overnight (you have to sleep a few times).
For me, this problem is a red herring. Both PVA and leather glue have a slight odor. Leather glue does not smell like a slaughterhouse. In fact, it smells far less than any other barn I’ve ever been to. For me, PVA doesn’t even have a particularly pleasant smell. So let’s say that after a few times of using both glues, you won’t notice the smell.
Both glues have a long service life. Although most manufacturers recommend using PVA within two years, I have found that it can last much longer if sealed in the bottle. Liquid skin glue typically lasts from 18 months to two years if kept sealed in a cool, dry environment. Although this can also be extended. (See “How to extend the shelf life” below.)
In other words, if you can’t use a bottle of glue in two years, maybe you should devote yourself to a different pastime.
How to extend the shelf life
Amany carpenters criticized liquid leather glue with a short shelf life. Chances are they only have themselves to blame.
Liquid leather glue should last from 18 months to two years if it has been properly stored, sealed and in a cool place. So if you’ve stored it properly and left it to rest for two years, maybe you should try doing a little more carpentry.
Another way to ruin the glue is to expose it to extreme heat. Think about how hot it can get in the car on a summer day – it will.
On the flip side of this equation, you can significantly increase the shelf life of liquid leather glue by keeping it sealed in a cool, dry place, such as a basement.
Franklin International’s glue technicians recently tested old bottles of Titebond’s liquid leather glue to see if it would heal. Surprisingly, properly stored 18-year-old hidden liquid glue bottles were well cared for.
Some fans of liquid leather glues recommend storing the glue in the refrigerator or freezer to extend its shelf life. It works – freezing doesn’t hurt the glue to hide – but the thawing process can cause water to condense on the glue if you’re not careful.
PVA is everywhere. And you can easily purchase PVA that has been factory modified to be water resistant, waterproof, ultra thick so that they don’t work, diluted to have a longer open time, with UV additives to find juicing and again and again.
Liquid leather glue is available in only one basic form, so it is up to you to change its chemical composition for different effects. But you can make it do many fantastic things if you are willing to learn.
Tricks for glue for hidden liquids
Hthe ide glue is made with skins, soft connective tissue and bones from farmyard animals (so no, it’s not vegan). Dry this material and you have hard pearls with leather glue, which can be immersed in water and heated to produce hot glue.
But how do you make this liquid at room temperature? Usually by adding urea, salt or other chemicals. (There are many recipes on the Internet for making your own liquid glue to hide the pearl from the pearls.)
All the glue for leather can be modified to make special tricks. Below is a short list of common changes. For a complete list, check out Stephen A. Shepherd’s “Hide Glue: Historical & Practical Applications” (Full Chisel Press).
■ Reverse glue to hide. In addition to using heat and moisture to reverse the bonding of leather glue, you can also use denatured alcohol or acetone. These chemicals dry the glue and make it fragile. A strong blow on the joint with a hammer will then release the bond. The use of alcohol or acetone is ideal for large-scale jobs (which would take a long time to warm up) because the liquid can be injected into the joint with a syringe. The hidden glue that has been dried can be renewed without problems with a new glue.
■ Add flexibility. The hidden glues do not creep in, ideal for aesthetic coating and folded lamination. If you want to add some flexibility to the joint so that it behaves more like a PVA, add glycerin to the glue (up to 5%) to add flexibility. Other old recipes that I haven’t tried say that adding a little sugar or molasses will also make the glue more flexible.
■ Make it water resistant. Add aluminum sulphate (up to 1%) to make the glue water resistant.
■ Crackle finish. To simulate a crazed or cracked finish, paint a base coat on your project. Then brush a layer of leather glue on the paint. The thicker the coat of glue, the more pronounced the crackling will be. Then paint a second coat of paint on the glue to begin the crackling effect.
■ Filling gaps. Mix some chalk on the leather glue until it turns into a putty that can be used to fill cracks and gaps. The amount of plaster needed depends on the moisture and moisture content of the glue, so start by adding small amounts of plaster.
■ Bonding of unusual materials. To glue teak or other oily woods together, rub the surfaces to be glued with a clove of garlic before applying the leather glue. You can also stick the wood to the brass by rubbing the brass with the garlic before applying the glue.
The biggest drawback of PVA glue is that it is not reversible. Once the glue hardens, it is difficult to undo the joint without destroying the wood in the process. Large amounts of heat and humidity can release the joint, especially if the joint is new, but it is a difficult job that could end up ruining the whole piece.
The glue for liquid leather is easily reversible. A little heat and humidity will liquefy the hidden glue that is older than you. This is one of the main reasons why I prefer to hide the glue.
Let’s say you glued a door. The next day you realize that everything is back and reversed. If you’ve used hidden glue, you can easily undo the assembly and fix everything.
