86 GEOLOGY OF OHIO. 



strument before, were able to get the same results. Occasionally a man 

 would be found whose eye failed to distinguish the minute shades of color 

 on which the value of the instrument depends, and to such a man the 

 pyrometer would be useless. 



By this careful and extended examination of the instrument, under- 

 taken with a view to determine its fitness to be used as an instrument of 

 daily reference in clay works, many valuable facts were ascertained which 

 will be presented in connection with the latter part of the work, but the 

 general conclusion arrived at is as follows : 1st. It is the lightest, most 

 portable and simplest of all the pyrometers of moderate price now in use 

 in this country. 2nd. Its use is recommended as an instrument of con- 

 trol for superintendents, managers and foremen in charge of clay factories, 

 and not directly in charge of the burning processes. By its use, any ir- 

 regularity in the progress of a burn, any retrogression of the heat by 

 carelessness, or any undue increase of the heat by ignorance can be de- 

 tected. 3rd. A fixed temperature reads the same on the Lunette by day 

 or night, where the unassisted eye is greatly deluded by light or darkness 

 in judging the heat of a kiln. 4th. This instrument or any other pyro- 

 meter is not recommended as a means of finishing the burning of a kiln 

 of any kind of ware, or it is not recommended to supersede the trial piece 

 system now in use, for the guidance of workmen in the performance of 

 their duties. The final heat to which a kiln of clay ware is to be sub- 

 jected is a matter of very great delicacy and can only be determined by 

 the most careful and vigilant attention and skill. 



In no clay working process now in operation is it possible to com- 

 mand such extreme regularity of composition of the materials that the 

 burning processes can be brought to a termination by means of the ac- 

 complishment of any special temperature or heat. The burning of each 

 lot of material must be concluded for itself and the use of trial pieces 

 made of the same mixtures at the same time under the same conditions, 

 and of the same bulk as the articles being burnt forms the safest and 

 easiest guide which the nature of the case admits. 



Dr. Seger, a German chemist and technologist, has perfected a system 

 of tests for temperature, which, while they are ingenious, are in reality 

 but little more than a modified form of the trial piece system. The theory 

 of the process involves making a fusible base mixture of ordinary potter's 

 materials, clay, flint and spar, and perhaps the glazing materials as well. 

 To separate portions of this base he adds successive portions of silica or 

 flint each larger than the last and increasing in regular order. These sep- 

 arate compounds are each infusible in proportion to the amount of silica 

 added to the fusible base. By taking a well known mixture of predeter- 

 mined thermal qualities far a starting point and another for a finishing 

 point and dividing the intervening space into as many aliquot parts as he 

 desires, he obtains a fairly regular set of melting points. 



