My grandmother stirred her lard and lye soap with a big spoon. It took hours of gentle heating and patient stirring until the soap finally reached "trace" and could be poured into the mold. Today, I use a stick blender (also called an immersion blender, wand blender, or stab mixer), to bring most batches of soap to trace in just a few minutes.
What is trace?
Fat and lye are immiscible, meaning they do not want to mix. If soap batter is not stirred, the fat will float on top of the lye, and a thin layer of soap will form where the lye and fat layers touch. When enough soap forms to become a barrier between the fat and lye, the saponification reaction will stop. To keep the saponification reaction going, the soap maker must stir the soap batter to break up the soap particles and mix the fat and lye layers.
Historically, soap makers used a paddle (called a crutch) or a large spoon to mix the soap batter. The batter had to be stirred continuously for hours -- a tedious, boring job. When enough enough particles of soap had formed, these particles emulsified the batter. This chemical emulsification helped the lye and fat to stay mixed without the need for more mechanical stirring.
This point in the soap making process is called "trace" -- the stage at which soap batter is chemically emulsified and obviously thicker. An emulsified batter will stay mixed without having to actually stir it. There are many good articles and videos that describe trace; here is one --
"All About Trace in Cold Process Soap" by Soap Queen TV by Bramble Berry
Why does a stick blender work so well for bringing soap batter to trace?
This process of reaching trace became faster and easier when immersion blenders became popular for home cooking (and home soap making) in the 1980s. (1)
Stick blenders are high intensity mixers that blast the fat and lye into extremely small droplets. These tiny droplets of lye and fat are able to saponify much easier and quicker than the much larger droplets created by mixing with a paddle, spoon, or whisk.
The high intensity mixing also breaks the newly forming soap into tiny particles. Very small flecks of soap are more effective as a chemical emulsifier, so the soap batter usually reaches trace much quicker when a stick blender is used.
How to best use a stick blender?
Some people, especially beginners, use their stick blender with a heavy hand. This can create more problems than it solves. My motto -- Stick blend less and hand stir more!
When making a typical batch of soap, I stick blend the batter in 3 or 4 bursts that are 2-3 seconds per burst. Between bursts, I hand stir the batter with a spatula and watch how the batter changes texture, color, and thickness. This whole process of watching, hand stirring, and brief bouts of stick blending might last a total of 3 to 5 minutes.
At the end of this time, the soap batter is usually emulsified, but it does not show any obvious signs of trace. Sometimes soap makers say the soap batter is "at emulsion" when it reaches this stage. These two videos show the subtle things to look for when deciding if the batter is "at emulsion" or not --
"Stickblending to Emulsion" by SMF Soap Videos
"Mini Drop Swirls" by Kapia Mera Soap Company (emulsion demo starts at 1:00 and ends at 1:25)
When the batter is at emulsion, I will split the soap into smaller portions for coloring. Each portion gets another 1 to 2 seconds of stick blending to mix the colorants into the batter. I might also stick blend the main batter in my soap pot a little more even if it does not get any colorant, just so all of the batter gets treated the same way.
By this time, the soap is getting obviously thicker and I usually do not do any more stick blending -- I want to move on to pouring the soap into the mold and finishing up.
Before pouring soap batter into its mold, I scrape the sides of the soap pot with a spatula and stir these scrapings into the main body of the batter. This ensures all of the batter has the same consistency before I pour. If this is not done, the scrapings may be slightly different than the main batter and they may show in the finished soap as unwanted streaks or odd textures.
What stick blenders are good for making soap?
Keep it inexpensive. I have Cuisinart and Hamilton Beach stick blenders for making lotion and soap; both cost under $30 US. If I was limited to one stick blender for soap and lotion, I could be quite happy with either one. Since I have both, I tend to use the slightly more powerful Cuisnart for soap. I use the Hamilton Beach more for lotions, because it seems a bit less prone to splashing when mixing smaller batches.
Choose stainless steel. I prefer stick blenders that have a stainless steel mixing end, rather than the ones with a plastic bell. Plastic can melt when doing some types of hot process soaps and is more prone to cracking from age and accident.