The reversibility of the leather glue saved me a few times in my shop and many more in the classroom. It also allowed me to make changes to the design of a project, for example by moving or changing moldings, without remaking the whole piece.
PVA does not stick to itself. So if you have a tenon that has dried PVA on it, for example, a new coat of liquid PVA will not stick to the old glue. You have to scrape off the old glue on the bare wood, so there is a good chance that you will remove some of the wood fibers and reduce the strength of the new joint.
The hidden glue, on the other hand, sticks to itself. If you have a disassembled tenon with dried leather glue on it, you can reactivate it by brushing a little hot water and then adding a new coat of fresh glue. The old and new glues will mix to create a new bond. The first time you do this, you will hear the angels singing.
A history of liquid glue
Liquid leather glue has been marketed since the beginning of the 20th century. Franklin International in Columbus, Ohio was probably the first to sell glue in the United States when the company was founded in 1935. And Franklin’s formula for glue has remained essentially the same for the past 81 years, according to officials at company.
But liquid leather glue has been known to carpenters for a long time. Somehow somebody understood that by adding urea (urine) to hot leather glue you could make it remain liquid at room temperature.
An unconfirmed story is that the first French apprentices tried to sabotage their master’s hot glue by urinating into it. But their stunt ended up producing glue for liquids instead.
Although it may be a tradition, there are several 19th century references to making liquid leather glue with the help of nitric or acetic acid.
But recent research suggests that liquid leather glue has been around for a long time.
In 2010 archaeologists published an article in the magazine Archaeometry this concluded that liquid leather glue was used in the construction of a mosque in Uzbekistan in 1540.
Some external “muqarnas” (a decorative feature of a dome that may resemble a honeycomb) have been found in place in the past 500 years only by animal glue and small dowels. When the glue was examined, some samples clearly showed that the glue had been mixed with the urine. The addition of up to 4% of urea makes the glue liquid at room temperature and facilitates its diffusion, according to the results of the archaeologists.
So if someone wonders how long your liquid glue will last, you can safely say that it can last 540 years or more.
Both standard PVA and leather glue are sensitive to water. Do not use them for garden furniture, cutting boards or anything else that can see more than the occasional splash of water.
If you need a water or water resistant glue, you will need a special PVA or a completely different glue, such as epoxy. Most PVA manufacturers produce a water-resistant PVA.
And you can make your leather glue water resistant by adding a little aluminum sulphate to the glue, a chemical that is usually used to regulate the acidity of the soil of houseplants or in tanning leathers. Add about 2 grams of aluminum sulphate (half a teaspoon in the real world) to an 8 ounce bottle of liquid leather glue to make it water resistant. Note that this additive will make the bond irreversible with water.
Transparency to finishes
If you don’t remove all the PVA from the surfaces of a project before applying a finish, the remains will appear as ugly white spots. The glue to hide, however, is transparent to many stains and finishes, including boiled linseed oil, most of the paints I have tried, lacquer and shellac.
To tell the truth, if you leave large drops of leather glue behind, your project will look ugly. But small smears of leather glue and fingerprints that are invisible before applying the finish will usually remain invisible.
One of the struggles with any adhesive is to get a complex assembly before the glue’s open time runs out. A large row of carcass dovetails is an excellent example. Many complex joints that come together quite easily when assembled dry hang before being completely closed when glue is added.
I found that liquid leather glue is far less likely to grab the joints during assembly. While some people have theorized that this is because leather glue contains less water than PVA, this is probably not the reason. The truth is that the water content of both glues is almost the same: PVA is 54% water; liquid leather glue contains 48 percent water, according to the glue experts at Franklin International.
The real reason why liquid leather glue is less likely to block joints in mid-flow is because of the longer open time of liquid skin. Liquid leather glue simply works slower than PVA.
Leather glue and standard PVA have approximately the same resistance at room temperature. You can perform all types of resistance and temperature tests, but the bottom line is this: a clean and good joint made with PVA or liquid leather glue is strong enough for furniture inside a house. The strength of the bond is not a problem with either of the two glues.
A wrinkle in this discussion concerns the use of “toothing planes” when using glue to hide for veneer. Old textbooks for woodworking say that it is necessary to dent all veneered surfaces before using any type of leather glue: liquid or hot. The logic is that the toothing of the surface will increase the bonding area and therefore the bond.
While some longtime carpenters still swear by the board (and I won’t contradict them), scientists at Franklin International claim something different. As you increase the bonding surface with a toothed plane, they say, you will also increase the incidence of wood sagging when the joint breaks due to shredded wood fibers (and I won’t contradict them either).
Since many veneered surfaces have survived hundreds of years (and others not), you will have to draw your own conclusions.
