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How to use the roots of ordinary rhubarb from the garden to make plant-based pink soap. This involves infusing dried rhubarb root in oil and using it to make soap. This rhubarb soap recipe creates beautiful soap bars that range in color from soft pink to coral pink.
Three years ago, I came across an interesting and somewhat exotic ingredient that we could use as a natural colorant for soap making – Himalayan rhubarb. It’s related to the rhubarb that we grow in our gardens as a fruit, but not the same. It’s a wild plant that creates rich mustard yellows and browns in natural fiber dyeing – that’s also the color you get when you infuse the powder in oil. However, use it in soap recipes, and you’ll get the most eye-popping magenta-red soap that you’ve ever seen. You can hardly believe that it’s a natural color!
Since discovering Himalayan rhubarb soap, I’ve wondered if you could also use garden rhubarb roots (Rheum rhabarbarum) to color soap. I finally got around to trying it out and am kicking myself that I didn’t do it sooner. Not only does it work, but the soft pastel shades of pink it gives are simply beautiful. The color is warmer than French pink clay when gelled, but ungelled, it is a soft, true pink. Best of all, this color is 100% from my garden and can be from yours, too. If you have a patch of rhubarb or a friend or family member who does, harvest some of the root in the dormant season and use it to make rhubarb soap.
Using Rhubarb Root in Soap
I’m going to take you through how to make rhubarb soap from scratch and how to harvest the roots and prepare them. That way, you can be self-sufficient in a natural pink soap colorant if you wish. Those who don’t grow their own rhubarb can find rhubarb powder for sale, but not usually your standard garden rhubarb. Usually, it will be Himalayan rhubarb (Rheum australe / Rheum emodi) or ornamental rhubarb (Rheum palmatum). These are different but related species of rhubarb that are used in herbalism and in natural dyes for fibers and fabrics. If you use them in soapmaking, your soap may turn out different shades of pink than what you see in these photos.
Rhubarb root, of any species, is not red, and you may initially wonder if this colorant will work. Trust me, it will be worth your time! However, you need to infuse the root in oil before using it to get vibrant, rosy tones. Add the powder directly to your soap batter, and you’ll get a yellowish color at best. Alternatively, you can put the rhubarb powder into your lye solution. Doing this extracts some, but not all, of the color potential from rhubarb powder, so the color will be weaker than if you had infused a liquid oil with the powder.
Rhubarb Root Soap Colors
There is a wide variance in intensity and hue when working with natural soap colorants, and rhubarb is no different. How much rhubarb powder you use, the quality of the rhubarb root, how much rhubarb-infused oil you use, and whether you gelled the soap are all factors in the final colors you get. With this recipe, I found that ungelled rhubarb soap stayed a softer and truer pink, while the gelled soap had a coral tinge to it.
Another challenge that some people have is that when making rhubarb soap, the inside of their bars is more of an orange color than pink. Often, this orange will morph into pink over the course of a few days, but sometimes it doesn’t. It’s one of those quirks of using natural soap ingredients! So far, my garden rhubarb soap has remained vibrant and light-fast, but I’ll update this article with a follow-up if I notice any changes.
Harvesting Rhubarb Root
Chances are that if you have a vegetable garden, you have a rhubarb plant or two. It’s a very common and easy-to-grow perennial crop that benefits from being divided every five or six years. While you’re dividing rhubarb plants, you could also harvest a couple of the roots to use in soapmaking. Taking just a little won’t hurt the plants at all, and you can dry and use the root material for up to two years. Plant material degrades over time, which is why growing your own herbs can ensure that you’re using the highest quality material.
Dividing rhubarb, or even digging up a root to harvest, should be done when the plant is dormant. Late autumn is a good time, as is late winter. Any time during the dormant season is fine, though, as long as the ground isn’t frozen. We aim to harvest herbal root crops during this time because the plant recovers quicker. The herbal or color potential in the roots is also strongest after summer has come to a close and the plant begins shutting down.
I have a video showing how to divide rhubarb if you’re new to it and aren’t quite sure what to do. While it focuses more on reinvigorating the plant and creating new plants from division, you can also swoop in and harvest rhubarb roots during the process. I wouldn’t cut the root too close to the crown but instead, leave a stump of a root at least three inches long. That should help avoid any issues with potential crown rot.
Oxalic Acid in Rhubarb Root
Before we get into how to dry and grind rhubarb root, let’s chat about safety. Rhubarb root is very high in oxalic acid, so much so that it’s toxic when ingested. Though some use it in herbalism, I personally wouldn’t play around with it. Rhubarb root can cause some pretty severe gastrointestinal issues, in the very least.
You may have heard of oxalic acid before, though. That’s because it’s the same organic compound that gives sorrel, spinach, and rhubarb stems its characteristic flavor. In small quantities, it’s quite tasty and harmless for most people. When using rhubarb root to make soap, only a small amount of the root extract makes it into the bars and is washed off the skin anyway. I don’t see any issue with using it in soap, but I’d be careful with using rhubarb root around food items.
Processing Rhubarb Root for Soap Making
Once you’ve harvested fresh rhubarb root, wash it thoroughly of the soil. Next, cut it up into the smallest size pieces that you can. You can use your ordinary kitchen knives and cutting boards if you wish, but ensure that you clean them thoroughly afterward. Next, dry the pieces until they are bone-dry and brittle. I spread mine out over greaseproof (wax) paper and left them in a dim and airy place for a month. You could use a food dehydrator if you wish, but again, clean it thoroughly afterward.
