Ch. 26] PROPERTIES OF FOUNDRY SANDS 477 



The settled material in the jar is then removed, dried, and screened, 

 usually with sieves of the U. S. Bureau of Standard Series, but some- 

 times with the W. S. Tyler sieves. 



Grain-fineness number. After a sieve test is made, it is customary 

 to calculate the average fineness, which represents the size of grain 

 that would be formed if all the material were formed of grains of uni- 

 form size. The method consists in multiplying the sand percent re- 

 tained on each sieve by a certain factor. The sum of these products is 

 divided by the sum of the grains, and the quotient represents the 

 average fineness. The chief disadvantage of the grain-fineness number 

 is that two sands may differ in grain-size distribution and thus differ 

 in physical properties for foundry purposes, yet have the same fineness 

 number. A better idea of the distribution may be gained by expressing 

 the fineness graphically by means of the grading-size curve (weight- 

 accumulation curve) or the size-frequency curve (histogram). The 

 grading-size or cumulative curve is an S-shaped curve which presents 

 percentage of particles passing given sieve sizes. The size-frequency 

 curve is a peak-shaped curve similar to the probability curve. 



The advantages of the cumulative curve over the size-frequency 

 curve are: (1) it gives a smooth curve; (2) sieves that retain little 

 material can be eliminated or other sizes added without distorting the 

 curve; (3) faulty sieves are indicated by a break in the curve when 

 different samples are sieved; (4) it is more practicable for specifying 

 types of sands required because it permits the specification limits to 

 be plotted as two curves rather than as points falling between two 

 limiting curves already plotted; and (5) the data for silt- and clay- 

 size particles obtained with the hydrometer can be plotted on the same 

 graph, as a continuation of the line presenting the sieve data. 



PROPERTIES OF FOUNDRY SANDS 



Sands used for molds and cores must possess certain properties, and 

 unless these are developed to the right degree the sand is useless for 

 foundry purposes. This fact is not always realized by those not 

 familiar with the subject. The properties which the sands must possess 

 are: 



(1) Sufficient cohesiveness to hold together when moist, and this 

 necessitates the presence of some bonding material. (Clay is the com- 

 mon natural bond and is present in the naturally bonded sands. If 

 the sand has no clay bond, it must be mixed with clay or some artificial 

 binder in the proper amounts. The types of clay commonly used are 



