"Keeping this up for days might be challenging fate," he answered.
Samuel stared at the pieces of copper shifting and changing shape in the air, then turning solid. At the same time, his body was listening to what his eyes could not see. He could feel the vibration and consistency of her magic projected into this space, could trace its cold lines and spires. They really felt sharp. He felt as though they might cut into him, if he were to press down on them.
"You are doing very well. Can you change its temperature?"

The flow of energy out of her left and right side was in balance. No disturbance, no wavering in concentration threatened its stability.
Yet he must not permit his vigilance to lessen; the circle he had engraved into the table was not meant for beginners; it was complex and large in diameter to allow for powerful transmutations. It also required a high volume of energy to upkeep.
He felt her lean in and increase the amount of power she was supplying. Samuel felt himself tense up. Here lay dangers. He closed his eyes and took measure of the invisible structure of magic between them, ascertaining its destructive potential.
They had already entered into the territory, he decided, where a sudden recoil of all the energy that was at work could harm them.

Samuel had the suspicion that, much like for himself, for Themis it was not keeping balance and focus that would present a challenge, but withstanding the strain transmutation put on the body. The longer they kept at this, the more grew the likelihood that she would experience stutters and sudden short drops in the supply of her magic, as her system struggled to keep up with the demand. His own body had adapted over the decades, but he had to remember that she was new to this.
Nonetheless, he did not want to caution her prematurely and take her out of her experience.
Samuel decided then that he would trust himself. When she found her limit, he would react quickly enough to prevent any harm. Maybe it was better if she felt it viscerally, that there were limits. Otherwise it would be so exhilarating, so easy to believe that there were no bounds to what could be accomplished. He knew that.