Animal glue is an

adhesive Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation. The use of adhesives offers certain advant ...

that is created by prolonged boiling of animal

connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...

in a process called rendering. In addition to being used as an adhesive it is used for

coating A coating is a covering that is applied to the surface of an object, usually referred to as the Substrate (materials science), substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquid ...

and

sizing Sizing or size is a substance that is applied to, or incorporated into, other materials—especially papers and textiles—to act as a protective filler or glaze. Sizing is used in papermaking and textile manufacturing to change the absorption ...

, in decorative composition ornaments, and as a

clarifying agent Clarifying agents are used to remove suspended solids from liquids by inducing flocculation, causing the solids to form larger aggregates that can be easily removed after they either float to the surface or sink to the bottom of the containment vess ...

. These

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

colloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...

glues are formed through

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

of the

collagen Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...

from skins, bones, tendons, and other tissues, similar to

gelatin Gelatin or gelatine (from la, gelatus meaning "stiff" or "frozen") is a translucent, colorless, flavorless food ingredient, commonly derived from collagen taken from animal body parts. It is brittle when dry and rubbery when moist. It may also ...

. The word ''collagen'' itself derives from

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

(), meaning 'glue'. These proteins form a molecular bond with the glued object. Conventionally, keratin glues, while made from animal parts like horns and hooves, are not considered animal glues as they are not collagen glues. Stereotypically, the animal in question is a

horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million y ...

, and horses that are put down are often said to have been "sent to the glue factory". However, other animals are also used, including cattle,

rabbit Rabbits, also known as bunnies or bunny rabbits, are small mammals in the family Leporidae (which also contains the hares) of the order Lagomorpha (which also contains the pikas). ''Oryctolagus cuniculus'' includes the European rabbit speci ...

s and

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...

History

Early uses

Animal glue has existed since ancient times, although its usage was not widespread. Glue deriving from horse tooth can be dated back nearly 6000 years, but no written records from these times can prove that they were fully or extensively used. The first known written procedures of making animal glue were written about 2000 BC. Between 1500 and 1000 BC, it was used for wood furnishings and mural paintings, found even on the caskets of

Egyptian Pharaohs Pharaoh (, ; Egyptian: ''pr ꜥꜣ''; cop, , Pǝrro; Biblical Hebrew: ''Parʿō'') is the vernacular term often used by modern authors for the kings of ancient Egypt who ruled as monarchs from the First Dynasty (c. 3150 BC) until the ...

. Evidence is in the form of stone carvings depicting glue preparation and use, primarily used for the pharaoh's tomb furniture. Egyptian records tell that animal glue would be made by melting it over a fire and then applied with a brush.

Ancient Greeks Ancient Greece ( el, Ἑλλάς, Hellás) was a northeastern Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of classical antiquity ( AD 600), that comprised a loose collection of cultu ...

and

Romans Roman or Romans most often refers to: *Rome, the capital city of Italy * Ancient Rome, Roman civilization from 8th century BC to 5th century AD *Roman people, the people of ancient Rome *''Epistle to the Romans'', shortened to ''Romans'', a lette ...

later used animal and fish glue to develop veneering and

marquetry Marquetry (also spelled as marqueterie; from the French ''marqueter'', to variegate) is the art and craft of applying pieces of veneer to a structure to form decorative patterns, designs or pictures. The technique may be applied to case furn ...

, the bonding of thin sections or layers of wood. Animal glue, known as () in Greek and in

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

, were made from the skins of bulls in antiquity. Broken pottery might also be repaired with the use of animal glues, filling the cracks to hide imperfections. About 906–618 BC, fish, ox horns and stag horns were used to produce adhesives and binders for pigments in

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...

. Animal glues were employed as binders in paint media during the

Tang Dynasty The Tang dynasty (, ; zh, t= ), or Tang Empire, was an Dynasties in Chinese history, imperial dynasty of China that ruled from 618 to 907 AD, with an Zhou dynasty (690–705), interregnum between 690 and 705. It was preceded by the Sui dyn ...

