Use of hide glues in piano construction and restoration have essentially been the universal adhesive right up to the advent of World War II. Postwar, PVA or yellow wood glue, epoxies, contact cements, and CA glues were adopted for various specialized uses. But modern piano technicians, restorers and repairers, keep returning to hide glues because of its reversibility and lack of creep, or movement under stress. While one can make a glue pot with a modern adjustable hot pot for cooking purposes very cheaply, it is instructive to look at the ways that hide glue was kept in its fluid state historically. After all, some of the best pianos and musical instruments ever made were produced without an electrical dial to achieve a 140 degree F optimal temperature. Historically, glue temperature was moderated by using a Bain-Marie, or double pot, with water in the outer pot, and the glue in the inner pot. Variables in controlling the temperature of the hide glue would include the size of the outer pot in relation to the inner vessel, the intensity of the heat source, the distance from the heat source to the outer pot, and the amount of time the heat source is engaged. Sources of heating the glue would include wood or coal shop stoves, open hearths, gas heaters, alcohol and kerosene lamps, and steam tables.
Glue pots have come in many forms, and made from a variety of materials. Early on, and up until the time of Christopher Gabriel, a major London tool and wood supplier from 1770 to 1822, many glue pots were made of lead, and were a single pot only, not a bain-marie. Lead had the advantage of retaining heat very well, giving the worker time to transport the glue away from a heat source, to the actual location of the bonding operation, if the work could not be moved adjacent to the heating source. In the United States and Britain, with the development of the cast iron industry in the early 1800s, glue pots were a logical candidate to be cast from iron. Iron distributes heat evenly and holds it–and mass producing these pots was relatively inexpensive–making them affordable to a wider market. Copper glue pots were prevalent in France starting in the 1700s, and to a lesser extent in England and the United States. Copper was used because of its ability to conduct heat, which means that the metal heats up quickly. Its also true, though, that copper releases heat just as quickly. But this is obviated by the use of an outer pot containing water, which holds the heat much longer. These pots were often made by individual coppersmiths, or in workshops, such as those within the copper making community of Villedieu, France. Many variations of construction can be seen in these pots historically; joining methods employed would include dovetailing, overlap dovetailing, folded joints, riveted joints, and soldering. The coppersmith would start using flat sheets of copper, bending and planishing into shape, using the above methods of joining. Other pots utilized copper sheets compressed into iron or steel forms, then the various components would be brazed together. Some copper pots would be made of ‘bright copper,’ meaning that the level of finish would be as refined as teapots and other copper vessels for the table and in cooking. Others, however, would be completed with less attention to detail, with an eye towards sturdiness. In England and the United States, starting in the mid 18th century and to the late 19th century, a significant number of copper glue pots were made just like teapots, with the exception of a spout and the addition of an inner pot for the glue.
Hide glue has many advantages, including the ability to control working time, or the timing of the setting process with urea. With heat and steam, the bond can easily be broken and cleaned; this is very useful during the replacement of some piano parts, and the retention of others. An example of this would be the hanging of multiple sets of hammers on the same set of shanks over the lifespan of the piano, especially when the original shanks are obsolete, such as a set of Thayer shanks. Multiple replacements of key bushings are another example. Also, hide glue is not as vulnerable to dry flow when compared to modern yellow wood glue or PVA, which can be subject to movement of structural components under shear stress load.
Top to bottom: obsolete Thayer shank with hammer; Thayer shank & flange; Knabe type shank & flange, with conventional knuckle; Steinway type shank & flange, also with conventional knuckle. Thayer was an Illinois-based piano action company. Picture provided because no images of Thayer grand parts are readily available online
“French cabinet makers use a glue pot with an inside pan made of glazed earthenware and divided radially into three divisions, in one of which is kept strong glue, in another weaker, and in the the third, water only, with a brush or piece of sponge for cleaning off superfluous glue from the work” From “The Practical Cabinet Maker and Furniture Designer’s Assistant,” by Frederick Thomas Hodgson in 1910.
Rustic and small copper glue pot, probably English. Copper conducts heat very efficiently, and also inhibits glue spoilage and mold, functioning as a biocide. Rust from the iron glue pots can really stain the glue; with copper glue pots, this is not a problem.
Here is a description of the kerosene lamp glue pot:
“Glue Pot.– A glue pot recently perfected consists of a circular kerosene lamp, made of tin, resting upon a tin bottom 8 1/2 inches in diameter. The lamp is fitted with a tin chimney in place of glass, and fitted with a small aperture, covered with mica, so as to see how to regulate the flame. The glue pot is made of copper, tinned on the inside and supported upon a rim setting up about six inches from the bottom of the lamp, the rim supported by three legs, soldered and riveted to the rim and bottom of lamp rest. The pot in which the bottom is placed has a portion of the bottom arched, to give more heating surface, and connecting with the chamber under the pot is a flue, passing out and up alongside of the pot, which carries off any smoke from the lamp, and also acts as a draft to the flame. This pot is five inches in diameter, and about six inches high. The pot for the reception of the glue is set in the same as an ordinary glue pot, and will hold about a quart of glue. The whole can be carried to any place where you wish to use it, and still have the heat kept up. The cost of oil is but a few cents a week. Another improvement is in the pot being of copper, tinned. It will not corrode and spoil the glue, as is the case with iron.” From “The Practical Cabinet Maker and Furniture Designer’s Assistant,” written by Frederick Thomas Hodgson in 1910.
