Soap is strictly defined as an alkaline salt of a fatty acid, although this definition may not be helpful to most readers. It is first important to distinguish soaps from other cleaning substances such as solvents and detergents. Soap is formed when a strong alkaline base is reacted with a fatty acid such as those found in tallow, palm oil, or coconut oil. The base used is almost always sodium hydroxide or potassium hydroxide. This process of reacting a fat or oil with a strong base to create soap is known as saponification.

Oils and fats are not composed of any one single molecule. Sodium Tallowate, when seen on a list of ingredients, is a convenient simplification. Rather than produce a list of all the individual fatty acids contained within tallow, manufacturers may list a simplier version, though many oils and/or fats do not have commonly used simple equivalents -- one does not see Sodium Lardate or Potassium Butterate, even though lard and butter may be used to make soap. Common fatty acids are stearic acid, palmitic acid, and oleic acid. These acids are differentiated based on the number of carbon atoms and the location/type of molecular bonds between hydrocarbon groups. Some fatty acids are modified natural products.

Saponification means soap-making process. During saponification, an alkaline molecule (usually a hydroxide / lye) breaks off a long hydrocarbon chain from the fatty acid molecule and combines with it, resulting in a soap molecule. More technically, saponification is ester hydrolysis to form a carboxylate and an alcohol. As a byproduct of this reaction, glycerin is produced from the breaking of the fatty acid to form soap molecules. The relative proportion of glycerin depends on the type of fat or oil used. Furthermore, it is worth noting that not all fats and oils are saponifiable, only fatty acids.

Soap can also be manufactured by pre-breaking (via hydrolysis) the fatty acid, usually a triglyceride, into its glycerine and smaller fatty acid components. The pre-split fatty acid reacts very quickly with the alkaline (hydroxide / lye). If this process is used, the soap reaction is a simple neutralization between an acid -- the fatty acid -- and a base -- the hydroxide. No glycerine is produced in this reaction because it was removed to produce the fatty acids.

Closely related to pre-broken fatty acids are other modified natural oils (sometimes listed as coconut derived surfactants) used to create synthetic surfactants such as sodium cocoyl isethionate and sodium laureth sulfate.

Detergents perform a similar function to soap, as they are both surfactants and can bind to water on one end and oil on the other end of the molecule, but are sometimes made from petroleum or an alcohol instead of natural lipids (all are long carbon chain molecules). Detergents do not leave soap scum or a bathtub ring, normally caused by a reaction with iron, calcium, magnesium, and other minerals found in hard water. Some body wash bars are not soap but rather a detergent, so they also do not leave soap scum or a bathtub ring.

Follow this link for more information on the Differences between Triple Milled Tallow and Glycerin soaps.

Sodium hydroxide (lye) makes a hard soap. Potassium hydroxide makes a softer or gelatinous soap. These two types are usually mixed to make a shaving cream soap. Sodium lye soaps are hard, and thus shaving soap pucks are made from a reaction of at least some sodium lye and lipids. Soap-making reactions (saponification) produce glycerin. Some soap manufacturers extract the glycerin, by distillation, for resale and use in other products. As glycerin is a product of a soap-making reaction, it is misleading to refer to a soap as glycerin-based, since glycerin is not a raw material for making soap, though it can be added to soap or any shaving product, and it may be the majority ingredient of a particular shaving cream or soap. Glycerin is a clear, viscous liquid that can be used as a moisturizer. It can be produced from vegetable or animal sources. Its relative expense compared to other moisturizers such as cocoa butter or jojoba oil often makes it cost-effective for soap manufacturers to extract glycerin and replace it with a cheaper alternative in their soaps.

Hard soaps can be made translucent by the addition of alcohol to ground soap particles after the soap-making process has completed. Glycerin has limited impact on the clarity of a soap. Liquid soaps and some cream soaps are translucent without the addition of alcohol, primarily due to a higher water content in the soap.

Most naturally occurring lipids (fats, oils, and some waxes) can be used in the soap making process. One common fat is tallow. Tallow is rendered beef or mutton fat (lard is rendered pork fat). Some people may avoid soap made from animal products. Palm oil is chemically very similar to tallow and thus is a common substitute when making all-vegetable soap. Palm oil often imparts a noticeable (possibly described as off) smell and, as a result, unscented all-vegetable soaps are uncommon.

