Two days earlier, Qin Fang had heard Doctor Li mention that saltpeter, while capable of making ice, was also a medicinal ingredient that could alter the properties of other medicines.

Saltpeter was also used to make firecrackers, so Qin Fang remained cautious about it.

Regardless of who it was, people tended to be very careful with medicines, even if they weren’t directly ingesting them.

After discovering that saltpeter could make ice, and as he and Yang Haiyan planned to share this method with the court, Qin Fang naturally took a more serious approach.

Qin Fang said, “Saltpeter can indeed make ice, but we don’t yet know if the ice it produces is safe to use. Sulfur, as Doctor Li mentioned before, is toxic even in its fumes, so too much exposure is harmful. We need to exercise caution with saltpeter. I’ll go to the Longevity Temple on Beast Mountain to consult Doctor Li.”

Hearing this, Yang Haiyan felt a warm sense of relief, grateful that Qin Fang had already considered this issue.

She remembered from reading novels and doing her own online research that, although saltpeter is only mildly toxic, excessive exposure is harmful.

It’s why, in modern times, saltpeter-based ice is always made with a water barrier.

However, in her current role as a village woman who had just stumbled upon this use for saltpeter while washing her hands, she wouldn’t logically know to avoid direct contact with the saltpeter.

Originally, Yang Haiyan had planned to discreetly remind Qin Fang to ask Doctor Li if the ice made from saltpeter was safe to consume.

As a physician, Doctor Li would certainly know about saltpeter’s toxicity and would likely advise against using the ice.

From there, they would naturally explore ways to avoid contamination, leading to the idea of using a water barrier.

However, she hadn’t expected that Qin Fang would think of it himself.

Yang Haiyan said, “Then go ahead. It’s wonderful that you noticed these things—I wouldn’t have thought of them.”

Qin Fang was clearly pleased with the compliment.

He broke into a smile, his teeth showing, but he quickly composed himself. “Then I’ll be off.”

It was already dinnertime, but Qin Fang left without eating and headed straight for Beast Mountain.

By the time he arrived, night had fallen, and the temple was brightly lit, visible from afar.

As soon as he entered the temple, Qin Fang sought out Doctor Li. “Doctor Li, I have something I’d like to consult you about. May we speak privately?”

Seeing Qin Fang’s serious expression, Doctor Li followed him to a secluded area. “I’m honored. What would you like to ask, General? Is it something about medicine?”

“It is, indeed,” Qin Fang replied, “and it concerns saltpeter. Today, my wife had an elder prepare firecrackers using sulfur and saltpeter to celebrate the New Year. In the process, she happened to discover another use for saltpeter—it can make ice.”

Doctor Li was astonished. “What?”

He was so surprised that he raised his voice.

Ice is a luxury not only for commoners but even for the wealthy, who often have limits on their ice supply.

It’s not a matter of wealth; it’s simply scarce.

If saltpeter could be used to make ice, the price of ice might plummet.

Qin Fang continued, “This is no trivial matter. I plan to report it to the court, so I ask that you keep this confidential. If word gets out before the court makes it public, it could cause issues.”

Doctor Li nodded, “Rest assured, General. I understand. You’re asking if the ice made from saltpeter is safe to use, correct?”

“That’s right,” Qin Fang replied. “You mentioned before that saltpeter is used in medicine, so I’m unsure if ice made from it would be safe.”

Doctor Li asked, “General, could you explain how saltpeter is used to make ice?”

This wasn’t a difficult question, so Qin Fang answered truthfully, “You just put the saltpeter into water, and after about half a day, the water will freeze. But the amount of saltpeter must be sufficient, and the amount of water should be controlled. Saltpeter lowers the temperature of the water, and over time, the water gets colder and eventually freezes.”

That was basically how it worked, although Qin Fang didn’t fully understand the details.

Doctor Li immediately said, “If that’s the case, then the saltpeter is mixed into the ice. Since saltpeter is toxic and it directly contacts the water and turns into ice, it must carry some of the toxicity. This ice cannot be used. To freeze so much water, you’d need a lot of saltpeter. A small amount of saltpeter might have a negligible effect, but the more saltpeter, the more poison it carries. So, ice made this way definitely isn’t safe to use.”

Qin Fang thought for a moment and said, “So, you mean that as long as the ice doesn’t directly touch the saltpeter and the saltpeter’s toxicity doesn’t mix with the ice, the ice should be safe to use?”

Doctor Li pondered this and agreed, “That seems reasonable, but we’d need to verify it first.”

Qin Fang said, “That’s simple. Come back to my residence with me; I have a way to test it.”

