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To make soap, you mix oil or some kind of fat with lye. The combination turns into soap. The process of soap formation is called "saponification." There is something to be said for mixing the oil and the lye in correct proportions. Too much lye can result in a product that is harsh and unpleasant to use, and too little can result in a greasy product. You also need to mix things very well.
When I first started looking at methods, most of the methods I saw were fairly large batches of soap being made in fairly large bowls. People tended to use a stick blender to mix.
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Personally, I found this to be a fairly messy proposition which is not very nice when you're dealing with concentrated lye. It's a little too easy to spray things where you don't want them. I wanted a technique that allowed me to make small batches without making too much of a mess. So I decided to scale things down and use a more or less "normal" kitchen blender to do the mixing. This type of blender has the advantage of having a lid which helps to confine the mess.
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To do a reasonable job getting the right proportions of ingredients I use an electronic kitchen scale that has a resolution of 1 gram.
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So I save my best shiny pennies so I can use them to test my scale. For example, ten pennies should weigh 25 grams or so. I like to test many different numbers of pennies to make sure that the scale is "linear." I find that the readings I get from my scale are consistent with the idea that a nice shiny modern penny weighs 2.5 grams to within a half of a percent or so. I wouldn't call this a calibration and if you have access to calibration standards, that may be preferable to using pennies, but this method can catch large unexpected errors that could lead to fairly unpleasant results. I think of it as a "reality check."
Lye is dangerous. Making soap is dangerous. It's fairly easy to make a case that you shouldn't do it. If you get lye on you, it can burn. If you get it in your eyes, you can go blind. So when working with lye, wear protective clothing such as a buttoned up long sleeve shirt, safety goggles, nitrile, latex or other safety gloves and a hat. If you do get lye in your face or anywhere else, go to the sink or shower and rinse with massive quantities of water. This means you need to have an easily accessible high volume water supply. You should know how to find it, turn it on and use it with your eyes closed.
I buy lye by the pound at the hardware store.
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Be sure the bottle says 100% lye. There is another bottle that looks very similar that says "lye based formula." Since I've never used anything but 100% lye I have no idea if it would work.
I mix my lye in a plastic reagent bottle.
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My bottle has a 2 liter capacity and I got it at a surplus store at the local university. It is made of polypropylene which is not attacked by lye.
FIRST THE WATER.
The order you do things in is important. First you put the water into the reagent bottle and then you add the lye crystals and mix. I like my lye solution to be in the vicinity of 40% lye. I buy lye by the pound, so that means I need around a pound and a half of water. That way I get something close to a 60-40 mix which is 40% lye. The problem is that when I buy a pound of lye I don't actually get a pound of lye, I get more. Sometimes significantly more, so we need to be careful about weighing things.
First weigh the reagent bottle (call it B).
Then add approximately a pound and a half or 680 grams of distilled water and weigh the total (call it B+W).
Then calculate the weight of distilled water actually added:
W=(B+W) - B
Then add the lye crystals and weigh the total (call it B+W+L). A funnel may be helpful at this point.
Then calculate the weight of the lye crystals:
L=(B+W+L) - (B+W)
Then calculate the concentration of lye:
C=L/(L+W)
Write this down and label the reagent bottle with this calculated concentration. It should be close to 0.4 or 40%.
It can take a significant amount of time for the lye crystals to completely dissolve in the water. I put the cap on the bottle but I don't tighten it completely. This is because as the lye dissolves, it heats the water and it's good to avoid a build up of pressure. I gently swirl the bottle from time to time until the crystals are completely dissolved. This can take more than one day. Then I tighten the lid. At this point, It is a good idea to weigh the bottle+water+lye again to see if anything has changed. If so, it means water has been absorbed from the atmosphere or water has evaporated from the solution. Update the concentration calculation if needed. At this point, assuming your lye concentration is greater than 40%, you may slowly and carefully add distilled water to make the concentration of lye as close to 40% as you like. This makes it easier to follow a fixed recipe.
My soap recipe is inspired by Aleppo soap, which is mostly olive and laurel oil. Although the essential oil from laurel is readily available, the kind of laurel oil I would want for soap making is mostly fatty oil with only a small fraction, maybe 3% essential oil. This oil is hard to come by here, so I ended up doing my own design. I use 80% olive oil and 20% coconut oil for the main fatty oils to get a pleasing texture and I add tea tree, eucalyptus and peppermint essential oils. Like laurel oil, they have been shown to have some antimicrobial properties, but it's not clear to me that the added ingredients have an antimicrobial effect in the context of soap. They seem to rinse away nicely. So I've come to the opinion that the antimicrobial aspect of the soap is perhaps mostly related to how well it cleans.
Like many DIY soapmakers, I calculate my recipe with Soapcalc, and I use a 5% "superfat" ratio. This calculation tells me how much lye solution to add to the oil to get the correct proportions. My most recent recipe (Aug. 27, 2015) which makes 9 soap cakes is as follows:
400g olive oil
100g coconut oil
172g 40% lye solution
2ml tea tree oil
2ml eucalyptus oil
2ml peppermint oil
Basically I preheat the oil in my main kitchen oven on "warm" which is around 80C or 175 degrees F. You need to preheat the oil so that the coconut oil melts and mixes with the olive oil. Sometimes I preheat the oil in the blender vessel itself. Then I add the lye solution and blend on the lowest setting. At around the 6 minute point I add the essential oils. The soap "batter" is usually done at around the 8 minute point. The exact time probably depends on your blender but the idea is to get something like a thick pancake batter that I can still pour into a mold. When it's done I pour the soap batter into small silicon muffin molds.
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At this point I put them into a toaster oven at 140-150 degrees F for around 4 hours. This helps to accelerate saponification which allows me to pop the soap out of the molds on the same day. In the future I'll probably leave this step out and just be a little more patient. I'll probably need to wait a day or two before taking the soap out of the molds. Then I wrap the soap in a coffee filter, mark the date and store it away. I let it age.
How long to let the soap age is an interesting question. I think many DIY soap makers like to think that their soap is ready for use in a week or so, but the people in Aleppo making Aleppo soap let their soap age for six months to a year. I think this is a good idea and this is what I like to do.
Here is a finished soap cake.
Enjoy!
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