“Glue pot and tea-kettles: Glue pots may be considered as the stepping-stones to tea kettle making, as the bodies are made the same size and way, and if a boy (or man either for that matter,) will take the necessary care and interest while being instructed in the work of making glue pots, he may attain the proficiency necessary to execute the best work, such as is demanded for bright goods. Let us begin to make two round bodies, the same as for 1-gallon glue pots and at the proper place we will branch off, finishing one a brown glue pot, and the other a bright tea-kettle, following throughout the methods in vogue when the coppersmith made his own mountings” –From “Art of Coppersmithing,” by John Fuller Sr., 4th edition, 1911.
John Fuller apprenticed in a coppersmithing shop, at the the age of nine, in 1842 in the town of Dorking, Surrey, England. He described how coppersmiths made their wares, at a time–up to around 1884 he explained later–of utilizing traditional methods with hand-made forms and mountings.
Traditionally designed cast iron Marietta glue pot, with white enamel or porcelain lined inner pot, to prevent glue staining. Made in great quantities by Marietta Mfg. Co., in Marietta, Pennsylvania. Kenrick & Sons, in England, also made many cast iron glue pots of a similar profile. Archibald Kenrick started a foundry in West Bromwich, U.K., in 1791, and Kenrick Co. is still in business today.
Warning: antique cast iron and enamel lined cookware from the the old Marietta foundry, and others, may contain lead in both the iron (to enhance malleability) and in the enamel. For glue pots and gluing purposes this would probably not be a significant issue.
Dionysius of Fourna, an Eastern Orthodox monk, wrote a manual of iconography “Hermeneia” between 1730 and 1734. Hermeneia included the following instructions on preparing hide glue from scratch. This material was taken directly from the Emily Carr University of Art and Design in British Columbia, Canada website.
“On the Making of Animal Collagen Glue–When you wish to make some glue, do thus: take some limed skins, and put them into lukewarm water to soak right through; wash and clean all the flesh tissue and dirt off them, and put them in to clean water in a copper vessel to boil. Watch for when they come to the boil and begin to thicken, and strain off either with a woven strainer or a cloth, otherwise they will burn, and then put in more water; repeat this two or three times, straining off until they are completely dissolved. If you cannot find limed skins, use unlimed ones, choosing the skin from the feet or ears of oxen, and any skins that cannot be put to any other use or are of little value. If they are thick it does not matter; buffaloes and ewes are also good. Deal with them thus: take some quick-lime and put it into a pestle with some water and mix it until it is all dissolved; put the skins in this solution and leave them for a week, until every single hair has come off them. Take them out, wash and clean them thoroughly and let them dry. Whenever you want to make glue, do as we have written above for you. But if you are in a hurry and have no lime, in order to soften the unlimed skins soak them and then put them on to boil for a short time; take them out and clean them well of any fat and flesh tissues and cut them up in a chopper so that they will be boiled more quickly; do not separate the pieces completely, but leave them joined together so that you will not be hindered when you come to strain them off. Boiling the skins like this you will obtain some glue. If you want to dry the glue, put the last of the glue alone on a low fire and let it boil until it coagulates, only watch it well as it may froth up a lot, and you must therefore be present when it is boiling so that when it froths up you can take it off the fire and put it in a vessel of cold water so that it touches the bottom in order to stop it rising. Put it back several times on to the fire until coagulates, and then take it off and leave it to cool. Stretch a piece of string in a bow-saw, cut them into small pieces and leave them on a board for two or three days until they begin to harden; then pass string through them and hang them up in the air to dry completely, and keep them for when you want to lay gesso. See that you always prepare glue in cold weather, as it smells if the weather is hot, and you will not make such good progress.” (Dionysius, 1734)
The text from Emily Carr University of Art and Design then described the modern method of making hide glue in basic outline form:
“This method [Dionysiyus’] of preparing glue was used up to the beginning of the 19th century, and in some places even later. Since that time, glue has been produced industrially.
The modern glue manufacturing process basically follows this procedure:
- Wash hides and bones to remove dirt;
- Soak in limewater for 60 to 90 days;
- Wash to remove hair and lime;
- Neutralize with acid, drain, wash and drain again;
- Add the raw material to water, heat to 45-50° C. (110-120° F.) for two to four hours (called an extraction);
- Drain off the dilute glue solution, evaporate, chill, dry and grind the resulting solids;
- Repeat the last two steps three to four times to extract all of the glue from the hides and bones with the temperature increased (20-25° F.) each time.
It is not worthwhile for an artist to try making glue using ancient techniques, such as the method described by Dionysius. The quality of this glue will be considerably lower than the quality of glues prepared industrially and using inferior quality glue can create problems in the final product, such as scaling, formations of cracks, and the like.”
“The Carpenter’s Glue Pot. (Fig. 219) an original, homemade example of which, constructed of lead for extra heat retention, as described by Moxon, is here shown. No. 20251. Still earlier than Moxon, in the “Probate Records of Essex County, Massachussetts, 1635-1681, 3 vols., George Francis Dow, editor. Salem Mass. The Essex Institute, 1916-1920, the carpenter’s glue pot is appraised at 1 shilling, in 1644, and at 1 shilling, 6 pence, in 1675. In these cases it seems unlikely that the glue pot could have been made thus cheaply of expensive lead, neither could it have been double-bottomed, as described in “The Circle of Mechanical Arts,”, by Martin, who, writing in 1813, says that the double-bottomed glue pot had just then come into use in England. The illustration also shows one such pot, factory-made, in three compartments, of cast iron, enclosing an inner receptacle of heat retaining alloy, of date about 1870, resembling in construction those now in use [1925-1929], in which the glue, in this inner kettle, escapes scorching by being heated by water boiled in the outer compartments.” From “Ancient Carpenters’ Tools,” by Henry C. Mercer in 1929.
And this section would not be complete without Scriabin’s “Vers La Flamme.”