All About Animal Glue

Animal glue is one of the first adhesives used by humankind. The basic adhesive substance of glue is the product of the hydrolysis of collagen — the protein substance contained in the tissues of living organisms. Collagen is an insoluble fibrous protein that occurs in vertebrates as the primary constituent of connective tissue fibrils and in bones and yields gelatin and glue on prolonged heating with water.

Glue largely consists of gelatin, but the collagen from which gelatin or glue is prepared is invariably associated with other protein material such as keratin, elastin, mucin, chondrin, etc., in addition to non-protein, organic material and inorganic salts that may or may not remain in the glue. Glue and gelatin merge into one another by imperceptible degrees. The difference is one of purity: the more impure form is called glue and is used only as an adhesive; the purer form, termed gelatin or size, is used when an especially fine adhesive or medium is required.

Glue is an organic colloidal substance of varying appearance, chemical composition and physical properties. It occurs in commerce in a wide variety of forms and colors. The colors range from all shades of white, yellow and brown, and glue may be transparent, translucent or opaque. Gelatin or glue-forming tissues occur in the bones, skins and intestines of all animals. These agglutinating materials are extracted with hot water, and the solution, on evaporation and cooling, yields a jelly-like substance — gelatin or glue. Glue is also prepared from fish bones, skins or bladders that give impure forms of bone gelatin, skin gelatin and isinglass.

The wide acceptance of glue as an adhesive stems from its unique ability to deposit a tacky viscous film from a warm water solution, which, upon cooling a few degrees, passes into a firm jelly state producing an immediate, moderately strong initial bond. Subsequent drying provides a permanent, strong, and resilient bond.

The use of glue as an adhesive dates from earliest recorded times. Whoever discovered that a strong adhesive could be produced by cooking pieces of animal hide, or perhaps bone, in water has never been ascertained, but archeological discoveries indicate that the Egyptians used glue more than 4,000 years ago. The practical manufacture of glue can be traced back directly to 1690 in the Netherlands. Shortly thereafter, or about 1700, the English began making glue and established its manufacture as a permanent industry. Elijah Upjohn is considered by some authorities to have been the first to manufacture glue in the United States, in 1808.

How to Use Glue

Animal hide and bone glues set in a two part process which first begins by cooling from 145° F. (63° C.) to room temperature, and then completely drying by evaporation during the next 12 to 24 hours. This allows the artist to use this glue to advantage, since it rapidly sets as it cools.

Collagen glues form a chemical (molecular) bond as well as a mechanical bond. This means that fresh animal glue will reactivate and chemically bond to previous animal glue surfaces, as well as forming a strong mechanical bond with wood surfaces and other natural fibers. Hide glue sticks to surfaces by an electrochemical attraction, or "specific adhesion." It is one of the few truly reversible glues, which can be changed from liquid to solid and back again with the addition or subtraction of heat and moisture.

The technique of making the different varieties of glue is practically identical. The differences lay only in the methods of purifying the end product and some special features of treating the raw material. The process of extracting glue from animal tissues consists of heating the raw material in water. Upon heating in water collagen is hydrolyzed forming a gelatinous substance that absorbs water to make a viscous solution with strong adhesive properties. The temperature of the solution must not exceed 70° to 80° C. (158° to 176° F.), otherwise the protein molecules are destroyed and the glue loses its adhesive properties.

When glue is soaked for some time in cold water, it softens and swells without dissolving, and, when again dried, resumes its original properties. When gently heated, it dissolves entirely in water, forming a thick, syrupy liquid with a characteristic but not disagreeable odor. Remelted glue is not so strong as that which is freshly prepared, and newly made glue is inferior to glue that has been in stock for some time. Glue losses strength continuously under the action of heat, and it is better to heat successive small amounts rather than to have a large pot cooking for a relatively long period. All glue solutions putrefy with time and lose their adhesive power.

Types of Collagen Glue

Fish glue is impure gelatin prepared from fish heads, bones and skins. The pure gelatin from fish bladders is known as isinglass. Glue made from the skins is the clearest and best. Usually, fish glue is marketed in liquid form, but it can sometimes be obtained in the form of cakes or broken sheets that are hygroscopic and readily soluble in water. As a liquid it contains a preservative and sometimes an essential oil to mask the odor. Fish glue is considered to be inferior to animal glues as an adhesive and is more easily spoiled by bacteria.

