Causticum - The Riddle Unravelled

For almost two hundred years, experts have disputed what actually forms when Hahnemann's preparation instructions for Causticum are followed. Karl Heinz Jansen and Dr med. Dirk Thomas Quak repeated the procedure numerous times in a modern research laboratory, even using glass vessels from Hahnemann's era, and analysed the distillates with present-day analytical methods. Their finding: the distillate is neither pure water nor a weak potassium hydroxide solution, but a basic aqueous solution of ammonium hydrogen silicates.

The Constantin Hering Foundation filmed the laboratory experiments. The film and the full paper are available here for download.

The Question

In the Chronic Diseases, Hahnemann describes exactly how Causticum is to be prepared: freshly burnt lime, roasted “double-acid sulphate of potash” (potassium bisulphate), hot water, a glass flask, the still-head sealed on with a moistened pig’s bladder, distilled to dryness. The distillate, he writes, is as clear as water, smells like caustic potash lye, tastes “puckering at the back of the tongue and exceedingly burning in the throat”, freezes below the freezing point of water, and promotes the putrefaction of preserved animal substances. At the same time, he demonstrates that it contains neither sulphuric acid nor lime.

By today's chemistry, this distillation should leave potassium hydroxide and gypsum behind in the flask, with only water passing over into the receiver. But water has none of the properties Hahnemann describes. Attempts at explanation range from a “chemical absurdity” (1841) to the assumption, still widespread today, put forward by Grimm (1989), that Causticum is a weak potassium hydroxide solution carried over into the receiver by bumping.

What Hahnemann Was Looking For

Like many chemists of his time, Hahnemann was searching for the “caustic principle” of alkalis: a substance that gives lime and the alkalis their corrosive property and that could be separated from them. Acid–base chemistry did not yet exist; the hydroxide ion was not postulated until 1887, 44 years after Hahnemann's death. His procedure was an attempt to release this “caustic substance” by saturating the base with an acid and to carry it over with the steam. It was precisely at this point that he was mistaken, by today's understanding: basic properties cannot be separated off as a substance in their own right.

The Experimental Setup

Following the original instructions, in each run, 50 g of slaked lime and 50 g of potassium sulphate were triturated together with 50 ml of boiling water in a porcelain mortar and distilled to dryness over about 90 minutes, the distillate being collected in fractions of around 10 ml. Three apparatus set-ups were used: a modern still made of Duran glass, a combination of a Duran flask and an alchemical still-head, and a flask of green soda-lime glass with a matching still-head dating from around 1830. The origin and preparation of the starting materials, the water quality, the sealing material, and above all the heating regime were varied — gentle heating in an oil bath or rapid heating with a heating mantle up to 420 °C, with continuous temperature monitoring.

Each fraction was analysed chromatographically for cations and anions (detection limit approximately 10 µg/l), as well as for amino acids and pH, and photometrically for silicates (detection limit approximately 20 µg/l). Before each experiment, the set-up was checked for blank values using a water distillation; the historical glassware was additionally eluted with ultrapure water over several days and showed no abnormalities.

What Was Found in the Distillate

• Modern still, pure chemicals: The distillate is indistinguishable from water. Hahnemann's smell test remains negative.
• Pig's bladder as seal: Ammonia is detectable in the distillate. Without the bladder, from the ammonium salts of natural limestone alone, 11.4 mg/l was found in the final fraction, corresponding to about 0.7 mg/l across the roughly 50 ml of distillate as a whole. With the bladder, the value in the final fraction rose to 60.6 mg/l, and to around 35 mg/l across the whole distillate; this amount has a distinct smell and shifts the pH into the alkaline range.
• Historical soda-lime glass: The glass releases silicates into the hot potassium hydroxide solution, which pass over with the steam. With added glass fragments, 5 to 6 mg/l were measured; in the historical flask without any addition, 3 to 4 mg/l.
• Heating regime: Gentle heating in an oil bath produced markedly smaller amounts of both ammonia and silicate than rapid heating.

Why the Bumping Hypothesis Does Not Hold Up

Hahnemann tested his distillate with two precipitation reactions: barium chloride for sulphate, and ammonium oxalate for calcium. Both tests were negative. Had reaction mixture been carried over during distillation by bumping, all ions would have been carried over in proportion to the original mixture — including sulphate and calcium.

The calculation here is unambiguous: a single splash of 50 µl would give around 325 mg/l of sulphate in the receiver. That is roughly 150 times higher than what Hahnemann's barium chloride test would still be able to detect. Conversely: a potassium hydroxide solution that could have remained undetected within these detection limits would have amounted to at most 0.8 mg/l. This would not be perceptible by taste (the taste threshold for potassium hydroxide lies between 1 and 50 mg/l), would not be caustic, and does not explain Hahnemann's description of the taste.

The second assumption — that potassium hydroxide might sublime under intense heat and pass into the receiver as a gas — likewise could not be confirmed. Potassium hydroxide melts at 360 °C and does not boil until 1327 °C; towards the end of the distillation, temperatures in the still-head and outlet tube are far below 100 °C, so that potassium hydroxide would have to crystallise out along the way. In the experiments, the flask was held at about 400 °C for several minutes during the dry phase without any increase in potassium concentration in the distillate.

The Process Step by Step

1. The starting materials are prepared. Limestone is burnt at about 1000 °C and slaked with water, producing slaked lime.