. They were similarly used on the

Terracotta Army The Terracotta Army is a collection of terracotta sculptures depicting the armies of Qin Shi Huang, the first emperor of China. It is a form of funerary art buried with the emperor in 210–209 BCE with the purpose of protecting the emperor in ...

figures. Records indicate that one of the essential components of

lampblack Carbon black (subtypes are acetylene black, channel black, furnace black, lamp black and thermal black) is a material produced by the incomplete combustion of coal and coal tar, vegetable matter, or petroleum products, including fuel oil, fluid ...

ink was proteinaceous glue. Ox glue and stag-horn glues bound particles of pigments together, acting as a preservative by forming a film over the surface as the ink dried. The Chinese, such as Kao Gong Ji, also researched glue for medicinal purposes.

Reemergence

The use of animal glue, as well as some other types of glues, largely vanished in Europe after the decline of the Western

Roman Empire The Roman Empire ( la, Imperium Romanum ; grc-gre, Βασιλεία τῶν Ῥωμαίων, Basileía tôn Rhōmaíōn) was the post-Republican period of ancient Rome. As a polity, it included large territorial holdings around the Mediterr ...

until the sixteenth to eighteenth centuries, when wooden furniture started to surge as a major craft. During the medieval ages, fish glue remained a source for painting and illuminating manuscripts. Since the 16th century, hide glue has been used in the construction of

violins The violin, sometimes known as a ''fiddle'', is a wooden chordophone (string instrument) in the violin family. Most violins have a hollow wooden body. It is the smallest and thus highest-pitched instrument (soprano) in the family in regular ...

. Native Americans would use hoof glue primarily as a binder and as a water-resistant coating by boiling it down from leftover animal parts and applying it to exposed surfaces. They occasionally used hide glue as paint to achieve patterns after applying pigments and tanning to hides. Hoof glue would be used for purposes aside from hides, such as a hair preservative. The Assiniboins preferred longer hair, so they would plaster the strands with a mixture of red earth and hoof glue. It would also be used to bind feathers and equipment together.

Glue industries

The first commercial glue factory opened in Holland circa 1700, manufacturing animal glue from hides. The United States’ first glue factory opened in 1899, established by the Milwaukee Tanning Industry. The L.D. Davis company thrived producing animal glue during the

Great Depression The Great Depression (19291939) was an economic shock that impacted most countries across the world. It was a period of economic depression that became evident after a major fall in stock prices in the United States. The economic contagio ...

after shifting its focus from stenciling, selling to local box makers and other users; L.D. Davis' animal glue formula for

bookbinding Bookbinding is the process of physically assembling a book of codex format from an ordered stack of ''signatures'', sheets of paper folded together into sections that are bound, along one edge, with a thick needle and strong thread. Cheaper, b ...

remains in production. During the 18th and 19th centuries, ranchers disposed of old animals – horses in particular – to glue factories. The advent of synthetic

s heralded the collapse of the animal glue industry.

Modern uses

Today, animal glues are sparsely industrialized, but still used for making and restoring

violin family The violin family of musical instruments was developed in Italy in the 16th century. At the time the name of this family of instruments was viole da braccio which was used to distinguish them from the viol family (viole ''da gamba''). The standa ...

instruments, paintings, illuminated

parchment Parchment is a writing material made from specially prepared untanned skins of animals—primarily sheep, calves, and goats. It has been used as a writing medium for over two millennia. Vellum is a finer quality parchment made from the skins of ...

manuscripts, and other artifacts.

Gelatin Gelatin or gelatine (from la, gelatus meaning "stiff" or "frozen") is a translucent, colorless, flavorless food ingredient, commonly derived from collagen taken from animal body parts. It is brittle when dry and rubbery when moist. It may also ...