Each lipid imparts characteristics to the final soap product. Most commercial bath and face soap, regardless of brand, color, or properties, is made from 80-20 soap base: 80% tallow, and 20% coconut oil. This base is usually shipped in small nuggets or noodles, and compressed (extruded or plodded) into soap blanks, along with any additives, at the manufacturer. The blanks are later cut/stamped into finished bars. 80-20 soap base is very hard, a product of its high tallow percentage and use of sodium lye instead of potassium lye or other caustic.

Olive oil is another common soap component. Its relative expense compared to beef tallow and coconut oil makes it less common in low-price soaps. Palmolive (Palm-olive) soap was so named because it was made exclusively from palm and olive oils.

Any excess oils left over after the saponification process may eventually oxidize and go rancid. As such, preservatives such as parabens or BHT are sometimes added to tallow and other oils or fats to slow this process. Tubes of shaving cream have less of a problem in this regard because the tube remains free of contact with outside air provided the tube is properly re-capped and squeezed without letting any outside air in.

Many soap makers add products to soap, during the saponification process or afterwards. These products may change the color of soap (titanium dioxide or Yellow #5), add fragrance, increase or decrease the foaming action of the soap (using clays such as bentonite, Montmorillonite, or kaolin, or chemical additives such as sodium laureth sulfate), or, as noted above, change the moisturizing properties of the soap. Some additives may be purely physical, such as pumice, ground oatmeal or pecan shells, or other exfoliating substances. Other common additives aid lathering in hard water, such as EDTA and related etidronates, phosphates (now banned because of environmental concerns), sodium citrate, sodium carbonate, and/or zeolites.

If the additives are oils and are saponifiable, and are added to the soap during the soap-making process, the additives (or at least the oil component of these products) will be reacted into soap and glycerin. If the additives are not saponifiable, they may be added at any time; if the products are saponifiable but are not added during the saponification process, they may be added during a milling process after the soap has cured and hardened.

Milling is a process by which soap is ground up, sometimes heated, and forced through a machine (the mill) with very tight or thin space, usually between two cylinders (often called rolls). In order to evenly distribute soap additives not present during saponification, soap additives may be mixed with soap or added as a coating, and then milled to physically redistribute the additives. Some additives are not totally or evenly dispersed during the first milling process, and thus some soaps are double milled or triple milled to ensure that the soap is uniform.

By contrast, hard-milled soap is soap without water or glycerin. Once the soap pieces and additives are uniformly milled, they are run through a plodder (also called an extruder) to compress the soap ribbons or noodles into a long bar or billet the approximate shape or cross-section of a soap bar. The bar is then cut into soap blanks and finally stamped into a finished bar. Hard milled soap lasts a long time because it is more difficult to dissolve in water, and because glycerin and water are effectively replaced with soap, in a bar of a given size.

Melt-and-Pour soaps usually contain about 50% soap/detergent and the remainder is solvents (such as alcohol), moisturizers and additives (often glycerin, as glycerin is a moisturizer, provides lubrication, and has a low melting point), or binders. It is possible that references to glycerin-based soaps refer to melt-and-pour with or without added glycerin.

Where do I keep the soap and how? Frequent question. Some like to keep the soap in a bowl or in a mug. Others keep them in their original container. You can use 2 bowls, one for soap, one for bowl lathering. What work best for you is the best answer.

It is recommended to cover the soap if you are not using it for a long period of time to prevent dust from getting on the soap. However, those can be washed off in a few seconds. For more information on shave sticks, please consult this page.

• All Shave Soap compendiums are available here.
  
• All Compendiums are available here.
  

• List of Tallow Based Soaps
  
• Shave creams
  
• How to mill any soap into a container
  
• Lather Tutorials
  
• Brands of Shave Soaps
  
• B&B Wiki page for tallow
  
• B&B Wiki page for water and hard water
  
• B&B Wiki page for lather
  
• Shave stick
  
• The New Guys Guide to Inexpensive Soaps and Creams