Product Code Database
Animal glue is an adhesive that is created by prolonged boiling of animal connective tissue in a process called rendering. In addition to being used as an adhesive, it is used for coating and sizing, in decorative composition ornaments, and as a clarifying agent.
These protein colloid glues are formed through hydrolysis of the collagen from skins, bones, tendons, and other tissues, similar to gelatin. The word collagen itself derives from Greek language κόλλα (), meaning 'glue'. These proteins form a molecular bond with the glued object.
Stereotypically, the animal in question is a horse, and horses that are euthanized are often said to have been "sent to the glue factory". However, other animals are also used, including cattle, and fish.
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. 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 Greece and Ancient Rome later used animal and fish glue to develop Wood veneer and marquetry, the bonding of thin sections or layers of wood. Animal glue, known as (ταυρόκολλα) in Greek and gluten taurinum in Latin, 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. Animal glues were employed as binders in paint media during the Tang dynasty. They were similarly used on the Terracotta Army figures. Records indicate that one of the essential components of lampblack 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.
Native Americans used 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 Assiniboine preferred longer hair, so they would plaster the strands with a mixture of red earth and hoof glue. It was also used to bind feathers and equipment together.
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. 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 obtain and use. Factories now produce other forms of adhesives, as the process for animal glue is complex. 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 Art restoration.
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 which may at some time need to be separated.Courtnall 1999, p. 63. 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, and rabbit-skin glue.
Where hide glue is in occasional use, excess glue may be held in a freezer, to prevent spoilage from the growth of . 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."
The glue liquor is then dried and chipped into pellets.
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. 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.
|
|
