From Valwiki

Ice, the Element

Made quite some time ago, production has basically been halted. This book, though written in Thuulstrea, can be found in teaching institutions across Kharlia. The author is Sornilzt Qualnryn. It explains ice and eventually how to harness it. It is considered a beginners book to ice magic.

Ice Formation In Fresh Water

In order for water to freeze it must reach its freezing point, which naturally implies heat loss. That heat loss occurs when the ambient temperature of the air is lower than the temperature of the water. The freezing point, of course, is 0 degrees. However, its maximum density is reached at 4 degrees. From an ecological perspective, the latter point is extremely important, since the deeper water which is located under the ice – except in some small lakes or ponds - does not freeze, which means that the creatures living there can survive winter under the ice. An example being: A lake whose temperature decreases to 4 degrees in the fall. Since density increases with the decrease in temperature, the entire lake – from the surface to its bottom – will reach 4 degrees after a certain length of time. When cooled the thin layer close to the surface of the lake becomes denser, therefore heavier, and sinks toward the bottom of the lake, and is replaced on top by “warmer” water which rises to the surface. If the temperature continues to drop and dips below 4 degrees, the layer close to the surface becomes less dense when approaching the freezing point; it then increases in volume instead of shrinking, and becomes “lighter” than the water below it at 4 degrees.. *There's some odd scribbles here, though it continues shortly after,* Heeence, that cooler layer will float on the surface and will continue to cool until ice is formed. Ice is lighter than water due to its larger volume, and that is why it floats. The water located under the ice below remains at 4 degrees, except for the layer just below the ice’s surface. That layer will approach the freezing point of 0 degrees. As it reaches that temperature, it turns into ice as well, making the layer of ice on the surface even thicker. The colder it gets, the thicker and the more solid the layer of ice becomes. If water behaved like other physical bodies, it would increase in density as it cools. If that were true, rivers, streams and lakes would be frozen from the surface right down to the bottom. In the summer, only the top layer would melt, while the deeper water could remain frozen throughout the year. In such conditions, it is obvious that aquatic life in our rivers and lakes would be impossible.

Salt Water, The Exception

Since the freezing point is lower in salt water than in fresh water and that the density of salt water continues to rise with the decrease in its temperature, ice is formed less easily in salt water than in fresh water. Due to its properties, salt water must be cooled to a greater extent before ice can begin to form; contrary to fresh water, an entire column of water must be cooled before ice can be formed on the top.