woolsey.]  CLAYS    OF    OHIO    VALLEY    IN    PENNSYLVANIA.  465 
of  them  renders  the  clay  valueless,  in  contradistinction  to  alumina  and 
silica,  which  are  nondetrimentals.  H.  A.  Wheeler"  says  fusibility 
depends  not  only  on  the  amount  of  detrimentals  present,  but  also 
largely  on  the  density  and  fineness  of  the  clay.  He  has  deduced  this 
formula  for  the  calculation  of  the  fusibility  of  clays: 
Fusibility  equals  p        t 
in  which  N  is  the  sum  of  the  nondetrimentals,  or  the  total  alumina, 
silica,  water,  and  titanic  acid;  D  the  sum  of  the  detrimentals;  D1  the 
sum  of  the  alkalies,  which  have  been  found  to  have  about  double  the 
fluxing  value  of  the  other  fluxes;  and  C  a  constant  depending  on  cer- 
tain limits  of  specific  gravity  and  fineness,  when  the  clays  to  be  compared 
differ  in  density  and  fineness,  it  is  necessary  to  give  C  a  value  depend- 
ent on  fineness  and  specific  gravit}^.  C  equals  1  when  the  clay  is  coarse 
grained  and  its  specific  gravity  exceeds  2.25;  C  equals  2  when  specific 
gravity  is  2  to  2.25;  C  equals  3  when  specific  gravity  is  1.75  to  2;  C 
equals  2  when  clay  is  fine  grained  and  specific  gravity  is  2.25  plus;  C 
equals  3  when  specific  gravity  is  2  to  2.25;  and  C  equals  4  when  specific 
gravity  is  1.75  to  2.25.  Not  enough  work  has  yet  been  done  to  estab- 
lish a  standard  of  fineness,  but  Wheeler  contends  that  his  is  the  only 
formula  applicable  indiscriminately  to  all  grades  of  clay. 
Refractoriness,  the  counterpart  of  fusibility,  is  also  an  essential 
property  of  clay;  for  a  clay,  whether  it  is  intended  to  be  fused  or  not, 
must  be  refractory  enough  not  to  melt  out  of  shape  or  to  combine  with 
the  alkaline  slags  of  metals  reduced  in  refractory  furnaces.6 
There  are  minor  properties  of  clay  which,  while  not  essential,  are 
commercially  of  great  importance  to  the  manufacturer,  viz,  fire  shrink- 
age, air  shrinkage,  color,  and  tensile  strength.  Fire  and  air  shrinkage 
are  both  due  to  loss  of  water  and  gases. 
Air  shrinkage  is  due  to  loss  of  mechanically  combined  water,  while 
fire  shrinkage  is  due  to  loss  of  chemically  combined  water  and  gases. 
Both  may  be  decreased  by  addition  of  sand  to  the  clay  mixture. 
The  color  of  a  clay  is  not  always  an  indication  of  its  quality.  This 
is  especially  true  when  the  color  is  due  to  carbonaceous  or  organic 
matter,  but  bright- red  clays  show  at  once  their  nonrefractory  charac- 
ter. Carbonaceous  clay  must  be  burned  slowly  in  order  that  the 
gases  generated  from  organic  matter  may  escape  before  the  clay  vitri- 
fies; otherwise  the  imprisoned  gases  will  cause  it  to  puff  up.  The 
colors  of  clays  are  generally  due  to  the  presence  of  iron,  and  the  color 
of  the  fired  product  depends  not  so  much  on  the  amount  of  iron  as  on 
the  length  of  burning. 
The  tensile  strength  of  clay  is  the  resistance  which  it  offers  to 
rupture  in  its  air-dried  condition.     It  stands  in  some  relation  to  plas- 
aEng.  and Min.  Jour.,  vol.  57,  Nos.  10,  11  (March,  1891).  p.  224,244. 
6 Hill,  R.  T.,  Mineral  Resources  U.  S.  for  1891:  U.  S.  Geological  Survey.  1893. 
Bull.  225—04 30 
