The Bath Ingredient Encyclopedia
Ten bath ingredients. What each one actually does, with the study linked. How much for your tub. What to skip.
This is the page we wanted every time a recipe said “add a cup of Epsom salt” and never said why, or how much for a small tub, or whether the thing it promised was real. Ten ingredients. For each one: what it is, what the research actually shows (with the study linked, not “studies show”), how much to use for your tub, and what to skip. Where the evidence is thin we say thin. Nothing here treats, cures or detoxes anything; a bath is a bath, and a good one is enough.
Salts only, with cost and grams? Use the full bath salt calculator.
Mami says: Pick one base (a salt, oats or milk), one scent (botanicals or dispersed oil), and stop. Three ingredients is a soak. Seven is a science fair.
How to read the evidence tags
Good evidence means at least one controlled human study of the ingredient in or on skin, or a recognised clinical guideline. Some evidence means expert guidance or studies of the ingredient in another form (extracts, lotions, inhalation). Thin evidence means we could not find a trial we would stand behind; the ingredient is pleasant, not proven. The tag is about the usual marketing claim, not about whether you should use the ingredient. Milk is “thin” and we love a milk bath.
The dose chart
One table, every ingredient, three tub sizes. Measure with a real cup the first few times. A “handful” is about a third of a cup, which is how everyone ends up under-dosing.
| Ingredient | Small tub (~25 gal) | Standard (~40 gal) | Deep (~60 gal) | Evidence for the usual claim |
|---|---|---|---|---|
| Epsom salt | 1¼ cups | 2 cups | 3 cups | Warm-bath sleep effect: good. Magnesium absorption: unsupported [1][2] |
| Magnesium chloride flakes | 1 cup | 1½ cups | 2¼ cups | Thin [1][3] |
| Dead Sea salt | ½–¾ cup | 1 cup | 1½ cups | Good for dry-skin barrier (one forearm trial) [4] |
| Colloidal oatmeal | ½ cup | 1 cup | 1½ cups | Good for itch and dryness [5][6] |
| Baking soda | ¼ cup | ¼–½ cup | ¾ cup | Some (expert guidance, no trial) [6] |
| Powdered milk | ¾ cup | 1–2 cups | 2½ cups | Thin; softness only |
| Honey | 1 tbsp | 2–3 tbsp | ¼ cup | Some (antimicrobial on skin; trace in a tub) [8] |
| Clay (kaolin/bentonite) | 3 tbsp | ¼–½ cup | ¾ cup | Thin; buy heavy-metal-tested [9][10] |
| Dried botanicals (bagged) | ⅓ cup | ½–¾ cup | 1 cup | Some (chamomile, lavender) [11][12] |
| Essential oils (dispersed) | 3–5 drops | 5–10 drops | 8–12 drops | Safety well established; always disperse [7] |
Epsom salt (magnesium sulfate) Some evidence
The one everyone has. Epsom salt is magnesium sulfate, a mineral salt that has been sold as a soaking aid for about as long as bags have had labels. It dissolves fast, it makes the water feel softer and heavier, and two cups of it is the difference between a bath and a soak.
What the research actually says. Here is the honest part: the idea that magnesium soaks through your skin and tops up your levels is not well supported. A 2017 review in Nutrients went through the literature on transdermal magnesium (sprays, flakes and salt baths) and concluded the claim is scientifically unsupported [1]. What a warm Epsom bath reliably does is be a warm bath: the Sleep Medicine Reviews meta-analysis found that water-based passive body heating at 104–109°F, taken one to two hours before bed, shortened the time it took people to fall asleep by about ten minutes and improved self-rated sleep quality [2]. That is the benefit you can bank. Treat the magnesium as a bonus you are not counting on.
- Standard 40-gallon tub: 2 cups. Small or half-filled: 1¼ cups. Deep soaker: 3 cups.
- Dissolve it under the running tap, not after you are in.
- Do not stack it with a second salt at full dose; pick one base salt per bath.
