J-type temp range is 32 <-> 1380 deg F 0 <-> 750 deg C.
K-type temp range is -300 <-> 2280 deg F -200 <-> 1250 deg C.
The temp error is the same for both (2.2 deg C or 0.75% whichever is greater), so the K-type is much more widely used and easier to find.
Problems with J type is iron is one of the metals so is prone to corrrosion in wet environments. Is less widely used and on the way out, as it were. Also, if embedded inside the brick it is not a concern with our use, but if you choose to leave the tip exposed passed the brick exceeding 1390 deg will induce permanent errors in the output. IE; no longer accurate.
For those who want more education, this is a good link.
Thermocouple application note from Pico Technology