There are some stick blenders that have zinc or aluminum bells and have internal seals that are not rated for exposure to strong alkalis like NaOH. These blenders are not acceptable for use with soap. One example is my expensive Bamix stick blender which fails on both counts. I use it strictly for food.
Removable mixing end is a plus. I also prefer stick blenders that have a mixing head that detaches from the motor. With a removable mixing head, I don't have to worry about the motor getting wet when I wash the mixing head. I can also put the mixing head in the dishwasher if I want.
Troubleshooting: My stick blender is overheating and my soap has not reached trace. What should I do?
Sometimes soap batter takes a long time to come to trace, even if a stick blender is used. I see this especially when I make liquid soap with potassium hydroxide (KOH).
If your soap is slow to trace, do not furiously stick blend until the machine overheats or your patience wears thin. My rule of thumb -- If I stick blend for better part of a minute, and the soap batter does not respond the way I think it should, I need to STOP and assess the situation. Some types of soap require a watchful waiting game, not a stick blender endurance contest.
Next time your soap batter is acting unusually slow, stick blend for a few seconds and then hand stir occasionally for a few minutes. If the batter is obviously separating after a few minutes, stick blend again for a few seconds and then hand stir occasionally for a few more minutes. Repeat as needed until the soap reaches a definite, stable trace.
You may find your soap will reach trace almost as fast by playing a waiting game rather than a stick blending contest.
Troubleshooting: I have been doing your "waiting game" for hours now and my batter is still not at trace! What now?
Check your measurements. Have you added too little alkali (KOH or NaOH) for the amount of fats you are using? Have you added too much water -- in other words, is your lye concentration too low?
Check if you used the correct alkali. Did you accidentally use KOH when you should have used NaOH? Or did you use sodium carbonate (washing soda) rather than NaOH? Either of these mistakes will prevent your soap batter from coming to trace.
Check the batter temperature. Is it fairly cool (under 100 F / 40 C)? If so, consider warming the batter to 120-160 F / 50-70 C to jump start a lazy saponification reaction.
Try a dose of benign neglect. If you cannot figure out what the problem is, cover the soap pot and put it in a safe place for the day or overnight. See what happens if the soap is left alone to do its thing.
Are there other ways to mix soap batter besides a stick blender?
Hand stirring with a whisk or spatula is still an acceptable, effective way to mix soap batter. Recipes that come to trace very quickly, such as pine tar soap, work best if they are only stirred by hand.
A regular kitchen blender works well for smaller batches. Many people are skeptical, but I know one full-time soap maker who uses inexpensive stand blenders for all of her soap making. More about using a stand blender to make soap...
A hand-held or stand mixer (the gadget used to make cookies or cake) is not as intense as a stick blender, but is another option to consider. A stand mixer has the advantage of being a hands-free option for soap makers with physical limitations. A drill fitted with a paint mixer is another lower-intensity mixing option that may fit your needs.
A final method suitable only for tiny batches (1 or 2 bars) is to shake the soap batter in a closed jar. I could not find anything online about the pros and cons of doing this, but this method is discussed in Kevin Dunn's book Scientific Soapmaking. (2)
More discussion about mixing methods --
https://www.soapmakingforum.com/threads/stick-blender-vs-stand-mixer.32739/
https://www.millersoap.com/SoapFAQs.html#8
https://www.soapmakingforum.com/threads/need-a-hands-free-soap-mixing-method.27032/
Got any tips if I want to stay old-school and only stir my soap batter by hand?
Use a recipe higher in myristic and lauric acids (coconut oil or palm kernel oil).
Use a higher lye concentration -- from 40% to 50% lye concentration may work well for a hand stirred batch. Note: Lye concentration is not the same thing as "water as % of oils".
Add grated or shredded bar soap to the soap batter. I suggest adding 1/2 to 1 ounce (15 to 30 grams) of soap per pound (500 grams) of oils to help emulsify the soap batter.
Add heat to keep the soap batter temperature between 120-160 F / 50-70 C.
Add a small amount of clove essential oil. The eugenol in clove essential oil is an accelerant.
References
(1) http://www.nytimes.com/1998/08/19/dining/test-kitchen-a-whirling-dervish-that-dips-right-into-your-pot.html
(2) Kevin M. Dunn. Scientific Soapmaking. Clavicula Press. 2010.
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