64 BULLETIN 1500, U. S. DEPARTMENT OF AGRICULTURE 
• 
Cover tlie blood albumin with the water and allow the mixture to stand for 
an hour or two. then stir slowly. Next, add the ammonium hydroxide with 
more stirring. Then sift in the paraformaldehyde powder while stirring the 
mixture rapidly. Paraformaldehyde should neither be poured in so rapidly as 
to form lumps nor so slowly that the mixture will thicken and coagulate before 
the required amount has been added. 
The mixture thickens cousiderably and usually reaches a consistency where 
stirring is difficult or impossible. However, the thickened mass will become 
fluid again in a short time upon standing at ordinary room temperatures and 
will return to a good working consistency in about an hour. It will remain in a 
good working condition for six or eight hours, but when the liquid finally sets 
and dries, as in a glue joint, it forms a hard, insoluble him. 
\Yhen the glue is pressed cold it has only moderate strength, and therefore 
this glue should not be used where maximum strength is required. If hot 
pressed, however, this glue is high in strength and very water resistant. 
WATER-RESISTANT ANIMAL GLUE FORMULA ^ 
Animal glue 100 
Water 225 
Oxalic acid 5.5 
Paraformaldehyde 10 
Soak the glue in the water until the granules or flakes have been softened. 
Melt the glue at about 140° F., and after that lower the temperature to between 
105° and 115°. Mix the small crystals of oxalic acid and the powdered para- 
formaldehyde together, and add the dry mixture to the glue. Stir the mixture 
until all of the oxalic acid has gone into solution, after which it is ready for 
use. The paraformaldehyde does not readily dissolve in the glue, and much of it 
remains as a finely divided solid during the working life of the glue. A certain 
amount of agitation is, therefore, necessary to keep it evenly distributed 
throughout the mixture. The paraformaldehyde used should be fine enough to 
pass through a No. 50-mesh sieve. 
The commercial paraformaldehydes for use in this formula should be of the 
slow-reacting kind. A fast-reacting paraformaldehyde appreciably shortens the 
working life of the glue mixture. 
If kept at a temperature not exceeding 115° F., and when the proper kind of 
paraformaldehyde is used, the glue will remain in a fluid condition for six to 
eight hours from the time of incorporating the paraformaldehyde and oxalic 
acid, after which it will set to a tough, firm jelly which can not be remelted. 
It is important to avoid heating the glue mixture much above 115° if a long 
working life is required. Organic decomposition of the glue will not seri- 
ously affect the quality of the glue at this temperature, since the chemicals used 
in preparation also act as preservatives. 
Both oxalic acid and paraformaldehyde are poisonous chemicals, and should 
therefore be handled with care. 
This glue combines the usual characteristics of an animal glue, and in addi- 
tion is highly water resistant. However, the water resistance varies directly 
with the temperature of the water to which the glued article is subjected. At 
room temperatures the glue is highly water resistant, but at 140° F. it is very 
low in water resistance. A conditioning period of about two weeks at ordinary 
room temperatures should be allowed for the joints to obtain their full water 
resistance when this glue is used. 
TESTING GLUES 
TESTS FOE ANIMAL GLUES 
Jelly Strength 
The jelly-strength test is based upon the assumption that the value of a glue 
is dependent upon the strength of the jelly formed upon cooling a sointion of a 
given concentration. It is a difficult test to make accurately and its use is 
30 Li\dauer. A C, and Hunt, G. M. water-resistant glue. (U. s. patent no. i506013.) 
U. S. Patent Office, Off. Gaz. 325 : 782. 1924. 
