When the conversion depends on the action of caustic soda alone, about 
7 percent caustic is customary, although variations of 6 percent to 10 per- 
cent are not uncommon. 
A temperature of about 150° F. will convert the starch without the 
addition of caustic soda. 
The amount of water used depends upon the variety and previous treat- 
ment of the starch. Processed starches or those containing added chemicals 
usually require a starch-water ratio of 1:1.2 to 1:2.5. If the raw starch 
is used, the proportion of water may be somewhat higher (such as 1:3.5). The 
use of a starch-water ratio of 1:14 to 1:20 is reported for the preparation 
of a vegetable-glue size. 
The caustic soda in the mixture produces the stringiness that is desir- 
able in this class of glues. The glue without caustic or other chemicals 
resembles paste, but good joint strengths can be obtained with it. The higher 
percentages of caustic cause the glue to set somewhat more slowly, and this 
allows the use of longer assembly times but requires a somewhat longer time 
in the clamps. Furthermore, the higher is the percentage of caustic, the 
greater is the danger of staining such woods as oak, maple, cherry, elm, ash, 
birch, and beech. For these reasons a compromise is usually made, under 
which the first method is most generally used. 
The equipment commonly used for the preparation of vegetable glue con- 
sists of a mechanical mixer surrounded by a steam-heated water jacket. It is 
possible, but not at all convenient, to prepare small batches of these glues 
over a water bath with hand stirring. The general procedure for conversion 
of starch into an adhesive is as follows: 
The required amount of water is poured into the mixer, the starch is 
added, and the mass is stirred until any lumps have been broken. The stirring 
is continued while the caustic soda (previously dissolved in about three times 
its weight of water) is slowly added. Steam is now turned into the water 
jacket, and the temperature is increased gradually (the stirring being con- 
tinuous) until the water in the jacket is somewhere between 190° and 200° F. 
(This is, of course, considerably in excess of the temperature required to 
convert the starch, but a considerable temperature gradient will exist 
between the water jacket and the center of the inner container, so that the 
average temperature of the starch suspension is not greatly in excess of 
150° F.) About one-half hour is required to bring the temperature of the 
water jacket from room temperature to 200° F. At the end of this time it will 
be noticed that the starch has changed in appearance from a white suspension 
to an amber-colored stringy paste. If picked up on a paddle and held to the 
light, it will appear almost transparent. When it reaches this stage, the 
conversion is stopped by turning off the steam and by replacing the hot water 
in the water jacket with cold. The stirring of the glue is continued until 
the temperature has been reduced to about that of the room. The glue is now 
ready for use. Details of the general procedure given above may be varied 
under the instructions of the glue manufacturer according to the chemical 
additions and to the grade or previous treatment of his starch. 
Report No. RJ0 -k- 
