Teasing

After saying that, Aya let go of Silas's hand and kept the bottle back.

A purification potion was a rank 6 potion that was hard to make. Only rank 6 alchemist or above would be able to make it. Some special alchemists can make it at rank 5 but it was rare.

For Aya, it was easy as long as she had enough materials.

The problem was, the materials were also rare and there were not many of them sold. Whenever there was some, Aya would buy them.

Because the purification potion was very important.

Moreover, a purification potion could only be made by either a rank 6 alchemist or an alchemist with light magic. Because the light magic could replace some of the materials. 

Aya is a light magician.

Aside from some materials that still have to exist, she replaced the rest with her own magic. It was more efficient and cost effective.