Ice was not widely applied on ancient battlefields, mainly because ancient battlefields were primarily in the Central Plains, where even in the depths of winter, temperatures were not particularly low, and the cryogenic properties of ice were not effectively utilized. The only documented practical application of ice to repel enemies, as recorded in historical texts, involved the renowned Song Dynasty general Yang Yanzhao—a historical figure, not a character from fictional tales.
In a certain location in Jizhou, Yang Yanzhao mobilized soldiers and civilians to brave the severe cold, drawing water to douse the city walls, creating an ice city to resist the Liao army's assault. Facing the Liao army under Empress Dowager Xiao, who personally led the charge and beat drums at the city gates, Yang Yanzhao seized the opportunity presented by a rare, century-level cold snap. He ordered his soldiers and civilians to repeatedly pour water onto the city walls, allowing it to freeze. The next day, the towering city walls transformed into an ice wall, smooth and incredibly strong. The Liao army launched a desperate general offensive, but arrows shot at the ice wall merely left white marks; siege ladders could not find purchase on the slippery surface and would slide off, easily pushed down by the defending Song troops. Liao soldiers, unable to scale the walls, were stalled for days with no improvement in weather conditions. Faced with this predicament, they were forced to retreat. The text here requires a scientific explanation. As everyone learned in junior high physics, water freezes around 0°C, but the hardness of ice varies with temperature. The lower the temperature, the greater the hardness of the ice. At 0°C, ice hardness is 1-2, easily broken, similar to unhardened glass. At -15°C, ice hardness is 2-3, comparable to pure copper. When the temperature drops to -40°C, ice hardness becomes 4, equivalent to steel. Further down to -50°C, hardness increases to 6, exceeding that of steel. At -70°C, ice hardness can reach 7, surpassing that of granite.
As water turns to ice, its volume increases. However, with further cooling, it conforms to the principle of thermal expansion and contraction. As ice cools, its volume shrinks, and the ice becomes denser and harder. Under extreme low temperatures, ice can be harder than steel or granite.
I estimate the current outdoor temperature is certainly below -30°C. The ice formed between the city gates possesses hardness close to that of steel, and the ample water prepared by Dongfang Jibai for me allows for the complete filling of the approximately one-meter wide gap between the gates. Generally, ice of a certain thickness at low temperatures can resist bullets and even cannonballs. For instance, at -25°C, ice becomes extremely hard and can be used as "armor" in warfare, with performance comparable to my soft body armor, and it is also lightweight.
In an extremely cold winter outdoors, if wood shavings or fibers are added to water before freezing, it greatly increases the ice's strength, providing even better protection. During World War II, Allied forces mixed wood fibers with water and froze them, creating a barrier layer that prevented ice from melting. This fiber-reinforced ice was exceptionally solid; a two-and-a-half-centimeter thick ice slab could support the weight of six men, while a ten-centimeter thick slab could withstand pistol and rifle bullets, significantly reducing Allied casualties in their battles against the Germans.
The ice at the city gates here is one meter thick. Forget about the rebel's modern weaponry; even if they brought Dai Lu's cannons here, they might not be able to break through.
I am truly impressed by Dongfang Jibai's execution capabilities. How did she clear the small icebergs at the water outlet in such a short time? I suspect they must have unique local methods for dealing with freezing conditions. It certainly wasn't by brute force hammering, and if it's as hard as iron, how could it be chiseled with iron tools? They likely used fire or other heating methods to melt the ice or raise the temperature before finding a way to break it.
Of course, defending the city gates solely with hardness is insufficient; it also requires resilience. Fortunately, ice possesses considerable tensile and compressive strength and load-bearing capacity. Scientific experiments have shown that ice at low temperatures has immense tensile strength, ranging from 12 to 15 kilograms per square centimeter, and compressive strength of 35 to 45 kilograms per square centimeter. As the temperature continues to drop and the ice layer thickens, its load-bearing capacity increases. A half-meter thick ice layer can easily support dozens of tons of tanks.
Therefore, it's futile whether the rebels use battering rams or horse-drawn towing to break open the gate. They cannot. They can only try to melt the ice between the gates, but that requires time!
It is this time that I am striving to secure!