32 



Hardwood Record — Veneer & Panel Section 



June 10. 1918 



Checking Up on Glue 



Lack of Standard Grades Makes Tests Difficult 



ipglJJll LUE users are showing an increasing desire to 

 l^^gH inform themselves as to the quaUty of the glue 

 ||'t/g|j^fc| they use. The following letter from a southern 

 sash and door mill indicates the attitude of many wood- 

 workers toward the question of glue specifications: 



The article "No Economy in Cheap Glue" in your issue of April 

 1 prompts us to check up. 



We would, therefore, consider it a great favor if you wo-ild 

 direct us to where we may obtain a glue specification for gen- 

 eral millwork and how to determine that we are getting what we 

 ask for. 



Unfortunately, our town does not boast of a commercial lab- 



essary formulas and apparatus. 



Trusting that we are not asking too much and thanking you 

 in advance for any advice you may offer. 



It is practically impossible to obtain a glue specifica- 

 tion for any industry in the woodworking field, because 

 there is no uniformity of grading standards among glue 

 manufacturers. TTiat is a plain statement of fact, with- 

 out any discussion as to the reasons for the lai-,k of 

 absolute standards throughout the glue making industry. 

 The only way to determine the best glue is to try it out 

 in actual practice. 



High test hide glues are best for what are termed 

 wood joints, because they set rapidly and make strong, 

 firm joints. A moderate test mixture of bone and hide 

 glue will do very well for veneered work. The higher 

 test glues are apt to set too quickly for panel making 

 in presses. If a spreading machine is used glue that 

 shows a tendency to foam should be avoided. For 

 veneered work there is also the very popular vegetable 

 glue, which usually runs in one grade, but special equip- 

 ment is required for its proper application. 



Between two or more glues of any one type actual 

 experiment is the only safe guide. Most glue salesmen 

 will give good advice, but the user should keep accurate 

 records of results, when possible, and once the right 

 glue is found there should be no changing at the whim 

 of some operator in the shop, or the claim of some com- 

 petitive salesman who has something "just as good" for 

 less money. 



In a general way it is known that good glue will go 

 farther and produce better results than poor glue, and 

 what glue-testing ought to tell is the exact amount of 

 difference between the good and the poor, giving figures 

 showing for each how much work may be done and to 

 what extent the work will differ in quality. When worked 

 out in a suitable way the test figures should represent 

 for the glue so treated the equivalent of that glue in 

 comparison with any and all other glues; first for vis- 

 cosity, as the basis of water taking or spreading capacity, 

 and next for jelly strength as the indication of binding 

 power. 



Viscosity tests or jelly tests, as properly made, require 

 a degree of experience, and an equipment hardly prac- 

 tical for use by the ordinary buyer and user of glue. 

 Certain tests, however, may be made that are of great 

 value in determining important facts about the glue it 

 is intended to buy and use. These tests may not be 

 used as a basis for the scientific grading of glue, but 

 they are of much value in deciding its purity and its 

 adaptability to the work at hand. 



In sampling animal glues it is well to take samples 

 from various parts of the barrel. Flake glue frequently 

 is made up of different varieties, and a single sample 

 may not be representative. Ground glue, in its very 

 nature, permits of easy adulteration. It should be ex- 

 amined in a good light for evidence of foreign substances. 

 Flake glue should be examined carefully for uniformity 

 of odor and general appearance. If bubbles show like 

 round blots on the surface of the glue, they usually indi- 

 cate decay during the manufacturing process. If this 

 glue is moistened and gives off a sour odor, there is addi- 

 tional proof of putrefaction. Such glue should be 

 avoided. 



Bubbles may appear within the glue flake (not on 

 the surface) without indicating decay. In fact, bubbles 

 are usually found in certain high grade glues, though 

 hardly ever in low grade bone or hide glues. Besides 

 being free from surface bubbles or blots, good glue is 

 smooth, though not necessarily glossy. Often very good 

 glue is dull or cloudy, and even inferior glues may have 

 a shiny surface. The surface should be uniform in coloj 

 and appearance. 



Generally speaking, color indications are not impor- 

 tant, though if the color of any particular lot of glue is 

 not nearly uniform, one may be suspicious of adultera- 

 tion. Bone glues are apt to be darker than hide glues, 

 but through an artificial clarifying process they may be 

 given the appearance of high grade glues. Frequently 

 oxide of zinc is added to glue. TTiis gives it a light 

 color and also makes a quick setting adhesive. It is not 

 harmful unless added in large quantities. In fact, for 

 certain work it is beneficial, and some panels are glued 

 with a mixture containing sufficient zinc oxide to make 

 the adhesive milk white. 



More or less is heard about acid or alkali in glue. The 

 best glues are neutral in this respect. To test for these 

 chemicals, dissolve a small quantity of glue in water and 

 dip a piece of litmus paper into the solution. Acid will 

 turn the paper red or violet, while alkali will turn it 

 blue. Litmus paper may be obtained at almost any 

 drug store. It is well to avoid glues with an excess of 

 acid, especially on oak or other woods with strong acid 

 qualities. The acids in the wood may unite with those 

 in the glue in such a way as to have a destructive etfect 



