Collection: Field Notes - Regenerative Materials | 

Series: Natural Ropes | 

Working out how to make an effective natural fibre rope preservative from first principles, and what the trials suggest about whether it should work

How I arrived at this

The eco-safety comparison post works through the environmental profiles of the treatments in the Atkins and Purser trials. The conclusion it arrives at is uncomfortable: the treatments that performed best in the trials — copper soap and coal tar combinations — are also the treatments that are most ecologically problematic in marine environments. Copper is a potent aquatic biocide, toxic to algae, molluscs, and crustaceans at low concentrations. Coal tar is rich in polycyclic aromatic hydrocarbons, several of which are persistent marine pollutants and human carcinogens. Creosote, the coal tar distillate used by Plymouth fishermen at the time of the trials, is now effectively banned for marine use across much of Europe.

Stockholm tar sits in a more defensible position — meaningful protection, manageable environmental profile — and it is what I use for most rope treatment. But the trials data for Stockholm tar alone, while useful, leaves a performance gap relative to the copper-tar combinations that is real rather than negligible. The question I kept returning to was whether there is a metal soap treatment that closes some of that gap without the copper's ecological burden.

The answer, suggested by the trial data and by the chemistry of metal soaps generally, is iron naphthenate. The commercial product Cuprinol Brown — the formulation Atkins and Purser describe as iron naphthenate, probably with some zinc naphthenate, in a tar oil carrier — held sisal rope at around 81% retained strength over seven and a half months in the Plymouth Sound trials. That is significantly better than Stockholm tar alone, and it uses iron rather than copper. Iron is an essential marine micronutrient rather than a biocide at the concentrations involved. The ecological case for iron over copper is clear.

Cuprinol Brown is not currently manufactured in a form I can obtain. So the question became whether I could make the functional equivalent.

What metal soaps are and how they are made

Metal soaps are the salts of fatty acids with metal ions. Ordinary household soap is a sodium or potassium salt of fatty acids — sodium oleate, potassium stearate. Replace the sodium or potassium with a metal like copper, iron, or aluminium, and you have a metal soap. The dressings and treatments post discusses copper naphthenate and copper oleate as the commercial products in this family. The same chemistry applies to iron.

The standard synthesis route for metal soaps is double decomposition. Make a sodium soap first — sodium oleate, produced by saponifying oleic acid with sodium hydroxide. Then add a metal salt solution — ferrous sulphate, in the case of iron oleate. The metal ions displace the sodium ions, and the metal soap precipitates out of solution:

2 NaOleate + FeSO₄ → Fe(Oleate)₂↓ + Na₂SO₄

The iron oleate sinks. The sodium sulphate stays in solution. Filter, wash, dissolve in your chosen carrier. In principle, straightforward. In practice, there are several variables that I have been working through, with mixed results.

Step one: making sodium oleate

Oleic acid is available from craft and soap-making suppliers — it is the primary fatty acid in olive oil and can be purchased in relatively pure form without difficulty. Sodium hydroxide is lye, standard soap-making supply, available in most hardware shops and from brewing suppliers.

The first question was solvent. Oleic acid does not dissolve in cold water without vigorous emulsification, which makes the saponification uneven and the resulting sodium oleate inconsistent. I tried water first, on the basis that it was simpler, and produced a lumpy partial emulsion that saponified incompletely. The sodium oleate that resulted had a grainy character and did not dissolve cleanly in the subsequent steps.

Bioethanol is better. Oleic acid dissolves readily in ethanol at room temperature, and sodium hydroxide dissolved in a small amount of water will saponify it cleanly in a homogeneous solution. The reaction produces sodium oleate as a soft, slightly translucent soap that is easy to work with. The complication is that ethanol in the sodium oleate solution interferes slightly with the subsequent precipitation step — it lowers the polarity of the aqueous phase and can slow or partially inhibit the iron soap forming as a clean precipitate.

The practical solution I am currently using: saponify in bioethanol, then warm the mixture gently with the lid off for fifteen to twenty minutes to drive off most of the ethanol before proceeding to the double decomposition. Adding the sodium oleate to a large volume of hot water at this stage means the precipitation happens in a predominantly aqueous environment, which produces a cleaner and more complete precipitate. Whether this optimisation actually matters for the end product's performance in rope I cannot yet say — I have not run a comparison between iron oleate made with and without the ethanol-driving step. It is on the list.

Step two: the double decomposition

Ferrous sulphate — iron(II) sulphate — is sold in crystalline form as a lawn conditioner and moss killer. It is cheap, widely available, and the right iron compound for the reaction. Dissolve it in hot water at a concentration of around 200–250g per litre — a dark green solution with a faint metallic smell.

Add the sodium oleate solution slowly to the ferrous sulphate solution while stirring. The iron oleate precipitates immediately as a brownish-green solid that collects at the bottom and sides of the vessel. The colour shift is satisfying in a way that suggests the reaction is proceeding correctly, though I hold that impression loosely — aesthetic satisfaction in chemistry is not a reliable quality indicator.

Filter through cloth, wash the precipitate with warm water several times to remove residual sulphate, and allow to drain. The precipitate at this stage is wet and relatively soft — it needs partial drying before it will dissolve cleanly in the carrier solvent.

The yield is reasonable but variable. I have not worked out a reliable relationship between input quantities and output mass, partly because the washing and drying steps remove material at variable rates depending on conditions. I am working toward a more controlled process and do not yet have one.

