Our sister site at Friends of Square Pianos has an excellent series of articles on the subject of glue suitable for restoration and repair of square pianos and instruments in general. We will not attempt to repeat this information here, and the reader is encouraged to visit and read all about the choices. That said, there remain some details on buying, mixing, and using hide glue, and alternate glues, that might be of use to both the experienced and less experienced restorer.
Firstly, we should note that hide glue is not the same thing as food grade gelatin, which in fact is made from bone. It is intended to EAT, not glue things up, and is unstable compared to hide glue. So we are looking for real hide glue, with numerous vending opportunities for you to buy from. Most of the supply houses for guitar and violin (luthier) will sell this glue, often at inflated prices! It is only $5 for 2 ounces at one EBay site, or about $88/kg, though from the supplier it can be had for 5% of this figure! Where you buy it will depend on where you live, but a thorough Internet search will turn up a good supplier with 15 minutes of work. Some possibilities are listed later in this article.
In general, hide glue is purchased by gram weight. This refers to the Bloom gelometer figure of merit, named for O.T. Bloom. Under carefully controlled conditions, a plunger is lowered into chilled and gelled glue, and the weight needed to reach a given set depression recorded. That weight defines the gram strength. The higher the strength, the longer the polymer chain and so theoretically, the stronger the glue bond. Much can go into a final bond strength though, so this is not a hard and fast rule. But in general, for gluing in a wrest plank you probably want a high gram-strength glue, and for putting leather on a hammer, perhaps something easier to handle is best. Hide glue is sold in gram-strengths between 60g and ~600g, and usually from about 192g to 400g. The higher strength glue is very viscous and gels quickly, the lower much less so, giving you a range of possibilities to consider.
The higher strength glues tend to be increasingly more cloudy and dark, but in use this is rarely an issue. We don’t typically see the glue joint.
As received it will come as granules that you will mix with water:
The ratio of water to granules is somewhat a personal choice and application sensitive. Hide glue is one of the few true glues that sticks well to itself. To produce a superior glue joint where end grain is glued to side grain, the joints are often sized with a thinned glue first, so you may need to make up size as thinly as 1 part (by volume) of glue to 3 or 4 parts water, though 1 to 3 is typical. For a strong joint with side grains together, the glue can be mixed by volume 1/1 up to 1/1.8 parts glue to water. A 1/1 mix is quite stiff even hot, so best used where droop is critical to avoid. I typically mix 1/1.4 by volume and thin or let thicken as needed.
Mix cold water with the granules thoroughly and set aside for about 4 hours to fully hydrate with the water. Mix what you can expect to use over the next few weeks at most. The residual fat in hide glue will grow mold and bacteria with time, quickly in fact, if not refrigerated after use. It is an ideal culture media. Residual fats can be removed by adding a pinch of lye, perhaps 1 part to 200 of the mixed glue, and allow the fat to precipitate to the bottom. Not a necessary step in most cases though.
To heat the glue for use, a number of commercial glue pots are available for the purpose where you can vary the temperature. We want to heat the glue up to about 140 F, and not much higher. Overheating quickly breaks down the polymer chains and renders the stuff brittle and useless. Too cool and it is stiff or rubbery. I favor the soup pots they sell at department stores for personal soup reheating. In this case, you put water in the pot, set the temp to low, and with your glue in a small sealable bowl used for food storage, just float it or rest it in the water bath. At temp it will stay in a steamy environment at ~140F and never get crusty or skim over. The pots can usually be bought for under $20.
On removal of the soundboard from an early Broadwood grand, I encountered the distinct smell of the stable coming from the heated and hydrated old glue. This glue obviously had animal urine added before use. The addition of urea to glue has been known since ancient times to prolong the working time. Putting it into the glue for gluing in the soundboard allowed more time to cinch in the big soundboard before the glue gelled. If the glue gells, and you continue to move the part, an inferior joint results, similar to a cold joint while soldering. It won’t hold nearly as well.