Isinglass is very pure fish gelatin from the swim bladders of a limited number of fish, chiefly the sturgeon, which is the main source of Russian isinglass. North American isinglass comes mainly from the hake and sometimes from the cod. It is nearly pure collagen. When soaked in cold water, it swells without losing its organized, fibrous, thread-like structure, but boiling converts it to gelatin, which, probably because of its formation, yields a very strong jelly.

From the many kinds of glue available, the best properties are found in sturgeon isinglass and rabbit skin glue (available from Natural Pigments). Isinglass is practically colorless, possesses significant adhesive strength, and yet preserves its elasticity, which is very important.

There are several varieties of glue made from mammals, each of which has its own name, such as rabbit skin, painter, hide and joiner glue. All these varieties are different in elasticity, adhering strength and color. Processing the bones, tendons and skins of rabbits yields rabbit skin glue, which is considered by some to be the finest glue for gesso and chalk grounds.

Next in quality is hide glue from the hides of almost any animal but primarily from cattle. This is simply because of the abundance of cattle hides due to the enormous worldwide consumption of beef and subsequent tanning of hides for leather. The trimmed hide pieces (too small or irregular shapes to provide useable leather) are shipped to the hide glue plant.

Of lesser quality for the purposes of making gesso is fish glue and gelatin. Fish glue is similar to sturgeon isinglass, but it is prepared from the heads, scale and bones of fish. Fish glue is somewhat inferior compared with sturgeon, because of its weaker adhesive strength and darker color, which can negatively affect the color of the gesso.

There are two basic grades of gelatin: food and technical. Both varieties, according to its adhesive strength, are inferior to sturgeon and skin glues, and only the technical grade can be used in gesso. For example, Knox Gelatin®, available from supermarkets in the U.S., is a food grade gelatin, but is not made from animal hides. It is made from rendering cattle bones. That means it is comprised of very light proteins that are edible, and which, in turn, do not make good adhesives. Granted, it has been rendered "pure" edible grade gelatin and as such retains a very small amount of fat. It is these fats that go rancid and cause the glue to stink as mold spores from the air begin to multiply in it.

All hide glue contains some fat. It comes with the product — even technical grades. To test a sample for fat just sprinkle a pinch of lye (sodium hydroxide or potassium hydroxide) into your gluepot — the fats are saponified and precipitate to the bottom. Animal hides contain the heavy-molecular proteins required for glue.

Properties of Animal and Fish Glue

Collagen glue is an adhesive consisting of organic colloids of a complex protein structure — essentially high polymer proteins. [1] These organic colloids are comprised of complex proteins found in animal hides, connective tissues and bones. This protein has two elements that define its characteristics: chondrin, which gives it adhesive strength, and gluten, which gives it gel strength (gelatin). [2] Collagen is the principal protein constituent of animal hide, connective tissue and bones. Collagen, animal glue, and gelatin are very closely related as to chemical composition. Gelatin is considered to be hydrolized collagen. There may be minor variations in the composition of collagens from different sources, as well as in the composition of animal glues imparted by variations in processing techniques; however, the composition of glues and gelatins having widely varying case histories are still very similar.

As a protein, animal glue is essentially composed of polyamides of certain alpha-amino acids. It is believed that these acids are not present in glue in the free state, but rather as residues which are joined together by the elimination of water to form long polypeptide chains. Glue is a polydisperse system containing mixtures of similar molecules of widely differing molecular weights. Because so wide a range of molecular weights is present, the molecular weight of glue is always an average, ranging from 20,000 to 250,000.

The main structural protein in collagen is tough and rigid molecule. It is made up of three protein chains twisted together to give a rigid, rope-like structure. Collagen combined with sodium hydroxide (NaOH) and heated relaxes the coiled molecules to make uncoiled molecules of gelatin. There is an optimal unwinding of the collagen molecules: too much and the gel strength is weakened; too little and the gelatin yield will be low.

Hide and bone glues make up the two major types of animal glue. Hide glue, which is by far the superior of the two, yields a fairly neutral pH in solution, usually in the range of 6.5 to 7.4, although wider variations are possible. Bone glue is generally acidic, having pH values of 5.8 to 6.3. A glue having a high acidity absorbs less water and tends to set more slowly than a glue having low acidity. A glue having a pH greater than 7.0 tends to foam, and has a shorter shelf life than a glue that is slightly acidic.

