Soft leather is AT 5, 6, 7 & 8 (with 7 & 8 being restricted to animals or magical armors)
AT 5 has no maneuver penalties; AT 6 requires no more than 4 ranks to negate (note that no matter how many ranks developed, you cannot eliminate the "minimum" penalties)
Rigid leather is AT 9, 10, 11 & 12 (with 11 & 12 being restricted to animals or magical armors)
Chain mail is AT 13, 14, 15 & 16
Plate Mail is AT 17, 18, 19 & 20