A rhubarb soap recipe that shows how to use Himalayan rhubarb to naturally color soap hot pink to red using the cold-process method
I share a lot of natural soap recipes, including ways to use natural ingredients to color cold-process soap. While most natural soap colorants yield soft pastel shades, a vivid hot pink or red has been elusive. You can get delicate shades of pink from cochineal or French pink clay, but nothing close to a true red. Then I chanced upon some folks experimenting with using Himalayan rhubarb – an extract used in natural fiber dyeing. I had to give it a whirl too.
Incredibly, the alkaline pH of soap morphs this yellow dye plant into something extraordinary. A stunning deep pink-red that lasts! This Himalayan rhubarb soap recipe will walk you through exactly what I did to create it – everything from sourcing the extract, to infusing it in oil, then using a simple eco-friendly soap recipe to achieve this incredible hue. I guarantee you that you will be overjoyed to see the final results.
Naturally Coloring Cold-Process Soap
Cold-process soap making is arguably the most popular soap-making method we use. You use a from-scratch soap recipe composed of one or more oils/fats and combine them with a strong alkali (lye). The natural chemical reaction that occurs transforms the original ingredients into a new compound – soap! Handmade soap is naturally alkaline, with a pH between 9-10, and so it can be a little troublesome to get plant-based natural colorants to stay.
What can happen in an alkaline environment is that ingredients like beetroot powder or juice turn brown. That’s why beets aren’t a great option for naturally coloring soap red or pink. Sometimes an ingredient can morph into a stunning color in an alkaline pH, though, and that’s the case with Himalayan rhubarb.
Using Rhubarb in Soap Making
Himalayan rhubarb Rheum australe (syn. Rheum emodi) is a plant that grows wild in India and Nepal. It can be challenging to find seeds or plants, but if you can, it will grow in the same conditions as ordinary garden rhubarb Rheum rhabarbarum. The stems of both are edible too, so you could use Himalayan rhubarb for your crumbles and rhubarb wine. Most people who grow Himalayan rhubarb do so because it’s more showy and ornamental than its more cultivated cousin. The flowers bloom as a panicle of reddish-purple spikes and are rather striking.
Though you might think the red stems of rhubarb would be used to make red soap, it’s actually the roots. Try to use the stems and you’ll end up with brown soap. Instead, infuse the fresh or dried roots into liquid oil and then use it in soap and you’ll hit those gorgeous rosy colors.
Also, though I’ve not yet tried it, ordinary garden rhubarb roots may be able to give you lovely pink soap too. The final soap color may not be as strong as with Himalayan rhubarb, but if you have some in your garden, why not give it a try? I know that I will once my rhubarb needs redividing. To use fresh rhubarb root, you need only a small piece, perhaps a ratio of 1:10 with light-colored olive oil. Slice finely and infuse for 2-4 weeks, or until the color is rich and golden like the photos below.
Sourcing Himalayan Rhubarb
First and foremost, Himalayan rhubarb is a plant used in natural fiber dyeing. In my book, A Woman’s Garden, I take you through many natural dye plants and even show you how to naturally color wool yarn with onion skins. In fiber dyeing, you can achieve some gorgeous golden shades with Himalayan rhubarb, just as in the case with those onion skins.
Though you won’t find the ingredient for sale at many soapmaking suppliers, you can find it through natural dyeing shops and artisans. Online is the best place to search, and the type I’m using comes from the Wonky Weaver in the UK. It comes as a brown powder and is sourced from sustainable and trusted sources.
When you’re looking for Himalayan rhubarb, please be aware that there are over sixty types of rhubarb out there! I cannot guarantee that the shades you’ll get with the others will be as vibrant as with Rheum australe (syn. Rheum emodi) so please be aware. And if you’re new to understanding what Latin names of plants mean, Himalayan rhubarb is known both as Rheum australe and Rheum emodi.
Himalayan Rhubarb Soap Recipe
- Mason jar
- Stainless steel pan for melting the solid oils
- A large bowl for measuring the liquid oils into
Himalayan Rhubarb Oil Infusion
- 500 g Olive pomace oil 17.64 oz
- 5 g Himalayan rhubarb powder 4 tsp
Make the Himalayan Rhubarb Oil Infusion
- One month before you make this recipe, mix together the olive oil and dried rhubarb root powder in a glass jar. Store it in a dim but warm place to infuse. Give it a shake that day and whenever you can remember. By the end of the month, the infused oil will be yellow.
- When you're ready to make soap, strain the oil through a sieve. It's best to do this when the powder sediment is not agitated so don't shake it beforehand. Bits of the rhubarb powder can cause a speckled effect instead of a homogenous color. Use this strained infused oil as all of the olive oil needed for the soap recipe. You'll have enough oil to make two 1-lb batches.
Prepare to Make 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 pan, the liquid oils into a jug, the water into another heat-proof jug, and the lye in another container such as a glass jar or ramekin.
Make the Lye Solution
- Next, dissolve the lye (sodium hydroxide) crystals in water. In an airy place, outdoors 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 outside in a safe place, or in a shallow basin of water (inside or outside) to cool.
Make Cold-Process Soap
- 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 first before pouring into the pan. Castor oil is pretty sticky and it's easier to pour when mixed with a lighter oil.
- Measure the temperatures of the lye solution and the oils. You should aim to cool them both to be about 120°F / 49°C. A digital thermometer is great for soapmaking but an infrared temperature gun is miles better. There's less mess and it's much quicker.
- Pour the lye solution into the pan of oils. I recommend pouring the liquid through a sieve to catch any potential undissolved lye. You're about to see the wonderful transformation of that pot of golden oils transforming into almost crimson!
- Dip your immersion blender into the pan and with it turned off, stir the mixture. Next, bring it to the center of the pan and with both your hands, hold it on the bottom of the pan and blitz it for just a couple seconds. Turn it off and stir the soap batter, using the blender as a spoon. Repeat until the mixture thickens up to 'Trace'. This is when the batter leaves a distinguishable trail on the surface. The consistency will be like thin custard.
- Working quickly, pour the soap into the mold. Give it a tap to settle it.
- For a truly vibrant soap through and through, your soap now needs to be gelled. It can be challenging to gel small batches in cavity molds by insulating with a towel. Instead, place the soap in an oven warmed to about 77°C (170°F) and keep it at that temperature for an hour. Turn the oven off then leave it there overnight.
- The next day, take the soap out of the oven and set someplace to rest for another day. Once 48 hours have passed, you can take the soap out of the mold. You can get around six decent-sized bars of soap from this batch. You will also notice the soap deepen in color after you take it out of the mold and begin to cure it.
- Cure the bars 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 fully evaporate out. 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 that you're using though — the closest best-by date is the best-by date of your soap. If your handmade soap is destined as gifts, check out these eco-friendly soap packaging ideas.
- Lastly, store this soap out of direct sunlight. Light can morph the vivid color to a less vibrant plum color. It might be a shade that you like though so you could always set a single bar out in a bright place for a couple of days and see what shade you end up with!