- Open cuts, a rash that is weeping, or a skin condition that flares with salt: skip it tonight.
- Pregnant, or managing a heart or kidney condition? Check with your doctor before making salt baths a habit.
- Keep it out of your eyes; it stings like seawater.
Magnesium chloride flakes Thin evidence
The expensive cousin. Magnesium chloride flakes are what most of the brine in the Dead Sea actually is, sold dry. They are pricier than Epsom, they clump the second the bag is open, and they are marketed hard as the “better absorbed” magnesium.
What the research actually says. The same 2017 review covers flakes and calls the absorption claim unsupported [1]. There is one small feasibility study on magnesium chloride sprayed on the skin, in people with fibromyalgia, where some participants reported feeling better; it had no control group and the authors described it only as grounds for a bigger trial [3]. The 2005 Dead Sea salt study (below) is the strongest skin-side evidence for magnesium-rich water, and that was about the skin barrier, not your magnesium levels [4].
- Standard tub: 1½ cups. Small: 1 cup. Deep: 2¼ cups.
- Flakes feel slippery and slightly “oily” on the skin; that is normal, not residue.
- Reseal the bag with a clip; they pull water from the air.
- Stings on broken skin more than Epsom does.
- Same pregnancy and heart/kidney note as Epsom: ask first.
Dead Sea salt Good evidence
The one with the best study behind it. Dead Sea salt is a mixed mineral salt: far less sodium chloride than sea salt, far more magnesium, calcium and potassium. The grain is heavy and it takes longer to dissolve.
What the research actually says. In a 2005 study in the International Journal of Dermatology, volunteers with atopic dry skin soaked one forearm in a 5% Dead Sea salt solution for 15 minutes and the other in tap water. The salt-soaked arm showed improved skin barrier function, better hydration, and less roughness and redness, and the authors attributed the effect to the high magnesium content [4]. Note what that study is: one forearm, a 5% solution, and people with dry, atopic skin. It is not “Dead Sea salt heals eczema,” and we will not write that. It is a well-run trial that says magnesium-rich salt water was kind to dry skin.
- Standard tub: 1 cup, half the Epsom dose, because the grain is heavier and the mineral load higher.
- Small: ½–¾ cup. Deep: 1½ cups.
- Give it a few extra minutes under the tap; swirl before you get in.
- Skin condition that is cracked or weeping: ask a dermatologist before salt soaks of any kind.
- Buy from a seller that names the source; “Dead Sea” on a label is not regulated.
Oats (colloidal oatmeal) Good evidence
The one your grandmother was right about. Colloidal oatmeal is just whole oats milled to a powder fine enough to stay suspended in water. In the US it is recognised in the FDA’s over-the-counter skin-protectant monograph, which is why it shows up on the back of so many “eczema” lotions.
What the research actually says. A 2015 paper in the Journal of Drugs in Dermatology showed that colloidal oat extracts reduced pro-inflammatory signals in lab assays, and that a colloidal-oat lotion significantly improved dryness, scaling, roughness and itch in a small group of women with dry, itchy legs [5]. (That study was run by a company that sells oat lotion; it is still the best mechanism paper there is, and it matches a very long record of use.) The Mayo Clinic lists colloidal oatmeal in the bath as one of the additives some people with eczema find relieves itching, alongside baking soda [6].
- Standard tub: 1 cup of finely blended oats, or one pre-measured colloidal packet. Small: ½ cup. Deep: 1½ cups.
- Blend rolled oats in a dry blender until they look like flour. Test a spoonful in a glass of water: it should turn milky, not sink.
- Lukewarm water, 10–15 minutes. Pat dry, moisturise while damp.
- Coeliac or wheat-allergic? Oats are usually fine on skin but are often milled alongside wheat; buy certified gluten-free if it matters to you.
- Children and babies: an oat bath is one of the gentler options, but check with your pediatrician for a rash you cannot name.
