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Open Source Apothecary At Home / Preparations
You can distil a compound that boils at 200 C in a still full of water at 100 C. The reason is partial pressure.
methodAn essential oil is the volatile, steam-carryable fraction of a plant, separated by distillation and (usually) less dense than water so it floats.
Why steam distillation works at all — the one piece of physics worth knowing. Limonene boils at 176 C and thymol at 232 C, temperatures that would destroy the plant before they were reached. Steam distillation does not need those temperatures. In a mixture of two immiscible liquids, each contributes its own vapour pressure independently and the mixture boils when the sum of the two vapour pressures equals atmospheric pressure. Water's vapour pressure at 100 C is 760 mmHg on its own, so the mixture starts to co-distil at a temperature where water contributes most of the total and the oil contributes the small remainder — in practice well below 100 C. The volatile compound is carried over in the steam stream at a temperature far below its own boiling point, and the condenser then liquefies the pair and they separate by density. This is why a pot of boiling water can extract a compound whose own boiling point is 232 C, and it is the whole justification for the method.
What the method cannot carry — the mineral rule. Non-volatile and ionic material does not enter the vapour stream at all. Potassium chloride does not vaporise until around 1,500 C and magnesium oxide around 3,600 C. A still at 78-100 C discards 100% of the minerals and the fixed acids. No essential oil is a mineral tonic, no distilled spirit is a source of electrolytes, and any product claiming otherwise is describing something other than distillation.
Yield, and why the numbers are small. Yield is a fraction of a percent to a few percent of plant dry weight for most species and is highest in the peel, bud, and seed, and lowest in leaf and root. Practical consequences:
Cold expression, and why citrus is different. Citrus peel oil is obtained by pressing rather than distilling, because its terpenes are present in large oil glands in the peel and the material is cheap. The consequence is chemically important: cold-pressed citrus peel oil retains furanocoumarins that steam distillation largely leaves behind. Bergamot, lime, lemon and bitter orange peel oils are phototoxic on skin exposed to sun; the distilled versions of the same fruit are much less so. "Bergamot oil" on a label does not tell you which one it is, and the difference is the difference between a pleasant scent and a chemical burn.
The hydrosol is a real co-product, not waste. The water that comes over with the oil — lavender water, rose water, orange flower water — contains dissolved volatile compounds at around 0.1% or less. It is pleasant, mildly antimicrobial, and useful as a skin and room spray and as a flavouring. It is not a substitute for the oil, and it spoils within weeks to months because it is dilute and near-neutral; keep it refrigerated and treat it as a fresh product.
What an essential oil is not. It is not the plant. It is a concentrated volatile fraction, chemically much simpler than the whole herb, which is why the clinical evidence for a whole-herb preparation does not transfer to the oil and why the oil pages on this site carry their own trials.
AGREEprep and GAPI metrics highlighted MAHD as a sustainable and greener method.PubMed 41592503 ↗
Additionally, it provides insights into method optimisation for chromatographic profiling of VOCs and supports the development of sustainable extraction methodologies for bioactive compound characterisation that focuses on industrial applications in the pharmaceutical, cosmetic, and food sectors.PubMed 41038076 ↗
Overall, this review emphasizes that the sustainable development of P. harmala should rely on quality-by-design strategies integrating botanical authentication, chemotyping, green extraction, and regulatory-oriented toxicological assessment.PubMed 42796530 ↗
This endeavor will not only streamline the production of premium-grade essential oils with improved safety measures but also pave the way for novel applications in various fields.PubMed 39154494 ↗
These developments are consistent with a circular bio-based economy, where an industrial byproduct derived from biological sources is repurposed for new applications.PubMed 39407589 ↗
Equipment
Basic steam-distillation procedure
1. Prepare the plant. Chop coarse to open the oil glands. Dried or slightly wilted material often gives better yields than turgid fresh material because the cell walls have begun to break down. Do not powder — it scorches and clogs. 2. Load the basket above the water line. The plant must not sit in the boiling water; it sits in the steam. A little water under the basket, plant packed loosely so steam passes through it. 3. Bring to a boil and distil gently. A hard rolling boil pushes water over and can scorch; the aim is a steady stream of vapour, not a vigorous one. 4. Condense. Cold water into the condenser jacket, the cooler the better; warm condenser water is the commonest cause of oily vapour escaping as smell. 5. Collect until the smell drops off. Practical rule: 30-90 minutes for leaf and flower, 2-3 hours for root, bark and wood. Time past the point where the distillate stops smelling of the plant is wasted heat. 6. Separate. Let the collected distillate stand; the oil floats (or sinks, for a few dense oils). Decant or draw off with a separating funnel. The remaining water is the hydrosol — keep it refrigerated if you keep it at all. 7. Dry the oil. Trace water in a fresh oil promotes hydrolysis and microbial growth. Drop in a little anhydrous sodium sulfate or magnesium sulfate, let it stand, then filter the oil off the salts. 8. Bottle small, fill high, store dark and cool. Essential oils oxidise and polymerise; the headspace over a half-empty bottle is the oxygen that does it. 12-24 months for most oils, shorter for citrus peel oils, which oxidise fastest.
Yield expectations — this is what "a lot of plant" means
| Material | Typical yield (dry weight) | |---|---| | Peppermint, lavender, rosemary (leaf/flower) | 0.5-1.5% | | Clove bud, cinnamon bark | 1-4% | | Citrus peel, cold-pressed | 0.5-2.5% | | Ginger, turmeric rhizome | 0.5-2% | | Rose petals | 0.01-0.05% (this is why rose oil is priced as it is) |
Cold expression of citrus peel — press the peel, collect the emulsion, let it separate, decant the oil off. It is a mechanical method and it retains furanocoumarins.
Dilution for use (adult skin)
| Strength | Drops per 30 mL (1 fl oz) carrier | Use | |---|---|---| | 1% | ~6 drops | face, sensitive skin, children over 12 | | 2% | ~12 drops | general body use | | 3% | ~18 drops | short-term, localised | | 5%+ | ~30 drops | specific, limited, practitioner-directed |
A drop from a standard orifice is about 0.05 mL, so 30 mL of carrier holds about 600 drops.