How to Make a Soap-Based Patch Lubricant
After a number of experiments I found that pure oil soap is a very efficient (and cheap) patch lubricant for
muzzleloaders. Here is how I make it.
Vegetable oil (canola, soya, olive, or similar)
Potassium hydroxide pellets
Deionized, distilled, or rain water
Lab beaker (borosilicate glass or stainless steel)
Lab stirrer (propeller type, no magnetic stirrer!)
Hot-air oven or baking oven
Stainless-steel or plastic spatula
Personal Protective equipment (safety glasses, rubber gloves, lab coat, etc.)
Before you start, keep in mind that potassium hydroxide and its solution is highly corrosive to skin and eyes. Read the Material Safety Data Sheet first! Use personal protective equipment! Do not use aluminium containers since they will corrode rapidly. I have run this reaction many times without any problems. Since what other people are doing is beyond my control, I do not assume responsibility and expressly disclaim liability for loss, damage, or injuries resulting from unqualified or sloppy working practice.
Vegetable oils are predominantly composed of triglycerides of C16-C18 fatty acids (mostly unsaturated) and have a saponification value of approx. 185-195 mg KOH / g. This means you need approx. 190 g of pure potassium hydroxide (or 380 g of the 50% solution) to turn 1 kg of oil into soap. I use only 185 g KOH (370 g of the 50% solution) per kg oil to avoid an excess of KOH. A soap thus obtained is less irritating to skin and eyes than one with a higher alkalinity.
Dissolve 50 parts (by weight) of potassium hydroxide (KOH) in 50 parts of deionized water with stirring. Caution! The mixture will get hot while the potassium hydroxide dissolves. Beware of possible boiling retardation! Do not breathe the fumes released by the hot solution! Observe that commercially available potassium hydroxide (lab grade preferred) may contain a few percents of water. The KOH content should be noted on the label (85-90 weight-% in most cases). If, for example, the product contains 90% active material, you have to dissolve 55.6 parts of it in 44.4 parts of water to obtain a 50% solution. Keep the solution in a closed container (preferably polyethylene) since KOH readily absorbs CO2 from the air and forms potassium carbonate which is not reactive enough for our purpose.
Weigh 500 g of vegetable oil into a beaker, warm it up to approx. 50°C (122 F), and slowly add 185 g of the 50% KOH solution with vigorous stirring. At first, the mixture has the appearance of an amber-colored emulsion. As the reaction progresses, the viscosity of the mixture rises gradually. At some point, the product stops flowing and assumes the consistency of a paste. This might take up to half an hour. Now, remove the stirrer and work the paste with a spatula for a few minutes. Then, cover the reaction vessel with a lid and put it into a hot-air (baking) oven. Set the temperature at 80°C (176 F) and let the mixture "mature" for about 6 hours. During this post-reaction, the paste becomes more transparent. A sample of the final product should be clearly soluble in deionized water (1-2 % concentration). If you observe oil droplets, the saponification is not complete yet or you have not added enough KOH. Do not use tap water for this test since the solution will become turbid due to the formation of insoluble calcium and magnesium soaps. The saponification reaction will not run smoothly in the presence of too much water, for example when using a more diluted KOH solution.
The reaction product is a mixture of potassium soaps, glycerol, and water. It may further contain small portions of mono and diglycerides, particularly when a substoichiometric amount of KOH has been used. It is the classic oil soap which found wide-spread use as a universal household cleaner in the days our (great)grandmothers.
I make my patch lubricant by dissolving 1 part of the soap paste thus obtained in 4 parts of warm (50°C) deionized water. This requires continuous stirring and may take a while. The final product is an almost clear, yellowish liquid. It is non-corrosive to steel and has excellent lubricating properties. It also makes an excellent bore cleaner for black-powder guns when diluted with more deionized water.
This is a refined version of my earlier corrosion test. Instead of immersing a steel sample in the lubricant, I soak a cotton patch with the lubricant, put it on a flat piece of carbon steel (degreased with acetone), and weigh it down with a lead ball (see photo). I further cover the samples with a glass jar in order to prevent a rapid evaporation of the water portion of the lubricant. This test assembly simulates the conditions in a barrel loaded with patch and ball. After 24 hours, I remove ball and patch and examine the steel surface for signs of corrosion. A patch lubricant failing this test (such as for example saliva) is not fit for use in my opinion.
There are liquid, ready-to-use "oil soaps" on the market. These are pre-diluted with water and may contain a number of additives, for example complexing agents (EDTA, phosphates), to make them usable with hard water (contains calcium and magnesium salts). I have used a few local products successfully but do not know how products sold in other countries perform. Check for yourself. Don't forget a corrosion test!
Will be continued...