Through Marcel Mathieu, Franklin gained a position in the Laboratoire Central des Services Chimiques de l'État, in Paris, working under Mering, starting in February of 1947. She would work in Paris for four years. Glynn reports that Franklin's time in Paris was the happiest of her life.

​Franklin befriended many of the other young researchers working under Mering. One of these friends was Vittorio Luzzati. Franklin and Luzzati shared a love of adventurous trips and of energetic debates. In fact, Luzzati and his wife Denise invited Franklin joined them on plenty of these vacations.

The director of research at BCURA (the ​British Coal Utilisation Research Association) was Dr. Donald Bangham, whom Franklin coauthored a journal article with ("Thermal Expansion of Coals and Carbonised Coals," 1946, shown right).

Franklin and Bangham were researching the thermal expansion of solid coal and charcoal. Thermal expansion is how substances change their shape and size with temperature, usually not including phase changes (eg. changes between water, liquid, and gas). Usually we look at thermal expansion in gasses (such as when a sealed balloon shrinks in the cold and gets bigger in the warmth), however, Franklin and Bangham were looking at the thermal expansion of solids. (While we don't think about it as much we still do see this in our everyday lives. A common example of this is pavement contracting in the winter and resulting in potholes in the spring.)

In this Franklin was studying the properties of pores of coals with varying carbon contents using measurements of true and apparent densities. True density is the actual density of a material. The apparent density, on the other hand, is the density measured using the volume of a liquid or gas displaced by the material. This works because the fluid should fill the holes in the sample of the material so it doesn't matter the material's shape. However, if the pores of the material are small enough, as they were with the coal samples in Franklin's experiment, the molecules of the fluid might not physically fit into the pores. Smaller molecules will fit into smaller pores while bigger ones will not, resulting in different apparent densities when measuring with different size molecules. The pore size not only affects the apparent densities when using different molecules, but also how fast they can seep into the pores. Franklin measured the apparent density of a coal in a particular fluid over time, not just at first, which allows molecules time to seep into the pores. This change in apparent density is called the drift.

In this article, she details her experiments studying cokes (carbonized coals) using apparent density measurements, again using a variety of liquids and gasses. She compared this apparent density data for varying types of cokes carbonized in a range of carbonization temperatures. 

This molecular sieve property which Franklin observed in these experiments, is of great importance today, as I mentioned previously. In part II, Franklin writes on several occasions that this property must be very important. In her abstract she wrote "This molecular sieve structure must be of importance in determining both the chemical behavior of the solid and the composition of the gas evolved during carbonization." In addition, in the last part of her results section, "Molecular Sieve Properties of Carbonized Coals," which specifically focuses on this property, she again writes how she believes the molecular sieve properties are of great importance.