Week 16 Desalination of iron Flashcards
CHLORIDE EXTRACTION FROM ARCHAEOLOGICAL IRON: COMPARATIVE TREATMENT EFFICIENCIES
David Watkinson
This paper compares four aqueous washing methods for extracting chloride from archaeological iron, specifically alkaline sulphite, Soxhlet hot wash, repeated boiling, and pressurized water. Results indicate that alkaline sulphite is the most efficient with an 87% extraction rate, followed by the combined treatments at 72%.
Samples for testing are chosen from archaeological objects, which introduces limitations in standardization.
Watkinson, D.E. (1996) why did he choose archaeological wrought iron
He chose archaeological wrought iron because it has a high degree of metallurgical similarity, even though it ranged from Roman through to early post-mediaeval periods. It is also likely to have a metallic core and originate from a moist, oxygenated burial environment, which would make it similar to other objects stored in the same way if they react in a similar manner. The nails chosen also have a fairly similar form, no matter what their size, which should introduce greater standardization of weight to surface area ratios.
how and why does chloride get into iron objects, and why does it need to be removed
Chloride can get into iron objects either through direct contact with chloride-containing substances, such as sea water or salt, or through environmental exposure to chloride-containing substances, such as industrial air pollution. Chloride accelerates the corrosion of iron and so needs to be removed to slow down the rate of corrosion and extend the life of the object.
what is alkaline sulphite and how does it help to extract chloride from iron
Alkaline sulphite is a chemical solution used to extract chloride ions from corroded iron objects. It works by breaking down the counter-ions that hold the chloride ions in solution, which allows the chloride ions to diffuse out of the object. The presence of sulphite also aids chloride removal by increasing the mobility of hydroxyl ions, which help to flush out the chloride ions.
how does chloride accelerate iron corrosion
Chloride ions act as a corrosion accelerator for iron because they are highly soluble and can penetrate deep into the metal surface. The chloride ions can react with iron to form iron chloride which is highly soluble in water. This increases the rate of corrosion by providing a source of electrons for the iron oxidation reaction and also by creating an electrochemical imbalance in the metal surface, leading to an increased rate of corrosion.
how deos a soxhlet hot wash work
A Soxhlet extractor is a device used in the extraction of compounds from solid material using a solvent. It is made up of a thimble of solid material placed inside a cylindrical glass container with a distillation head and condenser. The solvent is boiled in the distillation head and the vapors condense in the condenser and then run down the sides of the glass container, dissolving the desired compounds in the thimble. The solvent then passes through a siphon tube which returns it to the distillation head, repeating the cycle. The Soxhlet hot wash is a method for extracting compounds from solid material using a hot solvent. The solid material is placed in a Soxhlet extractor and the solvent is heated to a specific temperature and allowed to pass through the extractor. The hot solvent dissolves the desired compounds in the thimble, which can then be collected and analyzed.
tell me about the repeated boiling method for chloride extraction
This method involves the immersion of the iron object in boiling water, followed by a gradual cooling period. The process is repeated several times, allowing the water to extract any chloride ions present in the object. This method is often used in combination with other treatments, such as alkaline sulphite or pressurized water, to increase the efficacy of the treatment. The repeated boiling method is generally considered to be less efficient than other methods, such as alkaline sulphite, in terms of chloride extraction. However, it is still a viable option for chloride extraction and can be used to enhance the efficacy of other treatments.
Watkinson, D.E. (1996) what were the results, looking at the secondary treatments
The results of the secondary treatments showed that collectively, Soxhlet, boiling and pressure treatments removed extra chloride in 11 of the 12 tests carried out. This was never greater than 8%, and was 5% or less in seven of the 12 samples tested. No single one of these secondary treatments was seen to be more efficient than the others.
what is alkaline sulfite, its materials, and how does it facilitate the removal of chloride ions
Alkaline sulfite can be used in the conservation of metal objects and artifacts. It is typically used in the treatment of iron and other metal objects to remove corrosion-causing chloride ions. The alkaline sulfite is composed of sodium hydroxide, sodium sulfite, and water. The sodium hydroxide helps to raise the pH of the solution, which helps to remove chloride ions from the iron. The sodium sulfite helps to form a protective film over the metal surface, which prevents further oxidation and corrosion. The water acts as a solvent, allowing the sodium hydroxide and sodium sulfite to reach the metal surface. The combination of sodium hydroxide and sodium sulfite helps to create an environment that is conducive to the removal of chloride ions, as well as preventing further corrosion. The presence of highly mobile hydroxyl ions in the alkaline sulfite solution also helps to aid the removal of chloride ions, as the hydroxyl ions are attracted to the chloride ions and help to flush them away from the metal surface.
how does washing with De-aerated alkali work
De-aerated alkali washing works by preventing corrosion during treatment and by using hydroxyl as a counter ion to extract chloride from archaeological iron. The hydroxyl ion acts as an electron donor, which removes chloride ions from the surface of the iron object and prevents further corrosion. This makes the de-aerated alkali washing method one of the most effective and predictable chloride extraction methods.
whats ferrous hydroxychloride
Ferrous hydroxychloride (ß-Fe₂(OH)3C1) is a corrosion product of iron that can form in damp burial environments. It is composed of iron [II] and iron [III] solutions that have been hydrolyzed in the presence of chloride and incorporated into tunnels within its hollandite type crystal structure, as well as adsorbed on its surface. It has been identified in the dense corrosion product layer above the metal surface of archaeological iron objects.
what kind of corrosion products are formed on archeological iron
The corrosion products that are formed on archaeological iron include iron oxides (such as akaganeite (ß-FeOOH), ferric oxyhydroxide, ferrous hydroxide and ferric hydroxide), sulfates, chlorides, and alkaline sulphite reduction products.
whats the difference between ferrous hydroxide and ferric hydroxide
Ferrous hydroxide, or Fe(OH)₂, is a divalent iron-containing compound that is in a hydroxide ion state. Ferric hydroxide, or Fe(OH)3, is a trivalent iron-containing compound that is in a hydroxide ion state. The main difference between the two compounds is the number of valence electrons in each iron atom. Ferrous hydroxide has two valence electrons, while ferric hydroxide has three.
if the wash environment is de-oxygenated, is chloride better removed from iron, and why
Yes, chloride is better removed when the wash environment is de-oxygenated. This is because oxygen is known to interfere with the removal of chloride from the archaeological iron, as it can form compounds with the chloride ions that make them more difficult to remove. De-oxygenating the wash environment helps to minimize this interference and can thus help to maximize the amount of chloride removed from the iron.
An Investigation of Deterioration of Archaeological Iron: Quanyu Wang
This paper examines iron objects from British Roman and Anglo-Saxon sites, with the aim of determining suitable storage environments. Samples were randomly selected from two Roman sites and three Anglo-Saxon sites, and underwent various conservation treatments. The objects were stored in airtight containers with silica gel or in cardboard boxes or wooden trays in a glass cupboard. Recommendations are made for future storage of similar objects.