Another thing that can reduce the strength of the PVA and hide the glue is when the wood has been burnished, usually by a matte cutter in a machine. A burnished surface will generally appear shiny and sometimes black. The water in any glue has difficulty penetrating a burnished surface and the joint ends up being weak.
How to test a burnished surface? Easy. Put a drop of water on the surface to be joined. If the droplet sinks quickly, the wood is fine. If the drop remains on the surface for a long time, the surface is probably browned.
W. Patrick Edwards and “Old Brown Glue”
MPeople know W. Patrick Edwards for his extraordinary inlay work or for founding the American School of French Inlay in San Diego. But what few people know is that he also runs an empire of glue from his bathroom.
Like any furniture conservator, Marches or manufacturer of historical pieces, Edwards has long been a devotee of the glue for leather due to its numerous furnishing characteristics. But during the first part of his career there was always a task that he tried to make that hot glue was not suitable for: column veneers (especially those with a narrow radius).
After studying inlay and conservation in France in 1992-93, Edwards was invited to join an international inlay conservation team. And one year the team addressed the topic of how to lower the gel point of the hot leather glue so that it was liquid at room temperature.
The conclusion: add “thiourea” (a chemical similar to urea but carcinogenic) to the hot leather glue and this will reduce its gel point so that it is liquid at room temperature.
Edwards knew that thiourea was similar to urea and started cooking several batches of glue with urea and water in his California store. After 37 different recipes, he found the mixture that had the properties he wanted.
For the next 10 years he used his liquid glue for leather in his personal work. Eventually he veneered his columns (“It worked perfectly on the first try!”) And eventually he started using glue in about 80% of his works.
So Edwards gave President Brian Boggs a bottle. Boggs loved it and talked about it with Joel Moskowitz of Tools for Working Wood. Moskowitz called Edwards.
“Joel said,” I want to sell it, “said Edwards.” He said: ‘Put it in a small bottle and a large bottle and we will sell both.’ The rest is history. “
Today Edwards produces Old Brown Glue in a bathroom of his San Diego workshop using double boilers. It takes him about 90 minutes to fill 200 bottles and he produces hundreds a month.
“I’m a reluctant glue maker,” said Edwards. “I want to build, restore and teach. But (glueing) may be the thing I do full time after retiring. He is brainless. “
While the process of making it might seem brainless now, getting to the brainless part required a stroke of genius and persistence.
PVAs are flexible, which can be an advantage when gluing joints where there is a lot of movement of the crossed wood. A good example of this is an 8 ″ apron for a sideboard that is tenoned in the legs. Flexibility in that joint is good.
But flexibility is negative when it comes to bent veneers or laminations. Flexibility – called “sliding” – can cause gaps in veneer work or bent laminations to become slightly curved. These are two examples where the glue hidden in any shape really excels.
How to measure the strength and freshness of the glue
Leather glue tends to give off a strong ammonia smell when it has become bad. But is it possible to judge its freshness and strength before the glue becomes completely bad?
I recently spent a day with two glue scientists at Franklin International: Bob Behnke, technical support manager for Titebond products, and Dale Zimmerman, a technical support specialist. While both work for Franklin, both have always placed science first in my experience. They offered two ways to test the glue to hide to make sure it is still good.
Method 1: spread a thin layer of leather glue on a piece of writing paper. To make it dry quickly, place it on the floor in front of your refrigerator: the hot air expelled from the appliance will speed up drying. Or you can put it in an oven set at 150 ° F for 10 minutes to see if it will dry out.
After trying to force dry the glue, touch the glue film with your nail to see if it is hard. You can also fold the paper to see if the glue film will crack or burst – both are indicators that the glue is still good.
Method 2: Take three blocks of wood that measure 3/4“X 3/4“X 2”. Glue them together from one side to the other so that the central block is moved 1 “from the other two (see the photo for details). Block the group and let it rest overnight. In the morning, place the group on the bench and hit the protruding central block with a hammer to try to destroy the joint. You will quickly know if the glue is good (breaking the wood) or if it went wrong (immediate breaking of the glue).
Conclusions and personal notes
It is difficult to convince carpenters to switch to a new (or very old) glue, even if the facts are convincing. I spent many years demonstrating the benefits of both hot melt and liquid glue. The only method that seems to convert people is this: block a carpenter in a classroom where the only adhesive available is leather glue. They will use it successfully and will promptly accept it in their shop, usually side by side with their PVA.
Alright then. As I said before, few of us work in living history museums, so we don’t have to be purists. We can use any glue technology that we want to do the job.
Now, since I can’t lock myself in a shop with leather glue, you will have to take the first step. Order a bottle and try it out. It’s a small investment (around $ 8), but the payoff is huge, especially if you’ve ever accidentally put slot A into card B.
I know I did it.
Video: Watch reverse a hidden glue joint.
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