Animal glues are soluble only in water, and are insoluble in oils, waxes, organic solvents, and absolute alcohol; however, they may be emulsified in water-oil or oil-water systems under proper conditions. One of the more interesting properties of animal glue solutions is their ability to pass from a liquid to a jelled state upon cooling, and then revert to the liquid state upon re-heating.

The important properties of glue include its jelly strength or consistency (gel strength), viscosity, melting point, adhesive strength, tensile strength or elasticity, optical rotation, swelling capacity, rate of setting, foaming characteristics, reactions to grease (whether acid or alkaline), as well as appearance, odor, color and keeping characteristics. Of these, gel strength and viscosity are most often used for determining the grade of a particular glue.

Making Collagen Glue from Animals

These glues are made using a rather simple process, which hasnt changed much over the ages. The raw material is first conditioned in a water solution with lime (calcium hydroxide). Then the pH value is adjusted by adding a dilute mineral acid and rinsed in water. [3] Then the process of cooking begins, and while the material is cooked the water/protein solution is extracted and filtered. The protein that is collected by the filters is dried and ground up as a final product. The resulting glue is then tested as to viscosity (fluidity) and gel strength (stiffness of gel formation), and graded on a scale from 50 to 512. [4] Lower grades dry slower and are more flexible and higher grades dry faster and harder. Glue chip glass is made using hide glue with a 135 gram strength, which allows the glue to actually tear off the surface of the glass as it sets. Woodworkers choose between 160 and 250 gram strengths, which have slightly different working characteristics. The most popular is 190 gram strength that allows adequate working times when the wood is preheated to 95° F. (35° C.).

Painters of all epochs preferred to use skin glue, that is, glue obtained by processing the skins of animals. Good results can be had when using sturgeon and fish glue; however, you must strictly observe the ratio of the glue to water. Cracks can form in the gesso when the ratio of glue to water is not carefully followed.

In ancient times, artists prepared the glue themselves. Naturally, this glue was inferior in quality when compared to present-day glues prepared with more technologically effective and better purifying methods. Today, it is not difficult to buy high-quality glue, however, it is worthwhile to consider how it was prepared since times of antiquity. One common method is described in the book, Painters Manual or Hermeneia of Dionysius of Fourna, an icon painter of the 18th century:

On the Making of 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.

This method of preparing glue was used up to the beginning of the nineteenth century, and in some places even later. Since that time, glue has been produced industrially.

The modern glue manufacturing process basically follows this procedure:

  1. Wash hides and bones to remove dirt;
  2. Soak in limewater for 60 to 90 days;
  3. Wash to remove hair and lime;
  4. Neutralize with acid, drain, wash and drain again;
  5. Add the raw material to water, heat to 45–50° C. (110–120° F.) for two to four hours (called an extraction);
  6. Drain off the dilute glue solution, evaporate, chill, dry and grind the resulting solids;
  7. 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.




Fig. 1 The Manufacturing of hide glue, bone glue, and industrial gelatins.(Source: Milligan and Higgins)

Regardless of the source of the protein, the glue manufacturing process consists essentially of washing the stock, crushing or shredding the bones or hides, soaking in a lime solution to eliminate hair and flesh, boiling to extract the gelatinous material, gelling, and, finally, drying. The resulting hard, brittle sheets of glue are then broken into pieces or flakes, or ground into powder.

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.

Technical Requirements of Animal Glues for Applications

The wide acceptance of glue as an adhesive stems from its unique ability to deposit a tacky viscous film from a warm water solution, which, upon cooling a few degrees, passes into a firm jelly state producing an immediate, moderately strong initial bond. Subsequent drying provides a permanent, strong, and resilient bond.

The suitability of collagen glue for a specific application is determined by two characteristics, viscosity and bloom. The viscosity is measured by the time required to drain 100 ml of the glue in a 12.5% solution from a glass pipette that has been calibrated under rigorous conditions. The unit of measurement, millipascal seconds (MPS), is determined by own reference for the time of draining in seconds and constants from the calibrated pipette. The value for Bloom (also called gel or jelly strength) is obtained by measuring the weight required for a standard probe to penetrate a 12.5% glue solution. The mass needed to penetrate 4mm determines its rigidity and is measured in grams.

