"Your glass's stress comes from heating and cooling," Im said, standing beside the high-temperature furnace. "First, make a standard glass sheet. We'll map stress levels at different temperatures, then anneal it at the critical point."
Leon nodded, gathering sand, soda ash, and limestone. Then he paused. "Master, how do I measure the temperature? There's no ruler for heat like there is for length."
Im blinked, realizing he'd overlooked the basics. Leon's maturity had tricked him into assuming the boy knew such fundamentals. "I forgot. I'll teach you a temperature-sensing magic first. It's more of a mental skill—amplify temperature's effect on your mana, then quantify it."
"Can I cast it while maintaining Mage Hand?" Leon asked, uncertain. His past attempts at dual-casting had failed—he could only control his tentacles for one task at a time, like a novice gamer struggling to manage multiple units.
"It's simple once you master it," Im said, confident. "Mages are chosen for a reason—don't underestimate yourself."
Leon felt a twinge of pressure. Mages were rare, talented individuals—his only edge was Earth's knowledge and years of formal schooling. "Don't get complacent. You're not the protagonist," he muttered to himself.
The magic, called Temperature Sensing, wasn't a true spell but a mana application. Mana was sensitive to heat, but the change was too faint to detect—until amplified and quantified. Leon learned it by sunset, channeling a thread of mana through a spiral rune pattern in his mind. When he focused on a nearby clay cup, he "knew" its temperature instantly—no visual gauge, just a clear numerical sense, calibrated to the freezing and boiling points of water, like Celsius.
Memories flooded back: his older brother's mercury thermometer during a fever outbreak. Leon had failed to read it, thinking it was broken, and pressed it against a hot corn cob—only for it to explode, reading 42 degrees. He smiled at the memory, testing the magic on everything around him—the furnace, the table, even a spider crawling across the floor.
The next morning, they began the stress test. Leon melted the ingredients into glowing liquid glass, shaping it into a thin sheet with his Mage Hand. Im activated his Stress Sensing spell, watching a faint blue glow emanate from the glass. "Record the stress at every 10-degree drop," Im said.
Leon's tentacles held the glass steady while he maintained Temperature Sensing. The stress wasn't linear—spiking as the glass first solidified, peaking at 550 degrees, then gradually fading as it cooled further. They repeated the test twice to confirm the data.
"Here's the stress curve," Im said, handing Leon a parchment. "You don't need magic to anneal—just hold the glass at 550 degrees for half an hour to equalize internal temperature, then cool it 10 degrees every 15 minutes. That'll remove most stress."
"Why no magic?" Leon asked.
"Magic speeds it up, but slow cooling works if you're patient," Im explained. "Unfortunately, I can't build a dedicated annealing furnace right now—I'm swamped with the purification circle. We'll use the high-temperature furnace for now, even if it means waiting longer."
Leon nodded, understanding. Mages prioritized their own research, and Im's Purification Circle was far more critical than his glass experiments. Still, he was eager to create usable glass—starting with small crafts to practice.