TECHNICAL NOTE number 211 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY madison 5. Wisconsin revised December 1952 STRONG AND WEAK GLUE JOINTS The accompanying photographs of wood blocks glued with a good grade of animal glue and then sheared apart in tests at the Forest Products Laboratory show what may happen with the same glue rightly and wrongly used. The blocks that sheared apart at the glue line indicate poor glu- ing. Those in which failure occurred in the wood were good joints, as strong as the wood itself. The joints shown in the left-hand column were weak, since the failure was all in the glue. In joint A the glue was allowed to get too cold before pressure was applied. In joint B the glue was too thin and was squeezed out of the joint. In joint C the glue had dried before pressure was ap- plied. These three joints represent three of the most common errors in gluing practice. They are known as the chilled joint, the starved joint, and the dried joint. The joints shown in the rows to the right of A, B, and C are strong joints, each made by changing only one of the three conditions -- pressure, assembly time, or temperature. These are the three most importart variables in the gluing operation when animal glue is used. Thus a good joint can be made from chilled glue by increasing the pres- sure, or the glue may be kept from getting chilled and a good joint ob- tained if either the assembly time is decreased or the room temperature increased. (See the top row of photographs, left to right. ) If the glue is thin, starved joints may be avoided if the consistency of the thin glue is increased by increasing the assembly time, by decreas- ing the room temperature, or by mixing less water with the glue. De- creasing the gluing pressure may alBo improve the jomte, although such practice is not always recommended. ~ (See- -the middle row of photo- graphs. ) THREE COMMON TYPES OF WEAK ANIMAL GLUE JOINTS AND HOW THEY MAY BE AVOIDED POOR JOINTS THREE TYPES MADE UN- DER THREE DIFFERENT SETS OF CONDITIONS. GOOD JOINTS NDER SAME CONDITIONS AS POOR JOINTS TO LEFT EXCEPT VARIATION IN EACH CASE OF ONE OF THE THREE FACTORS ASSEMBLY TIME TEMPERATURE H-CHILLED JOINT Pressure 400 pound rfedi urn glue sprea Pressure of 150 pounds aquare inch. Al 1 othe ditions same as for / jer square Inch fter 12 minutes ssenibly. Room a applied closed t 75° F. ft it MUKflssiwa** B-STARVED JOINT Medium glue spread. Pressure of 150 pounds per square inch applied after 1 minute closed assembly . Room at 90° F. No glue remaining in joint. No wood failure. Pressure 25 pounds square inch. Allotherc ditions same as for Better joint than B, I this pressure not rec mended. C-DRIED JOINT Medium glue spread. Pressure of 150 pounds per square inch applied after 25 minutes closed assembly Wood heated 20 minutes at 120° F. Room at 80° F. Note shiny areae of dried J No amount of preseure will produce a good joint with other conditions same as for C. UNIVERSITY OF FLORIDA III III "3 "1262 09216 7427 No amount of pressure will produce a good joint irorn ariea giue, Dut by decreasing either the assembly time or the temperature to which the wood is subjected, a good joint can be made before the glue has dried out. (See the bottom row of photographs. ) Assembly time, room temperature, wood temperature, grade of glue, and glue -water ratio are chief among the factors affecting the consist- ency of an animal glue at the moment pressure is applied. Controlling the temperature of the wood and the assembly time is often the most practical method of insuring good glue joints. ZM31873-F Agr leu Iture-Mad Isor