Technical Data for Animal Glue

Industry Name

Animal and Parts


Millipascal seconds



Bone glue

bovine and porcine bones



Hide glue

bovine and porcine hides



Rabbit skin glue

bovine hides



Rabbit skin glue (genuine)

Rabbit hides



Bookbinders glue

bovine and porcine hides



Technical Specifications for Typical Applications of Animal Glue



Millipascal seconds










Book bindings



Hard bindings



Musical instruments






Agglutinating agent



Starch agent



Modifying Hide Glue

Animal glue in its raw form is not suitable for many of its applications. For example, it is much too brittle for use in bookbinding; therefore a plasticizer, such as glycerin, or a less expensive substitute such as sorbitol, often combined with glycols and tackifiers, are added to improve elasticity and resilience. These so-called flexible glues are usually prepared from high quality grades of hide glue, with the ratio of plasticizer(s) to dry glue controlling the degree of flexibility that is imparted. In addition, glue, being an organic material, is susceptible to mold; consequently preservatives, such as beta naphthol, or the safer phenols, e.g., p-phenyl phenol, are added to prevent mold and bacterial growth. Deodorants, such as terpinol, are also employed in commercial glues.

Hydrolyzed collagen glues are easily modified with a wide variety of additives. It can be formulated with water soluble materials, such as sorbitol, glycerol, sugars, syrups, metal salts and sulfonated oils. To make it more resistant to water add up to 10% by weight of alum (aluminum potassium sulfate), tannin or formaldehyde to the dry weight of glue. Common salt (sodium chloride) and potash (potassium carbonate) can prevent brittleness and crazing over time. Adding up to 5% by weight of glycerin can make the glue flexible enough to use on canvas. Adding 5–10% or more by weight of urea extends the gel time, and also increases flexibility, producing a liquid hide glue at room temperature. All of these additives reduce the actual strength of the glue, but the final result is still an adhesive that is stronger than most surfaces to which it is commonly applied. All protein glues contain some preservatives and foam control agents, which do not affect their working properties.

Working with Animal Glue

Working with traditional animal glues is a simple process. The glue in dry form has a unlimited shelf life, stored in a dry container and kept away from heat. To prepare the glue for use, just add cold water and let sit overnight. It is not really important how much water you add, as long as it completely covers the glue. If you mix by weight, use 1.8 parts of water to 1 part of glue. If you mix by volume, just cover the dry glue with more water than glue. Once all the water has been absorbed, put the gelled glue into a double boiler and cook it on a low heat. A variety of materials can be used for the double boiler, such as copper, iron enamel, glass, stainless steel or aluminum. Use a stainless steel meat thermometer to monitor the glue temperature. Keep it constantly at 145° F. (63° C.), and add more water as needed to replace that lost to evaporation. A foil cover can be kept loosely over the top of the glue pot while cooking to reduce evaporation. A good quality round bristle brush is best as an applicator.

Artists can make subjective tests to monitor the glue as it prepared. The odor should be pleasant if the glue is good, and smell bad if the glue is overheated or has been damaged by mold. The viscosity is measured by lifting the glue brush about a foot over the pot and letting the glue drip back down. It should be thin and liquid, with no lumps. You can test the strength by putting a small amount of hot glue between your finger and thumb, and rubbing together until it cools. The strength is then measured by pulling the finger and thumb apart several inches and looking at the protein strands, which appear, like spider webs. The longer the strands the stronger the glue. The color of protein glue when freshly cooked is light amber and it continues to darken as it is cooked. As long as it is not overheated it remains quite strong. If the glue temperature reaches 212 degrees it is ruined. If there is mold in the glue it can be very hard to see, but the glue will remain lumpy at operating temperature, and should be discarded. The glue pot and brushes must then be cleaned by boiling in water before a fresh batch is made. [6]

The Importance of Reversibility

Animal glues are the only easily reversible glues available to artists that have outstanding adhesion qualities. Modern synthetic glues convert from one chemical form to another by using a catalyst. Once converted these synthetic glues are difficult or impossible to undo. Since animal glues react to heat and moisture, they can be easily converted from liquid to solid and back again, even after a century or more of time. [7] This is a primary reason why these glues have continuously been used in the restoration field. The existing original glue can be softened or cleaned with warm water, and the new application of hot hide glue will completely bond with the previous glue.


