Medieval fire retardant?

In the last issue of the journal of the Historical Metallurgy society, there was an article on a Byzantine gold smiths manual from the 11th century. Amongst many other things in the manual was a recipe that translated, said:


Another mysterious method
Take unslaked lime, mix oil into the lime, wetting it as thoroughly once or twice. Then pour it into a distillation vessel. Also add a caustic solution [?] whereby you pour it all over until it is 2 fingers deep. Now fill this divine water into another flagon. After that, take a linen cloth and dampen it with this water and expose it to the fire. If the cloth burns, you know that it was not prepared well. Make the lime preparation again with other un-slaked lime and repeat as before until successful, ie until the fabric no longer catches fire.

My attention was caught by this, and I began to wonder what it was and whether it actually worked.
The article’s author only had this to say about the recipe:
Un-slaked lime has been a component of widely varied fire-proof products throughout the modern period. See the Curioser Kunstler (1 133/257/305/348)

Not enough for my curiosity. So I decided to try and replicate the recipe.
Now, at first sight it looks pretty silly. Oil and quicklime? The oil would prevent the quicklime from contact with water, but then if you distilled it with a caustic solution, what you would get might well be a solution of metallic salts, which when put onto cloth would result in, when dried, a layer of metal complex salts that I suppose would retard combustion a bit.

I set up my furnace as below, with the pot containing the ingredients sitting on top of some broken bricks, to raise it above the flames.



I mixed 30 grams of quicklime with olive oil, being what I had handy. Then I mixed some sodium hydroxide crystals with water, and poured that in as well. It started boiling over pretty quickly after I put it in the flames, and I lost a fair bit of vapour that did not condense out, although it was probably water. After about 20 minutes I stopped the process, and what was left in the bottom of the vessel was a lump of slightly foamed material.

What had come over was liquid, and smelt funny. It was perhaps a little bit more gloopy than I would expect for plain water.

So I dipped several pieces of linen into the liquid and dried them. They all burnt, although just a little bit slower than normal. This was not good, but not unexpected given the uncertain nature of what I was attempting. The recipe says to expose them to the fire after dipping them in the solution, but seeing as you can get the same effects by dipping the cloth in water, I figure that the liquid is supposed to form a coating. After all, a fire retardant coating would be useful on something like an apron, which comes close to a fire regularly, but it would be very impractical to keep it wet all the time.

After some googling and reading up on things, it appears that linseed oil would be the best to use, there being mention of its use in fireproof glue.
(http://www.wholesaleexporter.com/fireproof-glue.htm)
It also seems that anaerobically heating linseed oil to 300C makes it polymerize. I think that such a polymerized oil would not be so easily burnt, and would thus act as a fire retardant. In fact some polymers are sold as fire proof, because they are highly polymerized and their outer layers are already reacted with oxygen during the manufacturing stage, meaning there is nothing that will catch fire, unless you apply so much heat as to break down the polymer chains themselves.

So next time I shall use linseed oil, and boil it for longer at higher temperatures. This should help isolate the exact way this works. Possibly the action of the lime and sodium hydroxide solution is to keep the linseed oil isolated from the atmosphere enough to begin polymerizing. Or maybe not. It is frustratingly unclear.

Calcining lime and ashes

At last, doing what my blog is named after
I bought some garden lime a few weeks ago, which is basically crushed limestone. Limestone is calcium carbonate, CaCO3, and needs to be burnt in order to convert it to CO, calcium oxide, or quicklime. This is caustic (Alkaline), and used for destroying stuff. Also, if you mix it with water in the approved manner, you get lime mortar. Which then, over the years after it has been applied, absorbs CO2 from the air to become calcium carbonate again. All you are doing is making limestone into a form that is easily used for building, and then letting it turn back into limestone once in place.

I need it for several things. To start with, one or two odd chemical recipes require quicklime. (More on them this weekend) Furthermore, I want to see what I can produce and how much and how quickly, to get an idea of lime kilns and their work, and also whether I can make enough to do interesting things with. For example, it would be good to demonstrate making slaked lime (That’s the calcium hydroxide you get by adding water to the calcium oxide) and then using it to stick a couple of stones together.

Today’s take home fact is that lime mortar is re-usable, you just have to burn it.