, a form of animal glue, is found in many contemporary products, such as

gelatin dessert Gelatin desserts (also Jelly or Jello) are desserts made with a sweetened and flavoured processed collagen product ( gelatin). This kind of dessert was first recorded as jelly by Hannah Glasse in her 18th-century book '' The Art of Cookery'' ...

marshmallows Marshmallow (, ) is a type of confectionery that is typically made from sugar, water and gelatin whipped to a solid-but-soft consistency. It is used as a filling in baking or normally molded into shapes and coated with corn starch. The sugar c ...

, pharmaceutical capsules, and photographic film and is used to reinforce sinew wrappings, wood, leather, bark, and paper. Hide glue is also preferred by many

luthiers A luthier ( ; AmE also ) is a craftsperson who builds or repairs string instruments that have a neck and a sound box. The word "luthier" is originally French and comes from the French word for lute. The term was originally used for makers of ...

over synthetic glues for its reversibility, creep-resistance and tendency to pull joints closed as it cures. This adhesive is mostly used as glue, sizing, or varnish, although it is not as frequently used as other adhesives because it is

water-soluble In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solub ...

. Other aspects, such as difficulty of storage in a wet state, requirement for fresh raw materials (the animal skin cannot be rotten or grease-burned), make this product more difficult to find and use. Factories now produce other forms of adhesives, as the process for animal glue is complex and tricky to follow. Animal glues will also darken with age and shrink as they dry, giving them the potential to harm wood, paper, or works of art. Too much handling and too many changes in temperature or humidity could cause further harm. Some companies, such as those in Canada, still produce animal, hide and hoof glues from horses. Recently, animal glue has been replaced by other adhesives and plastics, but remains popular for

restoration Restoration is the act of restoring something to its original state and may refer to: * Conservation and restoration of cultural heritage ** Audio restoration ** Film restoration ** Image restoration ** Textile restoration * Restoration ecology ...

Types and uses

Animal glue was the most common woodworking glue for thousands of years until the advent of synthetic glues, such as

polyvinyl acetate Polyvinyl acetate (PVA, PVAc, poly(ethenyl ethanoate)), commonly known as wood glue, PVA glue, white glue, carpenter's glue, school glue, or Elmer's glue in the US, is a widely available adhesive used for porous materials like wood, paper, and ...

(PVA) and other resin glues, in the 20th century. Today it is used primarily in specialty applications, such as

lutherie A luthier ( ; AmE also ) is a craftsperson who builds or repairs string instruments that have a neck and a sound box. The word "luthier" is originally French and comes from the French word for lute. The term was originally used for makers of ...

, pipe organ building, piano repairs, and antique restoration. Glass artists take advantage of hide glue's ability to bond with glass. As the glue hardens it shrinks, chipping the glass. It has several advantages and disadvantages compared to other glues. The glue is applied hot, typically with a brush or spatula. Glue is kept hot in a glue pot, which may be an electric unit built for the purpose, a double boiler, or simply a saucepan or crock pot to provide a warm water bath for the container of glue. Most animal glues are soluble in water, useful for

joints A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw- ...

which may at some time need to be separated. Alcohol is sometimes applied to such joints to dehydrate the glue, making it more brittle and easier to crack apart. Steam can also be used to soften glue and separate joints. Specific types include ''hide glue'', ''bone glue'', ''fish glue'', ''

rabbit-skin glue Rabbit-skin glue is a sizing that also acts as an adhesive. It is essentially refined rabbit collagen, and was originally used as an ingredient in traditional gesso. History In traditional oil painting as practiced by the Renaissance painter, ...

''.

Hide glue

Hide glue is made from

animal hide A hide or skin is an animal skin treated for human use. The word "hide" is related to the German word "Haut" which means skin. The industry defines hides as "skins" of large animals ''e.g''. cow, buffalo; while skins refer to "skins" of smaller an ...