Fortunately for us, urea can be bought without the ammonia or other organics in common horse pee, and so an oderless glue is possible. Combining 1 part urea to 5 parts glue granule by weight before adding water will extend the glue work time substantially. It comes at the expense of stability though. Urea glue needs to be used in one or two settings. It will not tollerate multiple reheatings without serious degradation.
So how about the commercial hide glue like Titebond Liquid Hide Glue? In fact, if fresh this is a good alternative to hot hide glue for little repairs like gluing a damper pad back on, or even replacing a bit of loose veneer. But it does not have the same strength, old glue in the bottle looks and smells like freshly manufactured glue but is much weaker, and it may not have a use-before-date, which it must.
The yellow and white wood glues are solidifying rubber. No good for reversal, poor strength, subject to creep, and structurally unsound. Any idiot can glue a joint with yellow glue with the same results a professional will get. This means, by reflexive logic, that a professional who uses yellow wood glue will make the joints of an idiot! Stay away from the stuff. I built a fortepiano copy with it, spending perhaps 600 hours on the thing, only to watch it slide apart at critical stress points because of the wrong glue. After that the conversion in my thinking was complete.
I have weighed in on epoxy at the ‘friends’ site, but in general, for speciality applications they can find a use, though mostly they need to be avoided. They are not so strong as they claim, but where insects and other have damaged an otherwise fine bridge, where letting in new wood is difficult, perhaps Araldite and Torr-seal can be considered. I will say of Torr-Seal epoxy that it will lift a layer of polished glass before it comes apart itself.
The urethane glues like Gorilla Glue do cure hard and without creep. And are quite strong! But nothing prepares you for how messy they are, foaming, running, and staining. Not for pianos please!
Michael Cole had provided a helpful guide on glue and leather choices for the ‘Friends’ website, and so a permanent version of that is given here: Michael Cole has very kindly contributed the following piece
Some sources for hide glue include:
Lemuel Violins - a good source for multiple weights, and particularly the higher strength glues.
Luthiers Mercantile International - good all purpose 192 gram strength
Behlen can provide good all purpose hide glue
Old Brown Glue can provide a good room temp liquid hide glue with proper use-before-date
Bjorn provides the glue fanatic a resting place in which to abide with the great glue gods!
Seccotine (fish glue)
We now stoop to shamelessly steal from Wikipedia on the subject of fish glue:
Seccotine is a brand of refined liquid fish glue used for gluing paper and card and as a binder in gesso, which remains flexible after drying. It is also used in the mounting of preserved insect specimens, as it can be dissolved in water if the specimen must be removed for further investigation.
Seccotine originated in Ireland in the late 19th century, where it was patented by John Stevenson. The original manufacturers were McCaw, Stevenson & Orr of Belfast (founded 1876), but the product and its name were sold to the English Royal Sovereign Pencil Co. c.1967
It is one of the few tradenames to have its entry and etymology recorded in the Oxford English Dictionary (1933 edition) where the name is explained as “apparently suggested by Italian secco, dry”, though it is possible that the French sec (dry) may also have been an influence.
Lucy Coad, who’s restoration work is respected around the globe, has taken charge of the distribution of Seccotine, and can source it as needed. SECCOTINE GLUE info andMSDS sheet
http://www.laverdure.fr/COLLES/colles-animales/f5/sf49 (Remarkable shop!)
“You can get Kremer materials at http://www.apfitzpatrick.co.uk and Kremer’s hide glues are also far better than anything available from this country. I’m afraid trade adhesives made here are a sad story.”
*Editors note: I once worked closely consulting for a manufacturer of sticky tapes, and was given the stern lecture on the difference between adhesives and glue. An adhesive is a material that is designed to remain flexible and binds essentially by forming tiny vacuum cups with the material to be adhered to. A glue forms a rigid bond between two materials that is not intended to be released. In almost all cases, building and repairing instruments has us using glues.