Procedure:
Take two bits of scrap stainless steel from work. Place lime onto them. Get fire going, and heat up furnace to well over 1,000C. Hold at that temperature for half an hour. Stir lime occasionally. At this stage it should be so hot that any wood you put into the furnace bursts straight into flames.
Working a lime kiln must have been very hot, sweaty work. Holding over 1,000C for days at a time, with mixes of lime and coke or coal or wood tipped in the top would have been hard work. You can still see a lot of limekilns around Scotland, when local supplies of lime were exploited in the 18th and 19th centuries, in order to improve the farmland and build more houses. They are made of brick or stone, and are generally set against a bank or hill in order to make it easy to tip the mix of coal and limestone into the top of the furnace. After some time the material would have sunk to the bottom of the furnace, where I assume it was tapped to allow the lime out, although there would have to be holes to allow air in as well. I cannot find anything useful online about how lime kilns worked.

Regular readers will have noticed my grousing about a lack of reactions between some sand and boiled ashes. So during last week I decided to calcine the ashes I had in order to purify them. The end result was a much smaller amount of brown stuff. They lost 38% of their weight. Now, I am assuming that this was mostly carbon compounds that were oxidized readily in the over 1,000C temperatures that they reached.
The photo below shows the steel sitting in the furnace, which is glowing warmly.



Further information on Lime can be found at these places:

The Scottish Lime centre

Charlestown lime kilns

Sal Alkali and glass making

So, wondering what Sal Alkali is for? Simple- it and quartz sand are all you need to make glass. Oh, and a furnace capable of over 1,000 degrees, maybe a bit more. Have I mentioned that I like working with fire?
What I have done over the past few weeks was produce alkaline powder by burning beech wood. (see previous posts about this and sourcing the wood) Theophilus says to use ashes, but I wanted to see how much salt I could produce, since by the early 16th century glassmakers were certainly making the salt, rather than just using ashes. This has the advantage of meaning you have more control over the glass and what is in it, since simply using the ashes means you can end up with bits of earth and suchlike from the original material, such as bracken roots. (See previous post)

The end result was, not a lot of salt.
I faithfully took the ashes, and boiled them up in a pot, until they were dry, and had turned into a fine gray powder. This powder tested at a pH of about 10, so definitely had some caustic properties in it. I suspect it has a mix of sodium and potassium salts as well as bits of charcoal and other things.

So, to make glass, I weighed out 20grams of ordinary sand with 10 grams of the salt I had previously made. I mixed it up as much as possible, then placed it in the pot you can see in the picture below:


The furnace was set up with a 2 brick deep volume for the charcoal, and the bellows in their usual place. Then, with it lit, I bellowed it up to over 1,000C in an hour or so. It takes a long time to reach thermal equilibrium between the inside and outside of the furnace. With the crucible inside, and glowing red, I held it at 1030-1050 for 10 minutes, and stirred the sand/ salts mix occasionally. It definitely fizzed a little around the thousand mark, and oddly enough potassium carbonate melts at 980C…

This photo shows the pot in use:


The problem was that the pot actually cracked during use. It has since split into several parts, and it is clear that it is too thick, and could not cope with the different heating rates of different parts of the pot. Fortunately I did not lose any of the fritted mixture.

In this photo you can see the two ingredients and what they combined to make. The sand is at the top, the boiled ashes to the bottom right, and the fritted material to the bottom left. It doesn’t look very good I am afraid.


Next time I shall heat it up to over 1,000C, and mix in more boiled ashes. I suspect that the percentage of sodium and potassium salts might not be high enough, so if I mix more in I stand a better chance of getting the sand to melt. The salts melt at a lower temperature and act as a flux, taking the sand (silica) into solution at a much lower temperature. Otherwise I would have to go to at least 1600C for silica to melt. Which is impossible with my set up.
Next time I shall hopefully get a nice blob of glass.

What should I make and do for demonstrating medieval technology?

Any ideas? I have quite a lot of possibilities, involving various amounts of preparation and materials and explanation.

For starters, I could make Gold. Either mosaic gold, which may or may not require the use of mercury. Or I could tin plate lead. Or I could colour silver to make it look like gold, using either distilled eggs or a solution of sulphur and lime that apparently colours copper or silver yellow.

Then I could make some acid- nitric, HCl, Sulphuric. There is quite a list, but the problem is in demonstrating their uses, especially since I do not have any gold to experiment upon. I can mix them with some limestone, which would fizz satisfactorily.

I could distill eggs again. That would get me funny looks, but isn’t very entertaining except to hardened geeks. On the other hand small slow drips of yellow falling into a pot do catch peoples attention. It would be a good lead in to a discussion on the topic of early alchemy though.

