6 
simplification, with substantial benefits to both the producer and the 
user of animal glues. The specification for glues for Government use 
are now based upon these new methods of test. 
LIQUID GLUES 
Of the large class of glues sold in liquid form a high percentage are 
made from the heads, bones, skins, trimmings, and swimming blad- 
ders of fish, and the whole class is often referred to as fish glues. 
Some liquid glues, however, are made by treating animal glue with a 
chemical agent, usually an acid, and some are made from other raw 
materials. The relative merits of the glues made from these different 
materials have not been thoroughly studied. 
Most liquid glues are similar in properties to animal glues. Only 
the best liquid glues, however, are reliable enough and make strong 
enough joints for use in wood products. The ability to make strong 
joints is generally indicated by the viscosity or ** body " of the glue, 
thick glues giving high strength and thin glues low strength. A 
desirable property in a liquid glue is that of remaining in workable 
condition as long at it is in the container, but drying and setting 
quickly on wood surfaces. Other desirable properties of liquid glues 
and the ways in which they are determined are discussed in the 
Appendix. 
Liquid glues are commonly sold in small containers under various 
trade names, without other indication of grade or quality. Their 
chief advantage is that they come in prepared form, ready for im- 
mediate use. which makes them particularly suitable for patchwork 
and small gluing jobs. 
CASEIX AND VEGETABLE-PROTEIN GLUES 
CASEIX GLUES 
Casein glues have only recently been used in the woodworking 
industry in the United States, although they have been known here 
since about 1873 4 and have been used in Europe in bookbinding and 
cabinetwork for a much longer time. The World War greatly 
stimulated the use of these glues in America. "When airplanes were 
first manufactured in large numbers, requiring water-resistant mem- 
bers of plywood and laminated construction, a wide and important 
field was opened for casein glue, and its use has since expanded in 
other industries. 
Casein glues are made by mixing water and certain chemicals 
with casein* which is made from milk. They are often referred to 
as glue cements because when allowed to set they become entirely 
different in properties from the original mixture and most of them 
can not be redissolved by water. 
The main advantage of casein glues is their high water resistance 
or ability to retain strength when wet. Casein glues are mixed 
and used cold. The better casein glues produce joints which are 
adequate in strength for most woodworking purposes. At present, 
* Probably the first specific description of a casein glue in the American literature occurs 
in the following: Ross. J. H., and Ross, C. I>. frocessks OF preparing GLDB. (U. s. 
patent -NO. 18-6V24.) U. S. Patent Office, Off. Gaz., 10:508. 1S7G. (Filed August 11, 
1873.) 
