Evaporation and Polishing

Evaporation plays a greater role in polishing a shoe than most people realize.

Both the polish solvent, and the water used to press against the wax, vaporize. Not only does this remove the liquid solvent and the water from the surface, it also cools the surface of the wax slightly (like sweat evaporating off of the skin of a jogger).

Another interesting aspect is that the solvent and the water evaporate at different rates. The solvent (if it is Naphtha or Turpentine) evaporates about ten times faster than water. Isopropyl Alcohol (rubbing alcohol) evaporates about twice as fast as water, and four times slower that turpentine.

The faster the moisture evaporates the cooler the surface it was on becomes (it has to do with heat energy). Countering that action when polishing the shoe is the friction heat caused by the rubbing. So the less pressure rubbing required and the faster the evaporation, the easier it is to create a smooth coat of wax.

Of course the dissolving aspect of the solvent plays a role in this process as well, partially breaking down the wax of the previous coat, and allowing the current polish to spread. The quicker the solvent evaporates in this case the better.

One of the reasons I use orange oil as the solvent in the GlenKaren polish is that it evaporates about three times faster than turpentine when exposed to air.

One of the main factors in determining evaporation speed is Vapor Pressure, which is measured in millimeters of mercury (mmHg). The smaller the mmHg the more volatile (faster evaporating) the liquid is. Measuring the vapor pressure of the following liquids at a temperature of 68F/20C we see the following:

Orange Oil = 1.43 mmHg
Turpentine and Naphtha = 4 mmHg
Water = 17.5.8 mmHg
Isopropyl Alcohol = 33 mmHg

This would make it seem like water evaporates faster than Isopropyl Alcohol, which is not true. One of the other factors that can effect evaporation speed is Hydrogen Bonding within the molecular structure. Water (H2O) has very strong hydrogen bonds, which slows down its evaporation. Water has 4 times as many hydrogen bonds as Isopropyl Alcohol. Basically what it comes down to is that water evaporates about twice as slowly as Isopropyl Alcohol.

Orange oil and Turpentine are very similar in a lot of aspects:

  • Both orange oil and turpentine are solvents and considered volatile oils (evaporate quickly). With orange oil evaporating about 3 times faster.
  • Both come from natural sources; pine trees for turpentine, and oranges for orange oil. Both are biodegradable.
  • Turpentine has a flash point of 90F, whereas orange oil has a flash point of 113F. (both closed cup). A higher flash point is a little safer.
  • Solvent ability is measured in KB (Kauri-Butanol) Value. The higher the number the better the solvent ability. Turpentine has a KB value of 56, and orange oil has a KB value of 67. Making orange oil a better solvent by a small factor of about 1.2x.
  • Orange oil cost about twice as much as turpentine.
  • Orange oil smells like oranges, turpentine does not.