Carefully pulse it into tiny pieces or a powder when it’s fully dried. I recommend using a high-powered blender or coffee bean grinder. Store it in a jar in a dark place for up to two years, or continue with the recipe below. You don’t need much to create a truly beautiful pink soap!
Natural Pink Soap Colorants
There are many ways to make natural pink soap, including using clay, plants, and a type of insect (weird, I know!). Here are some of my other recipes showing you how to use pink soap colorants:
- 4 Ways to Make Madder Root Soap
- Cochineal Soap Recipe
- French Pink Clay Soap Recipe
- Pink Himalayan Salt Soap Recipe
- More Natural Soap Colorants
Garden Rhubarb Soap Recipe
- Stainless steel pan for melting the solid oils
- A large bowl for measuring the liquid oils into
- Rubber gloves
- Mason jar
- 500 g Olive oil (light colored) 1 pint / 500 ml
- 13 g Dried rhubarb powder/pieces 0.46 oz / 3 TBSP
- 3 tsp Lavender essential oil optional
Make rhubarb-infused oil
- To make pink rhubarb soap, you will first need to infuse the olive oil called for in the recipe with dried rhubarb root. The ratio of root to ratio is quite low, and infusing them is very easy. The process extracts the colored components from the root, but the pink color will not be evident until the lye solution is added to the oils during the soap-making process.
- Cold infusion instructions: Fill a Mason jar with the rhubarb powder and oil. Seal it and leave it in a dark cupboard for four or more weeks. I gave it a couple of shakes during that time, but you could shake yours more if you wish.
- Warm infusion instruction: Fill a Mason jar with the rhubarb powder and oil. Seal and gently heat it in a bain-marie for twelve hours. The easiest way to do this is to set a cloth at the bottom of a crock pot, half-fill the crock pot with water, and place the sealed Mason jar inside and on the cloth. Set the temperature to hot to warm the water, but reduce the heat to warm after one hour.
- When the time is up, strain the oil from the rhubarb root. It will have a yellowish tinge, but it won't be as yellow as when making Himalayan rhubarb soap. Discard the rhubarb pieces/powder and store the strained oil in a clean jar for as long as the original shelf-life of the oil. You can use it to make garden rhubarb soap immediately, and you should have enough to make two 1-lb batches of soap. The infused oil can replace all or some of the olive oil called for in soap recipes. The more of it you use, the pinker the soap will be.
Make Rhubarb Soap
- Prepare your workstation with your tools and equipment. Put on rubber gloves, eye protection, and an apron. Carefully pre-measure the ingredients. The solid oils into the pot, the liquid oils into a jug, the water into another heat-proof jug, and the lye in another container.
- Next, dissolve the lye (Sodium hydroxide) crystals in water. In an airy place (outdoors or by a window is best), pour the lye crystals into the water and stir well. There will be a lot of heat and steam, so be careful. Try not to breathe it in. Leave to cool. I tend to set my jug in a sink shallowly filled with cold water.
- Melt the solid oils in a stainless steel pan on very low heat. When melted, remove from the heat and set on a potholder. Pour in the liquid oils. If you have the olive and castor oils in the same container, stir them together before pouring them into the pan. Castor oil is sticky and easier to pour when mixed with lighter consistency oil.
- Measure the temperatures of the lye solution and the oils. You should aim to cool both to about 100°F (38°C).
- Pour the lye solution into the pan of oils. I recommend pouring the liquid through a small sieve to catch any potential undissolved lye.
- Next, we're going to bring the soap to 'trace.' It involves alternating stirring then pulsing the ingredients until they thicken to the consistency of warm custard. At trace, the soap batter will leave a distinguishable trail on the surface if you drizzle it from the immersion blender head. You will also see the pink color come to life in this step.
- Dip the stick blender into the pan, and with it turned off, stir the mixture together. Next, bring it to the center of the pan, and with both your hands, hold it against the bottom of the pan and blitz it for just a couple of seconds. Please don't move the blender around when turned on since it sends up splatters in small soap batches. Turn the blender off and stir the soap batter using as if it were a spoon. Repeat until the mixture thickens up to 'Trace.'
- Working quickly, stir the essential oil in if you're using it, then pour the soap into the mold. Give it a tap to settle it.
- You have a choice, now. To potentially deepen the soap color, you can encourage the soap to gel. I do this by placing the mold in a pre-warmed oven (warmed to 170°F) that's been turned off and leaving it there overnight. For a lighter true-pink, put the soap mold in the refrigerator overnight. The bottom shelf is best, and putting it in a large tupperware or covering it with clingfilm will stop the essential oil scent from contaminating the fridge.
- The next day, take the soap out of where it is and set it somewhere to rest for another day. Once 48 hours have passed, you can take the soap out of the mold and cut it into bars using a kitchen knife. You can get five to six decent-sized bars of soap from this batch.
- Cure the rhubarb soap for 28 days. Curing means leaving the bars spaced out on a protected surface out of direct sunlight and in an airy place. This allows the extra water content to evaporate out fully. Here are full instructions on how to cure soap.
- Once made, your soap will have a shelf-life of up to two years. Check the oil bottles and packaging that you're using, though. The closest best-by date is the best-by date of your soap.