This week a customer, who we'll call Mr. X, contacted me about a mold. It was a large production slab tray that is designed to hold around 96 ounces of melted soap base. Mr. X only filled the tray with 40 ounces of soap as he intended to make smaller sample-size soaps.
Since Mr. X did not fill the tray to its full capacity, the bottom of the mold did not sit completely flat and a very slight bowing occurred that caused the poured soap to shift to the outer edges of the mold (by about an 1/8"). Had Mr. X poured poured to full capacity, this would not have occurred. The weight of the soap would have given a even pour.
When Mr. X called to inform me that the molds were defective, I patiently explained that he needed to fill the mold to its full capacity to ensure an even pour. He was quick to point out that he had filled the mold to full capacity with water and some bowing still occurred (bowing means the bottom of the mold did not sit flat on the pouring surface). I explained that water was not as dense as soap and the same volume of water would not be the same volume as soap. In lay terms... soap weighs more than water. Mr. X had a hard time understanding this concept and insisted that whether he used water or soap, the bowing results would not change. This simply isn't true because liquid volume and weight measure differently.
Some liquids have more weight than others. One cup of syrup weighs more than one cup of water or milk. One cup of oil for the car weighs more than one cup of cooking oil. One cup of liquid mercury weighs more than water. When we read on the side of a bottle that it contains, for example, 8 oz. of cooking oil, soap, milk, or oil for the car, it does not mean that it weighs eight ounces. It means that there are eight FLUID OUNCES of the liquid inside. Fluid ounces are used to measure liquids because they have no shape. Volume is something entirely different. An object made of a substance with a density greater (such as a bar of soap) than the liquid (such as water) will sink in the liquid.
In the end, Mr. X could still not understand that the large tray mold filled with soap weighed in at 6 pounds; filled with water it weighed closer to 5 pounds. The difference was in the density of the liquid. Bottom line... soap weighs more than water. This is why soap sinks in water.
When we list the number of ounces per cavity for a mold, we are referring to the volume not weight. If the mold cavity is listed as holding 4 ounces, it means it will hold 4 liquid ounces whether it's soap or water. However, since soap is denser than water, you're finished soap will weigh more than 4 ounces.