Thermocouple tip contamination with other metals has no effect

There is the claim that contaminating the tip of a thermocouple with other metals will lead to a wrong reading. It is for example claimed that solder touching the tip will not allow you to read an accurate solder iron temperature. 

This is not the case, other metals at the thermocouple tip do not impact the measurement. The reason for this is that the Seebeck effect is an electrical potential that is occurring along a temperature gradient in a wire. That means between a hot end and a cold end of a wire. You need two different material with different gradients to be able to measure this gradient in form of a very small voltage signal. The metal at the tip does not impact this, because it is all at the same temperature, there is no temperature gradient and therefore no error.
The only purpose of the weld at the tip of a thermocouple is to make a durable electrical connection between the two wires. The connection can also be made by simply twisting the wires. In other cases the two wires are welded to a metal object, which then makes the electrical connection.

I will demonstrate this with a little experiment.

First I took a 2 ft thermocouple with Teflon PFA insulation, a Fluke calibration block set to 250 C and a NIST traceable K-type thermometer.

In the first step took the simple thermocouple, inserted it in the calibration block, gave it about 5 min to stabilize and read the temperature. I am showing the thermocouple tip and the measurement results. The reading of 251.1 degrees is right within the "special limits of error" or the "select grade" for Type K thermocouples, which is +/-1.1 degree C (standard error is +/- 2.2 C).

In a second step I wrapped the tip with copper wire and repeated the measurement

In step 3 I covered the tip with some leaded solder and repeated the measurement

As you can see, I got Identical readings in all 3 measurements, the differences in the decimal point are negligible fluctuations in the temperature and the reading.