144 GEOLOGY OF OHIO. 



The dirt from the pan is usually dropped into a bucket elevator and 

 carried up a sufficient height to get proper head room, so that the mate- 

 rial passes to all subsequent operations by gravity alone. 



The methods of screening clay in use are three: 



1st, by fixed inclined screens; 2nd, by rotary screens; 3rd, by 

 shaking or vibrating screens. 



The inclined screens are the simplest and cheapest and the most fre- 

 quent in use. The clay is merely delivered to the surface of a shute 

 about thirty inches wide by ten to fifteen feet in length. The bottom of 

 this shute is the screen material which may be sheet metal perforated 

 with round holes, or sheet metal perforated with parallel slots, or wire 

 cloth. The first material is the best on every account and the last is the 

 worst. 



The screen shute is given whatever pitch the clay needs to run down 

 freely. The greater the pitch the more rapid the movement of the clay 

 and the finer the screenings will be through any size screen; the less the 

 pitch, the slower the movement, and the nearer to the size of the open- 

 ings will the grains be. The objections to this screen are: 



1st. It requires a high building to give it proper room for opera- 

 tion. In a small brick plant it requires the use of an additional story to 

 the machinery building. In a sewer pipe shop this point is of- no weight 

 as the building is high enough, necessarily. 



2nd. The quantity which one screen, thirty inches by fifteen feet^ 

 can run through per day is limited, and for the production of a large 

 quantity of ground clay, a large number of screens must be maintained 

 and kept in order. One screen to one dry pan is the usual allowance, 

 yet if the conditions are anything like favorable the dry pan will over- 

 work two screens of this character. 



3rd. The screen requires frequent attention or it will become coated 

 with fine clay until it fails to do its work. One screen will require occa- 

 sional attention in good weather and frequent attention in bad weather. 

 Two screens take the time of a boy constantly to make them do their 

 maximum. 



The points in favor of this screen are', 1st, simplicity and cheapness; 

 2nd, no power required to operate it. 



The rotary screens are finding some considerable use in the state at 

 present. There is hardly a sewer pipe plant in the state that has not 

 , tried them at some time or other. They are finding more extensive use 

 in brick works than elsewhere. 



There are a number of different kinds of rotary screens. The fol- 

 lowing types have been seen and the workings observed: 



1. A cylinder or truncated cone made of perforated metal with its 

 frame work outside of the screen, so that material introduced at the 

 higher end works gradually down to the lower end and lies in the bottom 

 of the cylinder all the time it is in the screen. 