Test Method to Determine the Gel Strength with the Bloom Gelometer

For the test use a wide-mouth bottle that accepts a rubber stopper 42–45 mm in diameter. It should have a capacity of 155 ml and an overall height of 85 mm. The outside diameter should be 66 mm and the average internal diameter shall not vary by more than 1 mm from 59 mm. The stopper used should be tapered, cut in half and the upper portion perforated by plunging a red hot, 1-inch brad through it at the center. The upper half of the stopper will be used to obtain a snug fit in the neck of the bottle, the air vent serving to prevent the stopper from being blown out during the melting and heating of the sample. The test bottle and stopper should be clean and dry.

Weigh a quantity of glue equivalent to 7.5 grams and transfer it to the test bottle. Add 105 ml of distilled water at approximately 15° C. while thoroughly stirring the glue with a thin metal rod. Place the bottle in a cooler maintained at a temperature of 10° C. to 15° C. and allow the sample to soak for three hours. To prevent cracking, and in order to preserve uniformity in heating and cooling, the following procedure shall be used both when heating the sample to 60° C. and, after solution, when cooling prior to putting in a bath. Place the bottle for 15 minutes in a water bath kept at approximately 30° C. Bring the sample to a temperature of 60° C. in the melting bath, the temperature of which is not allowed to exceed 70°C. Determine the temperature of the sample with an accurate thermometer placed in the gelatin solution and carried by a stopper having a small hole off the center for this purpose. The time required to bring the sample up to temperature should not exceed 15 minutes. After closing the bottle with the stopper carrying the thermometer and before reaching the final temperature make the solution thoroughly uniform preferably by swirling the bottle a number of times, but avoiding any motion that will produce violent agitation of the solution. Place a finger over the hole in the stopper and invert the test bottle several times to mix in the water that has condensed on the walls of the bottle and the under side of the stopper. Then place the container in a totally enclosed chill bath maintained at a temperature of 10°C. ± 0.1° for not less than 16 but no more than 18 hours.

Measurement of the gel strength of the sample should be made with a Bloom Gelometer (See Fig. 1, Fig. 2 and Fig. 3) adjusted to give a 4 mm depression, and to deliver shot at the rate of 200 grams per 5 seconds, when the clam-shell arm E4 rests upon dog D1 and 800 grams of polished No. 12 chilled lead shot are in hopper 1. The Bloom Gelometer should be placed perfectly level on a rigid support. It can be leveled by turning thumb-screw nut G1 until the silver disc is well above the lower contact point A2, and by means of leveling screws T1 and T2 the base R1 shall be adjusted so that rod H3 hangs in the center of the hole in guide arm J2A. Brass contact point bracket A shall be set so that it will not come into contact with rod H3, nor shall the former be allowed to come in contact with screws holding J2A in position, otherwise a "short" in the circuit will result. The position of J2A shall not be changed as it must be kept directly under the point of suspension of spring F. The spring shall be of such a stiffness that between 2 and 3 grams placed on the pan H2 will bring the disc B from top contact A1 to lower contact A2. The weight of the pan system shall be counter-balanced by such a tension in the spring F that equilibrium is produced when contact disc B is just barely resting on the lower contact point A2. This condition shall be produced by first closing the electrical circuit by means of switch Q and then turning thumbscrew nut G1 until disc B on lowering makes the first perceptible electrical contact with lower contact point A2. When properly set, a slight vibration of the instrument will cause a succession of "makes" and "breaks" in the circuit producing a sound in mechanism D1, D7, D8 very much like that of a telegraph sounder. To make a test of the jelly strength of a given sample, the electrical circuit shall be closed by means of switch Q the jelly contained in the test bottle M shall be centered on platform N1 and by means of the rack and pinion elevating mechanism N2 the jelly shall be raised until disc B almost makes contact with contact point A1. Then with the fine adjustment on rack and pinion N3 the disc shall be brought into the lightest, but positive, electrical contact with contact point A1. This is indicated by sparking and the telegraph sounder effect noted above. The shot receiver K shall be quickly placed on pan H2 and immediately the arm E4 shall be raised to the predetermined position on one of the dogs D1 with a quick but uniform motion. The height to which arm E4 is raised regulates the velocity of the flow of shot, plunger L depressing the surface of the jelly until disc B makes contact with contact point A2. This closes the circuit which acts on the electromagnet C, moving the soft iron bar D8 and withdrawing the support of the dog from the arm E4, which immediately falls, thus cutting off the flow of shot by closing the clam-shell cut-off E-E1. All determinations should be made from the dog D1 and the results expressed to the nearest whole gram. The weight of shot delivered into the receiver K plus the weight of the shot receiver itself is the weight required to move the plunger L through a distance of 4 mm against the resistance of the jelly and, for the purpose of Clause 13, this weight shall be the Bloom gel strength of the sample.


