Seaweed is eaten in such diverse locales as Wales, Japan, and Belize. Agar, the gel upon which microbes are typically grown in a lab, and carrageenan, used to stabilize and thicken a diverse group of products including shampoo and soy milk, are derived from seaweed.
|Irish moss is a drink is made from Gracilaria seaweed (Source)|
Historically, certain seaweeds were collected, dried out, and then burned in large quantities to reduce them to ash. Two useful substances, sodium carbonate and iodine, could then be extracted from the ash.
Sodium carbonate has many applications but is perhaps best known for being an ingredient in laundry detergent (it's a builder, meaning it softens water and thus makes surfactants more effective at removing oil and grease) and silica-based glass (it serves as flux, drastically reducing the temperature required to melt the silica, also known as quartz sand). During the 18th century and early 19th century, most flat glass had a slight blue-green colour because it was being made using seaweed ash. The development of a cheaper means of sodium carbonate production (the Leblanc process) led to a decline in the use of seaweed in glass production. It's possible to identify seaweed glass by its relatively high strontium content, which is the result of this metal being present in seawater and absorbed by seaweed as it grows.
Iodine, a relative of the elements bromine and chlorine, was first discovered in the early 19th century when a French dude noticed that treating seaweed ash (after washing it with water to extract the sodium carbonate) with sulfuric acid produced a vibrant violet vapour (iode is French for violet-coloured). Here's a video to give you an idea of how this went down. As is the case with strontium, iodine is taken up by seaweed from the surrounding seawater as it grows. Iodine went on to be used as a disinfectant (after being dissolved in alcohol or water) and a means of treating goiter (thyroid enlargement due to iodine deficiency, which results in a swollen neck). It was obtained from seaweeds such as Fucus vesiculosus (bladderwrack), a common inhabitant of the coasts of northern Europe and traditionally used to treat thyroid problems. Iodine deficiency is particularly uncommon in Japan on account of seaweed featuring prominently in Japanese food. However, eating too much seaweed (often due to taking pills of it as a supplement) can cause the thyroid gland to go nuts (hyperthyroidism) on account of the large amount of iodine introduced into the body.
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