The thick Pliocene clay beds located along the mouth of the Guadalhorce River in Malaga were used in the Roman era for amphorae production related to the ever-growing production of garum and salted fish. As a first step in their archaeometric characterization, a series of firing tests on the potential raw material have been carried out. The main aim of this work is to obtain a suitable tool for estimating the firing temperatures of those amphorae, as well as to make a comprehensive evaluation of the potential of an innovative external-standard method for measuring the amorphous content of a pottery sample, the G-factor method. The obtained results emphasize a highly amorphous content, over 50 wt% in some samples, which varies with the firing temperature. This shows the potential of this technique, which allows straightforward fully quantitative mineralogical analysis and a mineralogical comparison even between samples fired at very different temperatures.