Fig. 2. The Bloom Gelometer (Front View)

A1. Set screw to hold adjustment screw in position. C. Electromagnet. C2. Adjustment screw for adjusting pitch of clam cut-off E. D. Guide bar of automatic shot control mechanism. D1. Dog. E. Clam-shell spout. E2 Adjusting screws to regulate closure. F. Spiral spring (No. 6 steel music wire). G1. Thumb screw nut. G2. Tension spring. H1. Pan arms. H2. Pan H3. Rod attached to pan arms supporting disc. B. I. Shot hopper with delivery tube. K. Short receiver. L. Plunger (12.7 mm in diameter). M. Test bottle. N2. Rack and pinion elevating mechanism. N3. Fine adjustment on rack and pinion. N4. Brake shoe on adjustment arm of N3. O. Battery box and batteries. R2. Pillar of gelometer. T1, T2. Leveling screws. T3. Spirit level.


Fig. 3. The Bloom Gelometer (Side View)

A Brass contact point bracket. A1. Upper contact point. A2. Lower contact point. A3. Wood fibre support for A. A5. Adjustment screw. B. Pure silver disc (5/8 inch dia. and 1/16 inch thick). C. Electromagnet. D1. Dog. D7. Hair spring coil to keep D8 in position. D8. Soft iron bar supporting dog D1. E1. Stationary clam-shell jaw. E2. Adjusting screws to regulate closure. E3. Weight. E4. Clam-shell arm. E5. Set screw to clamp weight to clam-shell arm. E6. Bearing on which cut-off mechanism turns. F. Spiral spring (No. 6 steel music wire). G. Adjustable support for spring F. G1. Thumb-screw nut. G2.Tension spring. H2. Pan. H3. Rod attached to pan arms supporting disc B. I. Shot hopper with delivery tube. I1. Bracket to hold lower end of shot delivery tube. J1. Upper supporting bracket attached to frame support R2. J2. Lower supporting bracket attached to frame support R2. J2A. Guide arm attached to J2. K. Shot receiver. L. Plunger (bevelled typre). The dimensions of the plunger are shown in Fig. 5. M. Test bottle. N. Elevating platform base. N1. Platform. N2. Rack and pinion elevating mechanism.


Fig. 4. Detail of Punger of the Bloom Gelometer

Comparing the Gel Strength of a Test Sample Against a Known Sample

Equal weights of the working sample and of the agreed sample, prepared in the same way as the working sample shall each be weighed and placed in two 150 ml beakers and 50 ml of cold distilled water shall be added to each from a pipette. The quantity of gelatin used (2 to 10 grams), the concentration, and the temperature at which the solutions are made, shall be the same in both cases and shall be the subject of agreement between the purchaser and vendor. A watch glass shall be placed over each beaker and the gelatin allowed to soak for about three hours until it is completely swollen. During soaking, the temperature of the room shall be from 15° C. to 20° C. Each beaker shall then be heated on a water bath for 15 minutes while the contents are gently stirred with a glass rod. The temperature of the water bath shall be not more than 70° C. and the temperature of the gelatin solution shall be not more than 60° C. If necessary the heating shall be continued until, on inspection through the bottom of the beaker, the gelatin appears to be completely dissolved. Each of the gelatin solutions shall be immediately poured into aluminum or porcelain cups. The cups shall be of exactly the same internal dimensions. After two minutes a lid shall be placed on each vessel. The cups shall be left to stand for 16 hours in a thermostatic bath maintained at a temperature agreed to by the purchaser and vendor. The lids shall be removed from the vessels and the two jellies compared by pressure with the finger or by any suitable instrument or procedure agreed to by the purchaser and vendor.

