530 MARINE PRODUCTS OF COMMERCE 



Liquid glues used for joining work should possess high tensile and shearing 

 strength under all ordinary weather conditions. Properties controlling strength in- 

 clude hygroscopicity, chemical composition, viscosity and flow at low temperatures. 

 If a hquid glue is to be used in cool rooms, its gel point must be relatively low. 

 The color and opacity of the liquid glue afi^ect the appearance of the joint. Clear 

 glues appear darker on wood than opaque glues, provided the opacity is not 

 caused by dark-colored pigments (i.e., blues, blacks, or browns). 



Gel Point. To determine the gel point of a liquid glue accurately it should 

 be cooled very slowly and the temperature at which flow ceases should be noted. 

 A simple rapid method, accurate within half a degree, is to cool rapidly a sample 

 of liquid glue in a test tube immersed in cold water, noting the gelling tempera- 

 ture and then melting the gel in warmer water. The average of the melting point 

 of the gel and the observed gel point is a close approximation of the true gel point. 



Viscosity. This property is easily determined in a Stormer viscosimeter if a 

 500-gram weight is substituted for the 100-gram capsule with which the standard 

 model of this machine is ordinarily equipped. Viscosity determinations are of the 

 most value when the viscosities of a given glue are determined at a series of 

 temperatures and plotted against the temperature on coordinate paper. The curve 

 gives much information as to the flow of the glue at various temperatures, the 

 concentration of the glue solution, and the presence of gel-inhibiting substances. 

 Moreover, the gel point is accurately indicated by the point at which the viscosity 

 reaches infinity. 



Moisture. The procedure for determining the percentage of moisture should 

 be uniform in all cases; heating for 5 hours at the boiling point of water will give 

 satisfactory results. 



Speed of Set. When liquid glue is first applied to a wood joint, the wood ab- 

 sorbs a certain amount of water; in a short time, usually a minute or two, the 

 liquid "sets" or gels and the joint attains some strength. This gelling is termed 

 "setting" because of absorption of moisture by the object to which the glue is 

 applied. The time elapsed between the application of the glue and the gelling 

 or preliminary drying of the liquid glue is known as the "speed of set." No accurate 

 methods have been proposed for determining this property. A rough approximation 

 of the relative speed of setting of various liquid glues may be gained by brushing 

 a thin layer on paper and noting the niunber of seconds elapsing before a skin 

 forms on the surface of the glue. Such results are of no value unless they are com- 

 pared with similar data obtained with glues of known setting properties. 



Ash Content and Composition of Ash. It is very difficult to obtain glue ash, 

 free from carbon, without volatilizing some chlorides and other easily volatilized 

 inorganic substances. For this reason, the simple ignition of a weighed sample of 

 glue does not give a true sample of ash. The samples used in the determination 

 of solid matter may also be taken for the ash determinations. They should be 

 moistened with standard calcium acetate solution and ignited according to the 

 method adopted by the Association of OflBcial Agricultural Chemists. 



Aside from indicating the total amount of inorganic materials in the glue the 

 ash content is of little value as an index of the quality. Examination of the ash 

 is of much more value to the chemist. The presence of large percentages of 

 chlorides indicates that the glue is hygroscopic. Most manufacturers of fish glues 

 consider this the best index of the hygroscopicity of a glue; therefore, a chloride 