Baking soda (sodium bicarbonate) Some evidence
The one that costs a dollar. Baking soda is mildly alkaline, which is why it feels silky in water and why people reach for it when their skin itches.
What the research actually says. There is no landmark trial on baking soda baths. What there is: the National Eczema Association suggests about a quarter cup in a bath to help relieve itching, and the Mayo Clinic repeats that guidance [6]. Dermatologists are not united on it (alkaline water can also dry some skin), so treat it as a cheap, low-risk thing to try, not a treatment.
- Standard tub: ¼–½ cup. Small: ¼ cup. Deep: ¾ cup.
- It is not a salt. Half a cup is plenty; it does not scale up the way Epsom does.
- Pairs well with oats. Do not pair it with a full dose of a second salt.
- If your skin feels tight or squeaky afterwards, that is the alkalinity; use less or skip it.
- Rinse off; do not leave it to dry on the skin.
Milk (powdered or liquid) Thin evidence
The one that photographs beautifully and does less than the photos suggest. Milk contains fat, protein and a little lactic acid, an alpha-hydroxy acid.
What the research actually says. Lactic acid is a real, studied skincare ingredient, but at the concentration a cup or two of milk reaches in forty gallons of water it is not doing meaningful exfoliation. There is no controlled trial of milk baths we would cite. What milk reliably adds is softness: the fat leaves a faint film that makes skin feel less tight after a hot soak. Enjoy it for that. One safety note from the Tisserand Institute: full-fat milk does not disperse essential oils, despite the folk advice, so do not use it as your carrier [7].
- Standard tub: 1–2 cups of whole powdered milk, whisked into a jug of hot water first. Small: ¾ cup. Deep: 2½ cups.
- Liquid milk works; you need a quart or more to notice it, and it is mostly a scent.
- Coconut milk powder is the dairy-free swap at the same amount.
- Dairy allergy on the skin is rare but real; patch test if you react to milk products.
- Rinse the tub after. Milk film and warm bathrooms are a smell waiting to happen.
Honey Some evidence
The sticky one. Honey is a humectant (it draws water toward itself) and it has a long, well-documented record as an antimicrobial, which is why medical-grade honey is used on wound dressings.
What the research actually says. A 2011 review in the Asian Pacific Journal of Tropical Biomedicine summarises honey’s antibacterial activity and the mechanisms behind it [8]. In a bath, though, two tablespoons in forty gallons is a trace; you are getting a little scent and a little softness, not a wound dressing. We include it because readers ask, and because a spoonful in a milk-and-oat soak is lovely.
- Standard tub: 2–3 tablespoons, stirred into a mug of hot water first so it actually disperses. Small: 1 tablespoon. Deep: ¼ cup.
- Raw or pasteurised makes no difference in a tub.
- Best paired with oats or milk; pointless with salts.
- It is sticky on the tub rim. Wipe it down before it sets.
- Do not use honey on infants under one year in any form; that rule is about ingestion, but it is a good habit to keep it out of baby baths entirely.
Clays (kaolin and bentonite) Thin evidence
The one with a safety story. Kaolin (white clay) is soft and gentle; bentonite swells in water and feels slicker. Both are sold for “detox” baths, which is a word we do not use.
What the research actually says. Clay does not pull toxins out through your skin; there is no mechanism for it and no trial showing it. What clay does in a bath is make the water silky and leave skin feeling smooth, in the same way a mask does. The thing worth knowing is the safety record: in 2016 the FDA warned consumers not to use two bentonite clay products, “Bentonite Me Baby” and “Best Bentonite Clay,” after lab analysis found elevated lead levels [9] [10]. Clay is dug out of the ground. Buy it from a seller that publishes heavy-metal testing, and never from a bulk listing with no brand on it.
- Standard tub: ¼–½ cup, whisked to a slurry in a bowl first. Small: 3 tablespoons. Deep: ¾ cup.
- Kaolin for sensitive skin; bentonite if you like the slip.
- Run the tap for a minute after draining so it does not settle in the pipes.
