The C-14 dating method was calibrated by comparing its results
with the results from another independent dating method
(the counting tree-rings - dendrochronology).
Quoted from Radiocarbon dating - Calibration:
To produce a curve that can be used to relate calendar years to radiocarbon years,
a sequence of securely dated samples is needed which can be tested to determine
their radiocarbon age. The study of tree rings led to the first such sequence:
individual pieces of wood show characteristic sequences of rings that vary
in thickness because of environmental factors such as the amount of rainfall
in a given year.
These factors affect all trees in an area, so examining tree-ring sequences
from old wood allows the identification of overlapping sequences.
In this way, an uninterrupted sequence of tree rings can be extended
far into the past. The first such published sequence, based on bristlecone pine tree rings,
was created by Wesley Ferguson.
Hans Suess used this data to publish the first calibration curve for radiocarbon dating in 1967.
The K-Ar dating method is based on the half-life of $^{40}K$ which is $1.248\cdot 10^9$ years.
This half-life could be determined in the laboratory by measuring two things:
- The isotope mixing ratios of natural potassium
can be determined with a mass spectrometer.
It contains $0.0117 \text{%}$ of $^{40}K$.
The other isotopes are not radioactive.
- The radioactive decay rate of a certain amount of natural potassium
can be determined with a radioactivity counter (e.g. a Geiger-Müller counter).
The decay rate is around 44 per second and per gram of potassium.
From these two measured numbers and Avogadro's constant
the half-life of $^{40}K$ could be calculated in a straightforward way.