Doctor Li agreed, “Yes, of course.”

The two returned to the general’s residence.

By now, it was already evening. As soon as Qin Fang arrived, he asked for a large basin and a bowl.

He filled both with water, placed the bowl inside the basin, and then added saltpeter into the basin.

Yang Haiyan heard about this and couldn’t help but ask, “Husband, what are you doing?”

Qin Fang explained, “Doctor Li said saltpeter is toxic, so if we directly use it to make ice, the ice likely carries some of the toxicity and can’t be used. So I thought, what if we separate the saltpeter from the water? If the water still freezes into ice without directly touching the saltpeter, would the ice then be safe?”

Yang Haiyan was stunned, looking at Qin Fang with amazement.

She had always felt a certain sense of superiority due to her knowledge from modern times and having read books about the past.

For example, when she first arrived at the border, she knew how to interact with certain people, like selling soap recipes to the estate of a noblewoman.

But in this moment, she was truly awestruck by the wisdom of the ancients.

No wonder saltpeter-based ice-making was discovered in ancient times.

Modern achievements are built on the foundation laid by ancient people, while the accomplishments of the ancients came from their own experimentation, without any existing references.

For example, ancient people started their education early, while at the same age, modern children are often still pampered.

Yang Haiyan herself had been coddled by her great-grandmother who raised her.

Now, she finally understood that she had always underestimated the wisdom of ancient people.

This realization made her feel uncomfortable, as it was the first time she acknowledged that her attitude and thoughts had been somewhat wrong.

Seeing Yang Haiyan staring at him, Qin Fang became a little nervous and asked cautiously, “Yan Yan, are you okay?”

Yang Haiyan walked up to him, gazed at him quietly for a while, then suddenly hugged him, saying, “Husband, you’re so smart.”

Qin Fang was bewildered.

What was going on?

Though he was happy to be praised and hugged by his wife, he felt a bit uneasy. “Yan… Yan Yan, don’t scare me.”

Yang Haiyan looked at his anxious expression and couldn’t help but laugh.

With her big eyes fixed on his face, she held his hands and said seriously, “Husband, I truly think you’re brilliant. Look, Doctor Li said saltpeter is toxic and can’t be used to directly make ice. Then you thought of using a barrier to separate the saltpeter while still making ice. How did you come up with such a clever idea?”

Qin Fang assessed her tone and, seeing that she seemed genuine, not just trying to make him feel better, he responded with a pleased hum, though trying to hide his joy. “Yan Yan, you’re too kind. If I were too dull, how could I deserve such a smart wife like you?”

Yang Haiyan was speechless. She didn’t know what to say to that. He was impossible!

Making ice with saltpeter took about half a day, roughly two hours.

By the time two hours passed, it was already midnight, and the ice had formed.

Qin Fang had someone fetch Doctor Li, who had stayed at the residence waiting for the results.

Doctor Li had been so intrigued by the saltpeter ice-making process that he hadn’t gone to sleep.

When summoned by Qin Fang, he immediately went to the inner courtyard.

Though it wasn’t proper for him to enter, with the general present, it was acceptable.

When Doctor Li entered and saw the ice in both the basin and the bowl, he was once again astonished.

Just as General Qin had said, saltpeter really could make ice.

Qin Fang said, “Doctor Li, as you can see, the ice in the basin formed with the saltpeter, while the ice in the bowl did not come into contact with the saltpeter. Could you test whether either ice contains any toxicity?”

Doctor Li replied, “Alright, I’ll test it.”

He took some ice from both the basin and the bowl.

Since the ice had just formed and was still relatively soft, it was easy to take samples.

After about fifteen minutes, Doctor Li returned with the results. “General, the ice in the bowl is non-toxic and safe to consume. However, the ice in the basin contains a small amount of toxicity.”

Qin Fang smiled with satisfaction. “Good.”

Not only was Qin Fang pleased, but Doctor Li was also excited. “It’s unfortunate that common people can’t easily obtain saltpeter, and even then, there isn’t much of it. Otherwise, this method of making ice could be widely used, and ordinary people could afford ice.”

Using saltpeter would lower the cost of making ice, but saltpeter was hard to come by.

In ancient times, minerals were not easily accessible, or explosives would have been widespread long ago.

Qin Fang agreed, “Indeed.”

Listening to their conversation, Yang Haiyan was reminded of salt extraction from seawater.

If salt could be extracted from seawater, could saltpeter be extracted from ice water?

Both salt and saltpeter were crystals, after all.

If saltpeter could be extracted from ice water, then it could be reused repeatedly. In that case, as long as people stored some saltpeter, they could use it to make ice every year.