- Buy tested clay only (see the FDA warnings above).
- Not for kids’ baths; not worth the lead question.
- Metal bowls and bentonite are a folk no-go; it is harmless, but a glass bowl is easier to clean.
Botanicals (chamomile, lavender, rose, calendula) Some evidence
The ones that make it a ritual. Dried flowers and herbs in a muslin bag, steeped in the hot water like a giant cup of tea. They add scent, colour and the feeling of having done something on purpose.
What the research actually says. Chamomile has a real evidence base as a herb: a 2010 review in Molecular Medicine Reports covers its traditional use for skin irritation and its anti-inflammatory compounds, mostly studied as extracts and topicals rather than bath infusions [11]. Lavender’s calming reputation is backed by a 2013 review in Evidence-Based Complementary and Alternative Medicine looking at lavender and the nervous system, largely through inhalation studies [12]. A bath infusion is a weak extract plus a lot of scent, which is exactly what you want from it.
- Standard tub: ½–¾ cup of dried botanicals in a muslin bag or a knotted sock. Small: ⅓ cup. Deep: 1 cup.
- Hang the bag under the running tap, then let it float. Ten minutes of steeping before you get in.
- Our Bath Tea 101 has three blends.
- Chamomile is in the daisy family; if ragweed sets you off, patch test first.
- Loose petals in the water look gorgeous and block drains. Bag them.
- Pregnant? Most bath infusions are fine, but check with your doctor on any herb you are using in quantity.
Essential oils Some evidence
The one most people use wrong. Essential oils do not dissolve in water. Drops added straight to the tub float as undiluted beads on the surface, and you then sit on them.
What the research actually says. The Tisserand Institute, which maintains the main safety reference for aromatherapy, is blunt about it: do not put drops of essential oil directly into a bath; mix them into a vegetable oil or a proper solubiliser first, because undiluted droplets cling to the skin and irritate, especially in sensitive areas and especially with citrus oils. They also note that neither full-fat milk nor Epsom salt disperses essential oils, whatever the recipe said [7]. On the benefit side, the lavender review above is the best-supported scent for winding down [12]. Everything else is personal preference, which is a fine reason.
- Standard tub: 5–10 drops total, stirred into 1 tablespoon of carrier oil (jojoba or fractionated coconut are least slippery) or a solubiliser, then added. Small: 3–5 drops. Deep: 8–12 drops.
- Add the dispersed oil after the tub is full, then swirl.
- Stir into salts only if the salts were first coated in carrier oil; the salt alone is not a dispersant.
- Phototoxic oils (bergamot, cold-pressed lime, lemon, grapefruit): skip them if you will be in the sun within 12–18 hours.
- Pregnancy and essential oils is a long topic; keep to low doses and check with your doctor or a qualified aromatherapist.
- Children: halve the dose at most, and skip peppermint and eucalyptus for small kids entirely.
- Any stinging: get out, rinse with plain water, and do not scrub.
What not to combine
- Two full-dose salts. Two cups of Epsom plus a cup of Dead Sea is a brine, not a better bath. Pick one base salt.
- Essential oils straight into salt, milk or water. None of the three disperses oil [7]. Carrier oil or a solubiliser, every time.
- Clay and a full dose of salt. Salt makes bentonite flocculate into lumps that sink and sit in the drain.
- Baking soda and vinegar. It fizzes, cancels itself out, and leaves you with slightly salty water.
- Bath bombs and whirlpool tubs. Oils, butters and glitter coat the jets. Salts and oats are fine; bombs and melts are not.
Water temperature, because it matters more than any ingredient
The sleep meta-analysis used 104–109°F and found the effect when the bath was taken one to two hours before bed [2]. The Mayo Clinic’s eczema guidance is lukewarm, not hot, for 5–10 minutes [6]. If you are pregnant, the American College of Obstetricians and Gynecologists advises against activities that push core temperature above about 102.2°F, which in practice means warm rather than hot and no long soaks in the first trimester; ask your doctor what is right for you [13]. For kids, keep it warm to the wrist, short, and plain unless a pediatrician has suggested otherwise.