Method for Determining the Solubility of a Partially Swollen Sheet

One lb. of the sheet gelatin shall be coarsely crushed with an iron pestle and mortar to pass a 3/8 in. test sieve conforming to I.S. 24:1950. Twenty ml of water at 27° C. shall be measured into a small basin and 20 grams of the crushed gelatin shall be added at once. The gelatin shall be stirred immediately and continuously with a suitable rod until all the water has been absorbed. The soaked gelatin shall be left to mature in a closed vessel containing water so that it is subjected to an atmosphere of 95 to 100% relative humidity for 24 hours at room temperature. Five hundred ml of water at 60° C. shall be placed in a beaker and the swollen gelatin added. The contents shall be stirred gently with a thermometer for 4 minutes, the temperature being maintained at 60° C., and then poured through a No. 200 test sieve conforming to I.S. 24:1950. The residue on the sieve shall be rinsed with cold water and transferred to a weighed stainless steel dish and then dried in an oven at 105° C. to constant weight. The stainless steel dish shall be as specified for the determination of moisture content in Appendix B. The weight so obtained shall be divided by 0.85 to allow for moisture content and the corrected figure shall be multiplied by 5. The result shall be expressed as per cent. insoluble residue of sheet gelatin swollen at 1:1. The figure obtained by subtracting the result from 100 shall be taken as the solubility of the partially swollen sheet.

Method for Determining Melting Point

A quantity of working sample equivalent to 7.5 grams of a sample shall be placed in a beaker and 105 ml of cold distilled water added. A watch glass shall be placed over the beaker and the gelatin allowed to soak for about three hours until it is completely swollen. During soaking the temperature of the room shall be from 15° C. to 20° C. The beaker shall then be heated on a water bath for 15 minutes while the contents are gently stirred with a glass rod. The temperature of the water bath shall be not more than 70° C. and the temperature of the gelatin solution shall be not more than 60° C. If necessary the heating shall be continued until, on inspection through the bottom of the beaker, the gelatin appears to be completely dissolved. The apparatus for determination of melting point shall consist of a brass bowl 22 mm in height, 17 mm external diameter at the top, and 15 mm external diameter at the bottom, and of such thickness that its weight is 7.0 grams. Into this shall be loosely fitted a glass rod 40 mm long and 3 mm in diameter which is flattened at one end to a disc of 9 mm diameter and fashioned at the other end into a hook. The gelatin solution shall be poured into the bowl, and the rod placed in a vertical position in the bowl with the disc resting on the base of the bowl. The bowl with the rod maintained in a vertical position shall be kept for 16 hours in a thermostat at 10° C. to allow the solution to set to a jelly. The apparatus shall be suspended in a beaker of water at 15° C. so that it is completely immersed. The beaker of water with the apparatus shall then be placed in a water bath at 20° C. and the water in the water bath heated so that the temperature of the water in the beaker rises at a rate of 1⁄4° C. per minute. The temperature of the water in the beaker at which the bowl falls from the rod shall be taken as the melting point of the gelatin. The melting point of the agreed sample shall be determined by treating an equal quantity in the same way.

Method for Determining Water Absorption

The working sample shall be sieved and the portion passing through a No. 22 test sieve, conforming to I.S. 24:1950, and retained on a No. 44 test sieve, conforming to I.S. 24:1950, shall be used for the test. If the original sample is of such a grist that it passes through a No. 44 test sieve, it shall be used without further treatment. Seven and a half gram of the powder so obtained shall be added to 210 ml distilled water at 10° C. in a 300 ml squat beaker. The contents shall be occasionally stirred with a glass rod at the earlier stages to prevent caking. The beaker and contents shall be kept at 10° C. for 16 hours. The supernatant water shall be poured off through a funnel fitted with a strainer of stretched damp muslin. The quantity of water passing through the funnel, allowing a drainage time of five minutes, shall be measured. The difference between this figure and 210 shall be taken as the water absorbed by the 7.5 grams of powder. The water absorbed by 7.5 grams of the agreed sample shall be determined in the same way.