Hardwood Kecord — Veneer & Panel Section 



25 



Some Methods of Testing Animal Glue 



Water-taking -Viscosity— Jelly-strength 



jT T HIS TIMi: there set-ms to he a particular desire 

 on the part of many Kluf ust-rs (or a more com- 

 prehensive knowled'^'c <>l mrthods used in de- 

 termining glue strength and quality in a reason- 

 ably accurate way. Unfortunately there arc no universal 

 standards for testing glues, but an effort has been made 

 to present in this article a few simple methods which may 

 be used in the average factory. 



A somewhat general method of testing glue consists in 

 determining the amount of water it will absorb. Aweighed 

 quantity of glue is covered with water, having a tem- 

 perature of about 60 degrees Fahrenheit, and allowed to 

 soak twelve hours, or until the glue has taken up all the 

 water it will. Then the excess water is drained off and 

 the soaked glue weighed. Other things being equal, the 

 greater the increase in weight the better the glue, pro- 

 viding it remains firm. Very good animal glue will 

 increase eight to ten times in weight, while common glues 

 will increase three to five times, and still remain firm to 

 the touch. If the glue, soaked in cool water, forms a 

 homogeneous mass it is apt to be fish glue, or a very low- 

 ^rade animal product that is not desirable for wood- 

 working operations. 



In making weight tests one should bear in mind the 

 fact that different atmospheric conditions may have more 

 or less effect on the uniformity of the tests, because in 

 damp weather the glue will have absorbed some mois- 

 ture. This absorption generally will not be large enough 

 to effect the practical results. .At the same time it is 

 always well to take test lots from near the center of the 

 barrel or bag. 



Any accurate scale may be used for weighing out glue 

 samples and making up test portions. The "valency" 

 type of scale with a capacity of 2 ' 4 pounds (troy weight), 

 or one kilogram in the metric system, on each pan, sensible 

 to the smallest practical weight, will be found very adapt- 

 able. While it is true that glue is handled commercially 

 by avoirdupois weight, test weights may be easiest handled 

 by means of the troy or the metric system, because of the 

 small amount used. So long as the proportions are 

 figured accurately it makes no difference what weigh- 

 ing system is used. 



A fairly simple apparatus may be had for gauging the 

 percentage of dry glue in a solution. This is not a 

 viscosity test, though it is sometimes referred to as such. 

 Two instruments, a glue hydrometer and a cylindrical 

 shaped vessel, make up this apparatus, (see figure II). 

 The cylinder may be of any waterproof material. Copper 

 vessels are most commonly used because of chemical 

 resisting and nonbreakable qualities. 



The principle of the operation of the hydrometer is 

 based on the physical law that a solid body floating in 

 a liquid will displace a quantity of the liquid equal in 



weight to the floating body. Hence, a body of constant 

 weight and proportion will always sink to the same extent 

 into a liquid of a certain density and to a greater or 

 lesser extent as the density of the liquid decreases or 

 increases. 



As a liquid expands or contracts with temperature the 

 density of the liquid varies in accordance and, there- 

 fore, when the hydrometer is constructed the scale must 

 be standardized for a certain temperature. The most 

 common glue standards are 150, 175 and 187 degrees 

 F. Owing to the fact that it is not always convenient to 

 have the liquid of the temperature for which the scale 

 of the instrument is arranged, for accurate work, it is 

 often necessary to apply a correction for temperature 

 variation, and for that purpose an auxiliary thermometer 

 is used to ascertain the temperature of the liquid when the 

 hydrometer is of plain form. To facilitate temperature 

 observations, however, the hydrometer is often made of 

 combined form, that is, a thermometer is combined with 

 the hydrometer; the thermometer portion usually being 

 in the body, or flotation bulb, of the instrument. In the 

 case of glue and other opaque liquids, however, the 

 thermometer scale portion of a combined hydrometer 

 is usually placed in the upper part of the hydrometer 





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