Questions people actually ask
Does Epsom salt absorb through the skin?
The 2017 review in Nutrients found the claim unsupported [1]. Enjoy the soak for the soak.
Can I use table salt instead of Epsom?
You can; it is sodium chloride rather than magnesium sulfate, so it is a different thing in the water. Our Epsom salt substitute finder gives the swap and the amount.
How much Epsom salt for a foot soak?
About ½ cup in a basin of warm water. The full foot-soak method is in the foot soak guide.
Is a bath with these ingredients safe during pregnancy?
Most are, at warm (not hot) temperatures, but clay, essential oils and large salt doses are worth a conversation with your doctor first [13]. This page is not medical advice.
Which ingredient is best for dry skin?
Oats and Dead Sea salt have the best skin-side evidence here [4][5]. Keep the water lukewarm and moisturise while damp.
Sources
- Gröber U, Werner T, Vormann J, Kisters K. Myth or Reality—Transdermal Magnesium? Nutrients. 2017;9(8):813.
- Haghayegh S, Khoshnevis S, Smolensky MH, Diller KR, Castriotta RJ. Before-bedtime passive body heating by warm shower or bath to improve sleep: a systematic review and meta-analysis. Sleep Medicine Reviews. 2019;46:124–135. doi:10.1016/j.smrv.2019.04.008. Summary: UT Austin Cockrell School.
- Engen DJ, McAllister SJ, Whipple MO, et al. Effects of transdermal magnesium chloride on quality of life for patients with fibromyalgia: a feasibility study. Journal of Integrative Medicine. 2015;13(5):306–313. PubMed.
- Proksch E, Nissen HP, Bremgartner M, Urquhart C. Bathing in a magnesium-rich Dead Sea salt solution improves skin barrier function, enhances skin hydration, and reduces inflammation in atopic dry skin. International Journal of Dermatology. 2005;44(2):151–157.
- Reynertson KA, Garay M, Nebus J, et al. Anti-inflammatory activities of colloidal oatmeal (Avena sativa) contribute to the effectiveness of oats in treatment of itch associated with dry, irritated skin. Journal of Drugs in Dermatology. 2015;14(1):43–48.
- Mayo Clinic. Atopic dermatitis: Proper bathing can reduce itching. (Cites American Academy of Dermatology and National Eczema Association guidance on bleach, baking soda and oatmeal baths.)
- Tisserand Institute. Bath safety: how to use essential oils safely in the bath.
- Mandal MD, Mandal S. Honey: its medicinal property and antibacterial activity. Asian Pacific Journal of Tropical Biomedicine. 2011;1(2):154–160.
- U.S. Food and Drug Administration, 28 Jan 2016. Consumer warning: “Bentonite Me Baby” by Alikay Naturals, risk of lead poisoning. Archived notice.
- U.S. Food and Drug Administration, 23 Mar 2016. Alert: “Best Bentonite Clay,” risk of lead poisoning. Archived notice.
- Srivastava JK, Shankar E, Gupta S. Chamomile: a herbal medicine of the past with a bright future. Molecular Medicine Reports. 2010;3(6):895–901.
- Koulivand PH, Khaleghi Ghadiri M, Gorji A. Lavender and the nervous system. Evidence-Based Complementary and Alternative Medicine. 2013;2013:681304.
- American College of Obstetricians and Gynecologists. Exercise during pregnancy (FAQ): avoid activities that raise core temperature, including hot tubs and saunas.
Last reviewed 28 October 2026. This page is editorial, not medical advice. If you are pregnant, bathing a child, or managing a skin or heart condition, check with your doctor before changing what goes in the water. Found an error or a better study? Write to [email protected] and we will fix it.
Cite or link this page freely. If you quote the dose chart, a link back to bathmami.com/ingredients/ is appreciated, never required.
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