Making Cold-Process Soap
The Michigan State College recipe from 1942: 13 ounces of lye in a quart of cold water, poured slowly into 6 pounds of melted fat at the right temperature for that fat, stirred 20 to 30 minutes until it holds its shape, molded in a cloth-lined box, cut with a wire, and cured for weeks. Plus how to rescue a batch that separates.
- 4 min
- 5 steps
- 3 questions
- Lesson 113 of 204
In this lesson
- The recipe
- Cut and cure
- If it separates
- Try it
Picking up where you left off.
The 1942 Michigan bulletin gave two recipes, a cold-process soap and a slower “two-day” soap, and noted you could also use the recipe printed on the lye can 1. The cold process is the classic.
The recipe
Cold-process soap 1:
- 13 ounces lye
- 1 quart cold water
- 6 pounds fat
- 3 tablespoons borax, if you like
- Dissolve the lye in the cold water, lye into water, with your goggles and gloves on 2. Let it cool to the temperature in the table below 1.
- Melt the fat and let it cool to its temperature 1.
- Combine. Pour the lye solution slowly into the fat, stirring slowly and evenly. Add the borax, if using. Keep stirring until the mixture is thick enough to hold its shape, perhaps 20 or 30 minutes 1.
- Mold. Pour it into a wooden or heavy cardboard box lined with a cloth wrung out of cold water, with the cloth’s edges hanging over the sides so you can lift the soap out 1.
The temperatures matter. The bulletin’s table, in degrees Fahrenheit 1:
| Fat | Melted fat | Lye solution |
|---|---|---|
| Tallow | 120–130 | 90–95 |
| Lard and tallow | 100–110 | 80–85 |
| Sweet lard or other soft fats | 80–85 | 70–75 |
| Soft rancid fat | 97–100 | 75–80 |
Quick check
Softer fats are combined cooler; the bulletin’s table lists four kinds.
Quick check
Then pour it into the cloth-lined mold.
Cut and cure
When the soap has hardened, cut it into bars with a knife, or loop a string or wire once around the block and pull the ends together 1.
Then wait. Soap must stand several weeks before it’s ready: aging finishes the combining of lye and fat and lets the soap dry. Don’t let new soap freeze 1. The bulletin’s test for a finished, neutral bar was touching it to the tip of the tongue to see if it “bites,” which means free lye is still there 1. Given what lye does, giving the soap its full cure is the better test.
Let the bars dry until they don’t wash away quickly in use. Homemade soap was recommended for laundry and dishwashing; dry bars can be grated or sliced into flakes to dissolve faster 1.
Playback is optional. If the player is unavailable, open the video at its source.
Quick check
Uncured soap can still contain free lye.
If it separates
Wrong temperatures or measurements can make the ingredients separate instead of forming soap. Don’t throw it away. Put the mixture in an enamel or iron kettle and melt it over low heat, stirring so it doesn’t stick. Add enough water to keep it like thick syrup, and boil it slowly until the lye, fat, and water have combined into soap the consistency of honey. Then mold it 1.
Try it
Make one batch with 6 pounds of lard and tallow mixed: fat at 100–110°F, lye solution at 80–85°F. Write down the temperatures and how long it took to hold its shape. Cut it the next day or two, stack the bars with air between them, and mark the date to use them six weeks out.
Lesson complete
Nice work.
Sources for this lesson
- 1Making Soap at Home (Extension Folder F-14, reprint). Michigan State College Extension Division. 1942. verifiedArchive copy; not current recommendations. 6 pounds of fat and 1 can of lye make 9 pounds of soap; fat from butchering makes excellent laundry soap; fryings can be clarified. Equipment: enamel, crockery, or glass utensils, wooden stick, household scales, wooden or heavy cardboard box lined with a wet cloth, thermometer. Clarifying fryings: boil with an equal amount of water, add 1 quart cold water per gallon, cool, lift off fat. Rendering: grind or cut fat, heat slowly in a little water, strain when cracklings settle. Mixed fats make the best soap; order: tallow and lard mixed, tallow, lard; poultry fat only combined with others. Cold process: 1 can lye (13 ounces), 1 quart cold water, 6 pounds fat, 3 tablespoons borax if desired. Temperatures: soft rancid fat 97-100 F with lye 75-80; sweet lard or soft fats 80-85 with 70-75; lard and tallow 100-110 with 80-85; tallow 120-130 with 90-95. Dissolve lye in cold water and cool; pour lye solution slowly into fat, stirring slowly and evenly, 20 or 30 minutes until it holds its shape; pour into box lined with cloth wrung out of cold water. Cut with knife or string or wire. Cure several weeks; aging completes the combining of lye and fat and dries the soap. Tongue test for neutral soap. Do not let new soap freeze. If it separates: melt with water to thick syrup, boil slowly until like honey, mold. Recommended for laundry and dishwashing.
- 2Back to Basics: Lye Safety Guide. Soap Queen (Bramble Berry). 2015. verifiedWear eye goggles, gloves, long sleeves, and long pants. Always add lye to water, never water to lye. Use sturdy heat-resistant plastic or glass; sodium hydroxide and aluminum produce hydrogen gas. Work with ventilation; some mix lye solution outdoors. On skin, flush with water at least 15 minutes and seek medical attention; vinegar is discouraged because acid on a base releases more heat. Label lye containers and keep them away from children and pets.