Chemistry Corner: How to make soap

Soap makes a unique and personal gift, especially when it is homemade. You can also sell your soap at a farmer’s market or just use it yourself and it is a great way to get rid of old grease and fat that would otherwise find it’s way to sewers or landfills.

The process uses caustic chemicals, so it is slightly dangerous, but if you follow general safety guidelines and keep your wits, you should be fine. You might make a mess, but it should be easy to clean up because most of the spills will be made of soap!

Ingredients you will need:
-A large amount of animal fat or vegetable oil (I added some olive oil to my recipe to make it milder).
-Lye (NaOH) crystals. You can get this at the hardware store, usually located next to the drain openers.
-Distilled water. You can use tap water, but I wanted to make sure there were no dissolved minerals that could ruin the soap.
-Essential oils for a scent. This is optional, but I think it helps. I use tea tree oil and jasmine oil. You can find these at a natural foods or local drug store.

Tools you will need:
-Safety equipment: Rubber gloves, goggles, and an OPEN bottle of vinegar within easy reach, just in case you need to neutralize any spilled lye. Lye is very basic and caustic, and it WILL burn your skin, or any part of your body, for that matter. If you have ever seen “Fight Club,” then you know what I mean.
-A large glass or ceramic bowl to melt the fat and mix everything in.
-An accurate scale. It does not matter whether you measure in grams or ounces, but you must be accurate.
-Something to stir with. You can always just use a big spoon, but it will take a long time and you will get tired quickly. I use a blender for mixing.
-Some kind of mold to pour the finished soap into. Besides a casserole pan or plastic bin, you can also use lengths of PVC or paper cups to mold the soap into round shapes (see the main photo).
-Notepad, pen, and calculator to make calculations.
-You will also need a few more tools such as bowls, scrapers, etc.

Render and Clean the Fat
Lay down some newspaper on a big table or counter space. You will need plenty of room to make soap. To make soap, you need some animal fat and oil. Vegetable oil will make a softer soap than saturated fat like lard or shortening, so it is a matter of personal preference whether you like a hard bar soap or a somewhat ‘squishier’ soap. Blending several kinds of oil or fat together can give you a wide variety of characteristics.

Most of my fat came from bacon and hamburger drippings, with some olive oil as well. It is important to make sure that you have each kind of fat separated and pure. Do not mix beef tallow and lard together until after you have weighed them, as it will throw off your calculations and your soap will not turn out right. Then you must render and clean the fat. Rendering is the process of melting the fat to separate it from water or bits of meat. In my case, I just fried my bacon and poured off the extra grease into a jar.

To clean the fat, you can either boil it, filter it, or both. I first filtered everything through an old sock and a coffee filter. Then I boiled it in a big, tall pot with half water and half fat. This gets rid of any water-soluble dirt or salt left in the fat. Boil it for about 20 minutes, then cover and let it cool in a fridge or cool location until the fat floats to the top and solidifies. You can then scoop out the purified fat and throw away the dirty water at the bottom. Scrape any dirt off the bottom of the fat.

Measurements and Calculations
A big part of making soap is precision. You need to weigh out ingredients precisely so you have them in the right proportions. Never measure by volume.

First weigh out the fat. I measured out 720g of lard and an additional 102g of olive oil.

In order to use the right amount of lye and water, you need a table of saponification values.
Since I am aiming for five percent residual fat at the end, I used values of 0.132 for lard, 0.129 for olive oil, and 0.38 for measuring the water.
720g lard x 0.132 = 95g NaOH
102g olive oil x 0.129 = 14.2g NaOH
Total: 108.2g NaOH
Now is a good time to put on your gloves and goggles. Weigh out the lye crystals into a bowl. It is alright to use plastic at this point because the crystals are not wet.
Calculate the total amount of water needed.
822g of total fat x 0.38 = 312.4g water
On this last measurement, you do not need to be too accurate. Just add a little extra water if you are not sure, since it will mostly evaporate in the end. Put the water in a smaller glass bowl so you can mix the lye in. I used a coffee pot because it is made of glass and has a pour spout.

Now carefully and slowly add the lye to the water. Never add water to lye, because it can spit out of the container and burn you. Stir slowly as you add the lye, making sure to dissolve all the crystals. The water will heat up due to an exothermic reaction with the lye, but if you mix it slowly enough, it will not heat up too much.

Mix Everything Together
Now comes the part where you actually make the soap. Keep your safety goggles and gloves on, because there is still a risk of chemicals splattering.
Make sure that both the fat and the lye/water mixture are warm, about 100 degrees Fahrenheit. This will keep the fat from solidifying before it actually turns into soap. You can heat them in the microwave if needed.

While stirring with a spoon, slowly pour the lye/water mixture into the fat. The fat will start to turn milky and thick as you stir. Once everything is combined, keep stirring hard. If you are using a blender, this step will go much quicker.

Once the soap is the thickness of pudding, test whether it is ready by trying to leave a ‘trace.’ Drip a trail from the spoon onto the surface of the soap. If it leaves a trail for a few seconds, that means it has traced and is ready to be molded. If the trail quickly drops back under the surface, it is not ready and you need to stir it more. Keep the mixture warm, and remember that you cannot stir it too much.

After it has traced, you can add whatever scent you want. You can also add ground spices or herbs. I added some nutmeg, ginger, and orange zest. Just do not add anything really scratchy, like coffee grounds, since you will end up making sand paper out of your soap. Also do not add anything with alcohol, acid, or any chemicals that might throw off the reaction.

You can now pour the soap into whatever mold you want. I used a simple casserole dish. After it solidifies, usually after a few hours, you can cut it into bars and wrap it. You can also imprint a brand or logo into the soap while it is still soft. It is good to keep most of the air away from the soap while it cures to prevent carbonic acid from forming on the surface due to the reaction with carbon dioxide in the air. Plastic wrap or wax paper works well for this.

Now you have to wait. It takes about 3 weeks for soap to fully cure. During this time, any excess lye will react with any remaining fat. If you use the soap before this time, it could irritate your skin.

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