(animal skin) and is often used in woodworking. It may be supplied as granules, flakes, or flat sheets, which have an indefinite shelf life if kept dry. It is dissolved in water, heated and applied warm, typically around 60 °C (140 °F). Warmer temperatures quickly destroy the strength of hide glue.Weisshaar 1988, p. 249. Commercial glue pots, simple water baths or double boilers may be used to keep the glue hot while in use. As hide glue cools, it gels quickly. At room temperature, prepared hide glue has the consistency of stiff gelatin, which is in fact a similar composition. Gelled hide glue does not have significant strength, so it is vital to apply the glue, fit the pieces, and hold them steady before the glue temperature drops much below 50 °C (120 °F). All glues have an ''open time'', the amount of time the glue remains liquid and workable. Joining parts after the open time is expired results in a weak bond. Hide glue's open time is usually a minute or less. In practice, this often means having to heat the pieces to be glued, and gluing in a very warm room, though these steps can be dispensed with if the glue and clamp operation can be carried out quickly. Where hide glue is in occasional use, excess glue may be held in a freezer, to prevent spoilage from the growth of

microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

s. Hide glue has some gap filling properties, although modern gap-filling adhesives, such as epoxy resin, are better in this regard. Hide glue that is liquid at room temperature is also possible through the addition of urea. In stress tests performed by Mark Schofield of Fine Woodworking Magazine, "liquid hide glue" compared favourably to normal hide glueSchofield, Mark
"How Strong is Your Glue?"
''Fine Woodworking Magazine'', v. 192, 36–40. 2007 in average strength of bond. "However, any liquid hide glue over six months old can be suspect because the urea eventually hydrolyzes the protein structure of the glue and weakens it – even though the product was 'protected' with various bactericides and fungicides during manufacture."

Production

Animal hides are soaked in water to produce "stock." The stock is then treated with

lime Lime commonly refers to: * Lime (fruit), a green citrus fruit * Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide * Lime (color), a color between yellow and green Lime may also refer to: Botany ...

to break down the hides. The hides are then rinsed to remove the lime, any residue being neutralized with a weak acid solution. The hides are heated, in water, to a carefully controlled temperature around . The "glue liquor" is then drawn off, more water added, and the process repeated at increasing temperatures. The glue liquor is then dried and chipped into pellets.

Properties

The significant disadvantages of hide glue – its thermal limitations, short open time, and vulnerability to micro-organisms – are offset by several advantages. Hide glue joints are reversible and repairable. Recently glued joints will release easily with the application of heat and steam. Hide glue sticks to itself, so the repairer can apply new hide glue to the joint and reclamp it. In contrast, PVA glues do not adhere to themselves once they are cured, so a successful repair requires removal of the old glue first – which usually requires removing some of the material being glued. Hide glue creates a somewhat brittle joint, so a strong shock will often cause a very clean break along the joint. In contrast, cleaving a joint glued with PVA will usually damage the surrounding material, creating an irregular break that is more difficult to repair. This brittleness is taken advantage of by instrument makers. For example, instruments in the violin family require periodic disassembly for repairs and maintenance. The top of a violin is easily removed by prying a palette knife between the top and ribs, and running it all around the joint. The brittleness allows the top to be removed, often without significant damage to the wood. Regluing the top only requires applying new hot hide glue to the joint. If the violin top were glued on with PVA glue, removing the top would require heat and steam to disassemble the joint (causing damage to the varnish), then wood would have to be removed from the joint to ensure no cured PVA glue was remaining before regluing the top. Hide glue also functions as its own clamp. Once the glue begins to gel, it pulls the joint together. Violin makers may glue the center seams of top and back plates together using a ''rubbed joint'' rather than using

clamps Clamp may refer to: Tools and devices *Brick clamp, an early method of baking bricks *Clamp (tool), a device or tool used to hold objects in a fixed relative position (many types listed) ** C-clamp ** C-clamp (stagecraft) **Riser clamp, a device ...

