When a starch suspension is overheated, there seems to be some danger 
of an undesirable chemical change in the nature of the paste. This change 
has been called caronelization, and it is characterized by a reddish-brown 
coloration in the glue. No attempt will be made to explain the exact nature 
of this change; the result is said to be a reduction in the adhesive ^ower 
of the glue. In the preparation of a starch adhesive, this caramelization 
is guarded against by: (l) Adding the caustic soda to the starch suspension 
before applying heat, (2) controlling the temperature of the water bath 
during the mixing operation, and (3) stopping the conversion when the adhe- 
sive has reached the transparent, amber- colored, stringy stage. 
Application of Vegetable Glue 
Vegetable glue is very viscous and difficult to spread by hand, but 
on a mechanical spreader it works very well. The mixtures commonly used dry 
slowly, and their working life is very long. In plant operations, the glue 
remaining from a day's run may be left in the spreader, the spreader be 
covered with a damp cloth at night, and fresh glue be added in the morning. 
Joints have been made in the laboratory with glue that has stood for k2 days 
after mixing, and the resulting strength has been entirely satisfactory. 
This slow rate of change in viscosity enables the manufacturer to use a long 
assembly time in gluing, which is particularly desirable in the production 
of plywood. However, this same characteristic of slow setting is sometimes 
a disadvantage in joint work where the stock is often under pressure for 
only a short time. 
Tests based on the more common types of vegetable glues have shown 
that the strength of joints increases slowly during the first 2 hours after 
pressure is applied. During the period of 2 to 6 hours after applying the 
pressure, the rate of increase is somewhat more rapid, and after about 8 
hours it becomes more gradual again. Joints of animal glues, casein glues, 
and some of the newer vegetable glues will increase in strength more rapidly. 
In thick stock, the joint will be strong enough for machining after about 
7 hours, but 2 hours in the press is sufficient, provided that the joint is 
allowed to condition further for a day before machining. In veneer work, 
the best results may be expected if the joints are left under pressure over- 
night and are then removed and allowed to dry or to condition thoroughly. 
Although most of the vegetable glues are slow in setting and all that 
have come to our attention are low in moisture resistance, attempts are being 
made constantly by the producers to improve their glues in these respects. 
Manufacturers have prepared starch glues that show a more rapid increase in 
joint strength than do the ordinary starch-caustic mixtures. The attempts to 
produce a water-resistant starch glue have not, so far as is known, met with 
success. 
Experiments conducted at the Forest Products Laboratory with a good 
grade of glue have shown that a spread of about k^> square feet of single glue 
line per pound of dry glue can be depended u^on to produce very satisfactory 
Report No. R30 -5- 