Casting metals is also a good one, I have yet to see someone do that at an event. I intend to be the first. (At least in medieval circles that I am in)
There are so many things to be made, from thimbles up to spurs, or belt buckles, and large objects like a mortar and pestle, and candlesticks. I have not seen many people with medieval candlesticks, however they would be expensive in terms of material, and also time to make the mould and then cast the metal into it. (Also, you wouldn’t actually carry them around with you on campaign.) Nevertheless, they are what I aspire to do.
Simpler things can be made using pewter, also including belt studs, buckles, plates, tankards etc.
Of course, I would have to keep people a sensible distance away, at least 3 feet I think. As long as it is small amounts of metal I am dealing with, avoid moisture etc, there is little danger.

Faking coins. What I envisage is this:
I am sitting at my furnace, with a few silver coins of the period of the event. I also have a clay mould of said coins, which I show to the crowd. I then cut up one of the coins, put about 2/3 of it into a crucible with some copper or tin to make up the weight, then melt it, and cast it into the clay. By doing this several times I can end up with more silver than I started with, and more coins, thus netting more money. Once I crack the mould open, I pass the fake coin round, but my assistant will have nipped out the back to get some people pretending to be the local law officers. I will scarper out the back just before the law gets there, who will then arrest the MoP holding the fake coin, for counterfeiting, and lead them away, possibly explaining loudly to the crowd the punishments for counterfeiting.
I like it. Entertaining and educational, all in one go. I know who to get the coins from as well, I have plenty of copper and can get some silver.

I could also do some glassmaking of small things, like glass phials or beads. This would only be tricky insofar as the weather would have to be good and a lot of heat would be required. It would however be do-able, since it appears that the melting point of potash and sand based glass is around 1,000C, which my furnace can easily reach. There are also the health and safety aspects of trying to blow glass, especially in front of people.

On a more industrial scale, it would be easy to make quicklime. I would set up my furnace like a chimney, and pile in limestone and charcoal. It would however smell a lot, necessitating a large field, and a strong wind to take the pollution away. The same goes for ore smelting. I could roast copper ores to remove the sulphur, smelt iron ore using charcoal, ore and limestone. In fact the latter has already been done at Rivaulx abbey:
Iron ore smelting

It would be nice to repeat it. I do know one or two people with spare fields.

The tricky bits
Apart from all the fun stuff, the other problem is holding the crowd’s attention. Now, I don’t expect them to be 8 deep around me, but I have to be able to hold peoples attention whilst doing preparation for my experiments. This may mean I need an assistant to do some stuff and hand me some tools and equipment as I need them. I shall probably require a bellow monkey, to be paid either with beer or whatever else comes handy. Failing that I need a pyromaniac whose content to watch the flames all day and pump up the heat when necessary. I’m sure there are one or two of them around.

Several of my experiments or demos can be done at once. For example, I could start something distilling on top of the furnace, after having put on some metal to melt in the furnace, and when all that is going on, I can be moulding some clay, or grinding something up. This would keep me always busy doing something, so that the passing MoPs might see something they are interested in. The drawback is that MoPs are often very shy. If you are busy, many of them will not interrupt, except the annoying ones with stupid questions. So I need to have ways of opening it all up to them, so they can ask questions.

The other way of doing it is to have a lecture of some sort (obviously not the same kind as at school) where I show people things. For example, I can have half made clay moulds for showing people, as well as used ones, and the raw materials for making them. This would be easy enough to do, and keep them interested for the 5 or 10 minutes necessary before something else happens. The same thing can be done with some raw material, eg uncovering it and explaining how it is made. I could scrape the Spanish green off copper whilst something is distilling in the background, for example.

Another approach related to lecturing might be to set specific times when I will be doing something, such as casting a hand bell at 11am. This would enable people to gather to watch, and save me having to start and perhaps stop activity at odd times. All this would require is a friendly events organizer including me on the program, or a chalkboard somewhere obvious with timings on it. I would start with a general introduction, show of tools and materials, and get stuck in, hopefully with them asking questions as I work.

Each approach has its ups and downs, and to be honest, I shall be winging it at first. Writing specific plans shall be left until after I have some experience to back up my opinions. Outlines however are useful for jogging the memory, and I shall be writing them in my medieval notebook, using a quill pen. In fact this post is an outline, written to solicit comments on what I might be better off doing.