. This technique involves coating half of the joint with hot hide glue, and then rubbing the other half against the joint until the hide glue starts to gel, at which point the glue becomes tacky. At this point the plate is set aside without clamps, and the hide glue pulls the joint together as it hardens. Hide glue regains its working properties after cooling if it is reheated. This property can be used when the glue's open time does not allow the joint to be glued normally. For example, a cello maker may not be able to glue and clamp a top to the instrument's ribs in the short one-minute open time available. Instead, the builder will lay a bead of glue along the ribs, and allow it to cool. The top is then clamped to the ribs. Moving a few inches at a time, the maker inserts a heated palette knife into the joint, heating the glue. When the glue is liquefied, the palette knife is removed, and the glue cools, creating a bond. A similar process can be used to glue veneers to a substrate. The veneer and/or the substrate is coated with hot hide glue. Once the glue is cold, the veneer is positioned on the substrate. A hot object such as a clothes iron is applied to the veneer, liquefying the underlying glue. When the iron is removed, the glue cools, bonding the veneer to the substrate. Hide glue joints do not creep under loads. PVA glues create plastic joints, which will creep over time if heavy loads are applied to them. Hide glue is supplied in many different gram strengths, each suited to specific applications. Instrument and cabinet builders will use a range from 120 to 200 gram strength. Some hide glues are sold without the gram strength specified. Experienced users avoid this glue as the glue may be too weak or strong for the expected application.

Rabbit-skin glue

Rabbit-skin glue Rabbit-skin glue is a sizing that also acts as an adhesive. It is essentially refined rabbit collagen, and was originally used as an ingredient in traditional gesso. History In traditional oil painting as practiced by the Renaissance painter, ...

is more flexible when dry than typical hide glues. It is used in the sizing or priming of oil painters' canvases. It also is used in bookbinding and as the adhesive component of some recipes for

gesso Gesso (; "chalk", from the la, gypsum, from el, γύψος) is a white paint mixture consisting of a binder mixed with chalk, gypsum, pigment, or any combination of these. It is used in painting as a preparation for any number of substrates suc ...

and compo.

Fish glue

Fish glue is made from the bones or tissues of fish.

Isinglass Isinglass () is a substance obtained from the dried swim bladders of fish. It is a form of collagen used mainly for the clarification or fining of some beer and wine. It can also be cooked into a paste for specialised gluing purposes. The E ...

is made specifically from the swim bladders, and is collagen-based. Fish glues were used in Ancient Egypt and Classical Antiquity in the Mediterranean; they continued to be used in Europe in Late Antiquity and the Medieval period, and are still used in niche applications today. It is brittle when dried, so it has sometimes been mixed with

plasticizer A plasticizer ( UK: plasticiser) is a substance that is added to a material to make it softer and more flexible, to increase its plasticity, to decrease its viscosity, and/or to decrease friction during its handling in manufacture. Plasticiz ...

s such as

molasses Molasses () is a viscous substance resulting from refining sugarcane or sugar beets into sugar. Molasses varies in the amount of sugar, method of extraction and age of the plant. Sugarcane molasses is primarily used to sweeten and flavour foods ...

and

honey Honey is a sweet and viscous substance made by several bees, the best-known of which are honey bees. Honey is made and stored to nourish bee colonies. Bees produce honey by gathering and then refining the sugary secretions of plants (primar ...

. It was used in art, woodworking,

, and for gluing paper and bone.

Notes

References

* *Patrick Spielman. ''Gluing and Clamping: A Woodworker's Handbook.'' Sterling Publishing, 1986. *{{cite book , title=Violin Restoration , last1=Weisshaar , first1=Hans , first2=Margaret , last2=Shipman , year=1988 , publisher=Weisshaar~Shipman , location=Los Angeles , isbn=0-9621861-0-4

External links

* http://woodtreks.com/animal-protein-hide-glues-how-to-make-select-history/1549/ Video on hide glue, by Keith Cruickshank * https://web.archive.org/web/20130522233935/http://www.oldbrownglue.com/articles.html Why Not Period Glue? - article by W. Patrick Edwards on hide glue * http://wpatrickedwards.blogspot.com/2012/01/why-use-reversible-glue.html - Why Use Reversible Glue? Woodworking adhesives Adhesives Animal products ja:膠 zh:明膠#製法