Step three: the carrier

This is where most of the performance uncertainty lives. The trials data for Cuprinol Brown describes a tar oil carrier — a product described as "a specially good tar oil" that the authors suggest is central to the treatment's effectiveness. They are probably right. The iron soap alone, dissolved in a neutral solvent, would provide biocidal activity but not the physical exclusion of water that a tar carrier provides, and not the lubrication that the tar fraction delivers to the fibre bundle.

Stockholm tar as carrier is the logical choice for a natural materials system. Warmed to around 50–55°C, it will dissolve iron oleate reasonably well at concentrations of around 10–15% by weight — approximately 100–150g of dry iron oleate per litre of warmed tar. The resulting preparation is darker than Stockholm tar alone, slightly more viscous, with a smell that is still predominantly pine resin.

I have applied this preparation to hemp and manila rope using the immersion method — rope dried to below 15% moisture content, treatment warmed to around 55°C, immersion for a minimum of four hours. The rope emerges darker than Stockholm tar alone would produce, with what seems like slightly better surface consolidation. Whether it performs better in sustained seawater exposure I cannot yet say with confidence. I have rope in salt water exposure testing now, but the results are months away from being interpretable.

The honest position at this stage is that the chemistry is sound, the preparation is achievable without specialist equipment, and the preliminary results are encouraging. Whether the performance matches the trials data for Cuprinol Brown is unknown, and will remain unknown until I have data to compare.

The zinc question

The trials describe Cuprinol Brown as likely containing iron naphthenate with probably some zinc naphthenate. Zinc oleate can be made by exactly the same double decomposition route, substituting zinc sulphate for the ferrous sulphate. I have not added zinc to my preparation, partly because zinc accumulates in marine sediments and has documented toxicity to aquatic invertebrates — the ecological case against zinc is less clear-cut than against copper, but it exists — and partly because I want to understand what the iron soap alone does before introducing another variable.

Whether zinc adds meaningfully to the iron soap's performance in the way that the original Cuprinol Brown formulation apparently intended is something I cannot determine from the trials data, because the iron-only and iron-zinc variants were not separately tested. Atkins and Purser describe the commercial product; they do not reverse-engineer its composition and test variants. That gap is one I may try to address eventually, but it is not the current priority.

The naphthenic acid question

Naphthenate is a salt of naphthenic acids — complex cycloalkane carboxylic acids derived from petroleum. The commercial Cuprinol products in the trials used naphthenate rather than oleate as the fatty acid component. Oleate is derived from plant oils and is the more natural-materials-compatible choice, but the trials compared copper naphthenate and copper oleate directly and found them broadly comparable in performance, which suggests the specific fatty acid chain matters less than the metal ion and the carrier.

I am using oleate because oleic acid is readily available in a form I am confident about. The equivalent naphthenic acids are petroleum-derived and harder to source in the small quantities that would be useful for experimentation. Whether iron naphthenate would perform better than iron oleate in rope preservation is an open question that the trials data does not settle, and that I have not tested directly.

A note on the reference

The foundational reference for understanding metal soap synthesis is Whitmore and Lauro's 1930 paper in Industrial and Engineering Chemistry — "Metallic Soaps: Their Uses, Preparation, and Properties" — which remains the standard citation for the double decomposition pathway. It is paywalled through the ACS but accessible through most university library systems. For a modern, open-access overview of metal soap chemistry including the synthesis pathways and their variants, Russo et al.'s 2023 paper in npj Heritage Science — written in the context of metal soap formation in historical paintings, of all things — is freely available and covers the relevant chemistry clearly.

Neither source addresses rope preservation directly. The connection between metal soap chemistry and rope preservation runs through the Atkins and Purser trials, which describe the commercial products without the synthesis detail, and the subsequent work in this series in trying to reconstruct a synthesis that produces something functionally equivalent. That connection is the gap this post is trying to bridge, and it is a work in progress rather than a completed one.

Where things stand

The preparation is repeatable. The process is accessible without specialist equipment. The preliminary results on treated rope are encouraging without being conclusive. The key unknowns are: whether iron oleate in Stockholm tar carrier performs comparably to iron naphthenate in tar oil carrier under sustained seawater conditions; whether yield and concentration consistency can be improved with a more controlled process; and whether adding a zinc component improves performance enough to justify the additional ecological concern.

I expect to have better answers to the first question in the coming season. The second is a matter of refining the process, which is ongoing. The third may wait until the first two are clearer.

What I can say with confidence is that making a metal soap preservative for rope is not a specialist operation requiring unusual chemistry knowledge or equipment. It requires oleic acid, sodium hydroxide, ferrous sulphate, and Stockholm tar — all of which are available without difficulty and without specialist sourcing — plus patience with a process that is messy in the way that most genuinely useful chemistry is messy, and willingness to hold the results as provisional until there is enough evidence to be more definite. Those are constraints I can work within.

Sources: W.R.G. Atkins and J. Purser, The Preservation of Fibre Ropes for Use in Sea-Water, Journal of the Marine Biological Association of the United Kingdom (1936). W.R.G. Atkins, The Preservation of Fishing Nets by Treatment with Copper Soaps and Other Substances, Journal of the Marine Biological Association of the United Kingdom (1928). W.F. Whitmore and M. Lauro, Metallic Soaps: Their Uses, Preparation, and Properties, Industrial and Engineering Chemistry (1930). Silvia Russo, Laura Brambilla and Edith Joseph, But aren't all soaps metal soaps? A review of metal soap applications and occurrence in cultural heritage, npj Heritage Science (2023).

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