Distilled water
Distilled or deionised water is water with its dissolved minerals taken out, used to dissolve the lye and to dilute liquid soap. In a bar recipe the minerals in tap water cost very little soap, but they cloud liquid and transparent soap and can bring the trace metals that speed rancid orange spots.
- Why distilled or deionised
- Rain water, boiled water and softened water
- Water as a process material
- How much water
Identity
Aqua (Water)
- Formula
- H2O
- CAS
- 7732-18-5
- EC
- 231-791-2
- Also called
- deionised water, demineralised water, purified water
Guides
Why distilled or deionised
Tap water carries calcium and magnesium, and each of those ions takes two soap molecules out of solution as insoluble calcium or magnesium soap, the "insoluble soaps of lime and magnesia" of Thomssen's 1922 manual. The World Health Organization still defines hardness by that reaction: hard water needs more soap to lather.
In a bar recipe the loss is tiny. A 1000 g olive oil soap at a 33 % lye solution uses 261.2 g of water, and at a very hard 300 mg/L that water can spoil about 0.48 g of soap in a 1390 g batch. It will not change the lather or the hardness of the bar, and it is not the white powder on a curing bar, which is soda ash.
It shows in liquid soap, where the paste is diluted with several times its weight of water. The liquid soap guide's worked batch takes 2082 g of dilution water, about eight times the water in that bar, and at 300 mg/L the same arithmetic turns roughly 4 g of soap into scum that stays in the bottle as haze or sediment; Thomssen warned that hard water forms insoluble soaps that settle out. It shows in transparent soap too: Simmons and Appleton warned that hard water "is liable to produce opaque streaks of lime soap", and Failor calls for distilled or soft water.
The other reason is trace metals. The US Environmental Protection Agency's guideline levels for drinking water are 1.0 mg/L of copper and 0.3 mg/L of iron. At those levels the bar's 261.2 g of water would put about 0.19 ppm of copper into the batch, close to the 0.2 ppm that a Lever Brothers patent found enough to spoil the colour and smell of soap. Those are upper limits, not typical values, and oils bring metals of their own, so tap water is a risk to reduce rather than a certain cause of orange spots. Tap water also carries chlorine or chloramine as a disinfectant, at no more than 4.0 mg/L under US rules; we found no evidence that it harms soap, and distilled water takes the question away.
Distilled and deionised water both do the job: ISO 3696 lists single distillation, deionisation and reverse osmosis as ways to make its grade 3 laboratory water. Buy water sold for the purpose, with nothing added.
Rain water, boiled water and softened water
Rain water is not a free substitute. The US Centers for Disease Control and Prevention warns that collected rainwater picks up bird droppings, dirt and germs from the roof, and chemicals such as lead and copper, and copper is one of the metals that speed rancidity in soap.
Boiling does not soften water. Hardness comes as carbonate, or temporary, hardness and non-carbonate, or permanent, hardness (WHO). Heating throws some of the carbonate out as scale, the fur in a kettle, but leaves the non-carbonate hardness in the water, and boiling does nothing about copper or iron.
Water from an ion-exchange softener has had its calcium and magnesium swapped for sodium (WHO), which soap tolerates, so it is kinder to soap than hard tap water. It is still tap water in every other respect.
Water as a process material
Water does more than dissolve the lye. It dilutes liquid soap paste, weighed and hot, and distilled for the reasons above. It makes up a large part of cream soap: ours carries water at 60 % of the oils. It softens the shreds when soap is rebatched or hand milled, and the rebatching guide gives amounts. It dissolves the sugar for transparent soap, and Failor uses distilled water there too. And a quick rinse or a steamer takes soda ash off a curing bar, because sodium carbonate dissolves readily in water.
The tests on this site all start with distilled water. A pH reading is taken on a 1 % solution, 1 g of soap in 99 g of distilled water that has been boiled and cooled, because dissolved carbon dioxide pulls the reading down; the standard laboratory guide, ASTM D1172, uses freshly boiled water for the same reason. Dunn's purity titration for lye dissolves citric acid in 500 mL of distilled water. The minerals and dissolved gas in tap water would skew both.
How much water
Water takes no part in saponification, so the amount is a choice, stated in the calculator as a lye concentration, a water-to-lye ratio or water as a percentage of the oils. It starts at a 33 % solution. Sodium hydroxide cannot be dissolved much beyond about 52 % at 20 °C, so a 50 % solution is the practical floor for water. The water and lye concentration guide covers what more or less water does to trace, cure and soda ash.
Sources
- Hardness in Drinking-water: background document for development of WHO Guidelines for Drinking-water Quality (WHO/HSE/WSH/10.01/10/Rev/1) , World Health Organization (2011) . Hardness as the capacity of water to react with soap; carbonate (temporary) and non-carbonate (permanent) hardness; heating forms insoluble carbonates as scale; ion-exchange softening replaces calcium and magnesium with sodium.
- The Soap-Maker's Manual , E. G. Thomssen, D. Van Nostrand (Project Gutenberg edition) (1922) . Insoluble soaps of lime and magnesia; hard water forms insoluble soaps that settle out of liquid soap.
- The Handbook of Soap Manufacture , W. H. Simmons and H. A. Appleton, Scott, Greenwood & Son (Project Gutenberg edition) (1908) . Hard water is liable to produce opaque streaks of lime soap in transparent soap.
- Making Transparent Soap , Catherine Failor, Storey Publishing (2000) . Distilled or soft water for clear soap, including the sugar solution.
- Secondary Drinking Water Standards: Guidance for Nuisance Chemicals , US Environmental Protection Agency . Copper 1.0 mg/L; iron 0.3 mg/L.
- National Primary Drinking Water Regulations , US Environmental Protection Agency . Maximum residual disinfectant level 4.0 mg/L for chlorine and for chloramines (as Cl2).
- Superfatted soap bar containing a synergistic preservative mixture of EHDP and EDTA (US 3,511,783) , Lever Brothers Company (1970) . As little as 0.2 ppm of copper has a significant effect on the colour and odour of soap bars.
- Collecting Rainwater and Your Health: An Overview , US Centers for Disease Control and Prevention . Bird droppings, dirt and germs from the roof, and chemicals such as asbestos, lead or copper, can get into collected rainwater.
- ISO 3696:1987 Water for analytical laboratory use: Specification and test methods , International Organization for Standardization (1987) . Grade 3 water may be produced by single distillation, by deionisation or by reverse osmosis.
- ASTM D1172, Standard Guide for pH of Aqueous Solutions of Soaps and Detergents , ASTM International . Freshly boiled water and a fixed temperature.
- Little White Lyes (Handcrafted Soap and Cosmetic Guild conference) , Kevin M. Dunn (2015) . Purity titration: about 10 g of citric acid in 500 mL of distilled water with phenolphthalein.
- OECD SIDS Initial Assessment Report: Sodium hydroxide (SIAM 14) , OECD and UNEP (2002) . Maximum solubility about 52 % by weight at 20 °C.
- Lye calculator , soapformulator.com . Worked figures from the hard water guide: 1000 g of olive oil at a 33 % solution takes 261.2 g of water, which at 300 mg/L spoils about 0.48 g of soap in a 1390 g batch and, at the EPA copper level, brings about 0.19 ppm of copper. The liquid soap guide's worked batch takes 2082 g of dilution water; roughly 4 g of scum is the same two-soaps-per-ion arithmetic. Our cream soap carries water at 60 % of the oils; the calculator starts at a 33 % solution.