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What ingredient in Ivory Soap makes it float?

First, a little history is in order.

First, a little history is in order. Soap making is one of the oldest of all chemical processes. There is even a reference to soap in the Bible in Jeremiah 2:22. "For though thou wash thee with nitre, and take thee much soap, yet thine iniquity is marked before me, saith the Lord God." The soap referred to probably was a mixture of a kind of clay known as Fuller's earth and nitre, or sodium carbonate. It would have been a good cleaning agent but it wasn't soap as we know it. Real soap was first used widely by the Romans although according to Pliny, the Phoenicians were aware of the cleaning properties of a mixture of boiled goat fat and wood ashes. These are typical ingredients in soaps.

But it was the Roman ladies who used to go to a particular place on the banks of the Tiber River to wash their clothes who deserve a lot of the credit for the introduction of soap. They found that the clay in this location had remarkable cleaning properties. The river bank was located at the bottom of Sapo Hill, the place where animal sacrifices to the heathen gods were traditionally carried out. Animal fat from the offerings had apparently reacted with hot wood ashes to form a substance that was washed down by the rains to the river bank where it was absorbed by clay. This substance was soap. Slowly people realized that soap could be produced by heating any form of fat with ashes. Ashes provided the alkaline conditions needed to convert fat into soap.

By the Middle Ages, soap making had been established as an industry with town like Savana, Castile and Bristol becoming important centers. The product was heavily taxed and was expensive and therefore was not used often. Indeed, a friend of Queen Elizabeth I remarked that she "hath a bath every three months whether she needed it or no." In 1789 a French physician by the name of Nicolas Leblanc introduced a process whereby soda, the necessary ingredient from the wood ashes, could be mass produced from brine. Now the fat could be stirred with soda in large kettles to readily produce soap.

In America the Procter and Gamble Company got into soap manufacturing in a big way when cottonseed oil became available as a cheap source of fat. In 1879 a worker at P&G took a lunch break and forgot to turn off the the stirring machine that mixed the fat with soda. When he returned he noticed that the soap that had formed was lighter than usual because air had been beaten into it due to the extensive stirring. The product was still effective so the company decided to use it. It was cut up into bars and sold. Soon Procter and Gamble began to get letters from consumers who wanted more of the miraculous soap that floated. They no longer had to grope in their bath tubs for lost soap! The company quickly capitalized on this lucky accident and began to promote Ivory as a floating soap. When the soap was chemically analyzed to ensure that it could be reproduced, it became apparent that the long stirring time had allowed for all of the fat and soda to be converted to soap leaving behind little unused reagents. In fact it turned out the soap was 99 and 44/100 percent pure. And another advertising slogan was born. But this number of course does not take into account the ingredient that makes Ivory soap float- which of course is simply air!  

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