CaCO₃ → CaO + CO₂ · CaO + H₂O → Ca(OH)₂

The “double-acid sulphate of potash” is heated until it glows. It loses water and sulphur trioxide, leaving potassium sulphate behind.

2 KHSO₄ → K₂S₂O₇ + H₂O → K₂SO₄ + SO₃

2. Potassium hydroxide solution forms in the mortar. Slaked lime, potassium sulphate and hot water react, with the evolution of heat, to form potassium hydroxide solution and gypsum.

K₂SO₄ + Ca(OH)₂ + 2 H₂O → 2 KOH + CaSO₄ · 2 H₂O

From 50 g each of the two salts, calculation gives 574 mmol of potassium hydroxide (32.3 g) and 287 mmol of gypsum (49.4 g); 383 mmol of lime remains in excess. This “magma” is placed in the flask.

3. On heating, the solution becomes ever more caustic. The water gradually evaporates, and the potassium hydroxide solution in the flask becomes more concentrated and hotter. The conditions for the next step therefore only arise in the course of the distillation and are most pronounced at the end.

4. The hot solution attacks the glass and dissolves out silicates. Soda-lime glass, of the kind available to Hahnemann, is attacked by hot potassium hydroxide solution; silicon dioxide passes into solution from the glass surface as silicic acid. The potassium sulphate in the mixture acts as a mediator here, because it brings poorly soluble metal oxides into solution.

SiO₂ + 2 H₂O → Si(OH)₄

How much dissolves depends strongly on pH and temperature: at pH 7 and 25 °C it is around 100 mg/l, at pH above 12 and 100 °C around 10,000 mg/l. Silicic acid is volatile with steam and rises with the vapour. Modern stills made of Duran glass release practically nothing; this is precisely why their distillate remains pure water.

5. Protein drips back into the solution from the seal. The soaked pig's bladder is placed around the neck of the flask and the still-head set on top; part of the bladder lies against the inside and extends into the vapour space. Heated along with the glass, fats and proteins are released, creep along the glass surface and drip back into the flask. In the hot potassium hydroxide solution the proteins are broken down, producing ammonia, alongside which lower amines may also occur. This arrangement, with the sealing material on the inside, produces the largest amounts of ammonia: 60.6 mg/l in the final fraction, compared with 11.4 mg/l without the bladder.

6. Ammonia and silicic acid rise together. Both are volatile, pass with the water vapour into the still-head and run over the spout into the receiver. The salts in the flask — gypsum and excess lime — remain behind. This explains why Hahnemann's tests for sulphate and calcium were negative.

7. Ammonium silicate forms in the condensate. Ammonia reacts with water to form ammonium and hydroxide ions and keeps the pH above 9 to 10. In this range, the silicic acid remains deprotonated and in solution, rather than condensing into polysilicic acids; the solution remains clear.

NH₃ + H₂O ⇌ NH₄⁺ + OH⁻

2 NH₄OH + Si(OH)₄ → (NH₄)₂SiO₃ + 3 H₂O

8. The distillate remains stable, for the time being. The excess ammonia stabilises the dissolved silicates. If the solution is acidified or warmed, ammonia escapes and the liberated silicic acid polymerises into a gelatinous silica gel. This behaviour is consistent with the observation that, in one experiment using the historical apparatus, crystals precipitated in the first fraction after 24 hours of cooling; these dissolved again in alkali and gave a positive reaction for silicate.

What the fractions show: The ammonia content rises from fraction to fraction, because the seal remains hot for longer and the solution becomes more concentrated; the silicate values, by contrast, remain largely constant. Gentle heating in an oil bath produces markedly less of both than rapid heating, which confirms the sequence described above.

The Result

Two components of the historical apparatus thus together produce what Hahnemann describes: the pig's-bladder seal the ammonia, the soda-lime glass the silicic acid. In the distillate, the two combine to form soluble ammonium hydrogen silicates.

These compounds have the properties Hahnemann attributes to his Causticum: completely water-soluble, volatile with steam, colourless, similar in smell to caustic potash lye, astringent and burning in the mouth, lowering the freezing point, promoting putrefaction, free of sulphate and of calcium. The authors' own taste test on their distillate confirms Hahnemann's description, down to the drying sensation in the mouth and throat that persists for hours.

The authors present this as a hypothesis, not as conclusive proof: the analysis shows ammonia and silicates in the distillate and rules out potassium hydroxide solution as an explanation; the formation of the ammonium hydrogen silicates themselves is inferred from the measured values and from known chemistry.

What This Finding Means

The remedy picture of Causticum is unaffected by this finding; it rests on Hahnemann's provings and on the documented use of the remedy since then. What is new is the answer to the question of which substance was actually being proved. Without realising it, Hahnemann produced a compound unknown to the chemistry of his time and made it available as a medicinal substance. For present-day preparation, this means: without soda-lime glass and without the addition of protein, no Causticum in Hahnemann's sense is formed - only distilled water.

Publication, Film and Funding

• Publication: Karl Heinz Jansen, Dr med. Dirk Thomas Quak: Causticum: Neue Wege zu einer alten Wahrheit [Causticum: new approaches to an old truth; in German]. jaqu-invent, Institut für homöopathische und naturmedizinische Forschung GbR, Fürstenfeldbruck, Germany.
• Film: The laboratory experiments, filmed by the Constantin Hering Foundation.
• Funding: The research on Causticum was funded by the Sanddorf-Stiftung (Sanddorf Foundation), Regensburg.

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Authors: qk | Rev.: glt | Editor: pz | last modified September 25, 2026