^ Facts illustrative of the 



crude iron. Soft cast iron very hot will shrink and contract 

 less than iron equally hot but of a harder quality, or, which 

 is the same thing, than iron containing less carbon. 



In casting cylinders, pipes, and other hollow machinery, 

 the effects of expansion and contraction are manifested with- 

 out any great degree of shrinkage appearing. 



The diameter of the mould in all these castings is gene- 

 rally made fi-oni l-8th to 3-l6ths per foot in diameter larger 

 than the casting is wished to be ; while the space or vacuity 

 left betwixt the exterior and interior of the mould, called 

 the thickness, is made less than the. strength in metal is 

 wished to be. 



When the cylinder is cold, however, the diameter, if pro- 

 perly allowed, will be found correct by the operation of con- 

 traction in coolinsr; while the thickness in metal will be 

 found increased, Qiough still correct, by an expansion or 

 separation of the exterior and interior parts of the mould. 

 This last is bv the moulder called strahiing ; and if great 

 care is not taken to compress the sand firmly round about 

 the mould, the thickness is sometimes increased so much 

 as to render the manufacture unsaleable. Should this ne- 

 cessary precaution be slightly performed, and the thickness 

 considerably increased, the usual expansion which takes 

 place W'hen the metal passes into the solid state becomes so 

 extensive as to effect a permanent increase of the diameter 

 of the casting, and destroy its use*. The united effects of 

 these two causes force the sand to assume an elevated pos- 

 ture all round the mould, and occasion violent rents and 

 fissures, which become immediately filled with pale blue 

 fiame, accompanied by a crackling noise like the snapping 

 of electric matter. 



Shrinkage in tliese castings, particularly if large, would 

 flfFect the solidity of the vessel by taking place to a consi- 

 derable extent upon the upper surface, immediately where the 

 runner discharges the metal into the mould. This is in a 

 great measine counteracted by feeding these gates or runners ^ 

 after the mould is filled, walh several ladles full of fiuid iron, 

 and keeping the communication open to the edge of the 



* The additional thickness always takes place to the exterior of the 

 tiiould. The pressure can more easily act with effect against the concave 

 than the convex lide of the mould. The moulder is fully aware of this 

 jn the act .if cooling;, particularly if the metal has been very hot and of a 

 •iharp quality. After he conceives the iron fairly consolidated through- 

 out, he cuts iv.o openings at least in the core or interior pirt of the mould, 

 and penetrates to I he red-liot iurfaee. This gives scoue to the contrac- 

 tion of the ves-fl, and preicrvfs the casting frequently from being de- 

 tiroyed. 



casting 



