It does not sound correct from your description of the calibration. Generally, a linearization would encompass bottom of range, top of range, and some points in between to prove linearity. Saying that a device is linear while only checking the lower 25% of range does not sound kosher BUT...
Before you accuse them of improper calibration, there are other factors to take into account are:
a) your calibration provider may be following the controller manufacturer's procedures;
b) your calibration provider may be using a calibrator to check the controller without the need to warm the oven at all, and only warms the oven to verify their findings and check the thermocouple/thermister/PRT/whatever.
c) The controller may be a multi-range controller, and they performed linearization in a different range band, then did a bias check in the range that you actually use.
I would contact the manufacturer of the controller or oven and ask them for their recommended calibration procedure and compare their suggested calibration points and method against what your provider is giving you. At a bare minimum, I would tag along and ask questions when they are in the next time. It annoys the technician (trust me, it does!), but it is well within your rights as a concerned and responsible customer.
Another thing that really helps the calibration provider is complete documentation of what you require. Customer requirements are the most important thing on the P.O. to a cal lab (other than the Bill To address), but the standard paperwork that I saw said "Calibrate IAW ISO/QS9000", which really just means that your calibration must be traceable and according to whatever procedures that the provider has documented as satisfactory. Put your specific requirements on the P.O.(ex: Calibrate oven at 600ºC ±10°C and perform linearity calibration IAW...), in bold print with flashing LED's. If your 'contract' requests calibration at 600°C, and they don't do it, they are in the wrong, and you have documentation to back it up. Then you can request a corrective action and that is NEVER a good thing to receive from a customer!
[This message has been edited by Ryan Wilde (edited 30 May 2001).]