Sunday, 13 September 2015

Chalk Lamps?

Carved Chalk Bowls found at Grimes Graves
How people in Neolithic and Bronze age Britain generated light is still a mystery, it is assumed the people used ceramic lamps fuelled by animal fat as their main source of light. There is good reason to make this assumption as stone lamps fuelled by animal fat go back as far as the Palaeolithic and continue in use till the 18th century changing from stone to pottery after its invention. Also they were the main form of light throughout most of antiquity and medieval times.

Palaeolithic Stone Lamp from La Mouthe, Dordogne
When the Neolithic Flint Mine Grimes Graves in Suffolk England was excavated 45 small bowl like objects made from chalk were discovered. They were found at different locations and in both Neolithic and Bronze age shafts (similar objects have also been found on Thanet in Kent and in Wiltshire). One explanation proposed for these bowls was that they were lamps used by the miners and as no other lighting source had been found, it seemed to answer the question of what light did the minors work by. However the interpretation from the moment it was made proved controversial. 

Wiltshire Chalk Bowl
An alternative theory was already in existence that miners worked by natural light which reflected down the shaft off the white flint walls. This though is contradicted by the 19th century flint miners working for the Brandon Gun Flint Industry in the same area, using similar shafts and techniques who did not use natural light, instead mining by candle light.  Other challenges to the vessel being lamps are based on manufacturing material, stating a chalk vessel would simply not make a good lamp.  However the interpretation of these chalk bowls as lamps was backed up in the 1980's when some cups under analyses showed to have residue of tallow in them. However as the cups showed no other sign of usage as lamps such as burn marks from the wick on the rim or charring, so the results of this analyses were disputed.

A second study conducted in 2011 which examined 11 of the 45 bowls contradicted the results, finding no residue of animal fat (but residue of Gum Arabic and other modern substances not around in the Neolithic/Bronze Age, suggesting the artefacts had been contaminated after they were found). This study concluded the bowls were most likely not lamps but did accept the possibility animal fat may not be present because it had not survived the millennia. The survey however did note the yellowing of the  chalk and may have been as a result of a burning flame inside turning the white flint yellow. The study also argued that these bowls  were of limited use as mining lamps as they would only have a burn time of around an hour.
Thanet Chalk Bowls
So I decided to conduct an experiment to see if I could add any information to the mystery myself. Not having the laboratory and chemical analyses capabilities of the previous two studies I decided to go in a different direction and conduct a prolonged usage test. If they were lamps they would have been in continual long term use, burning for hours on end for weeks or months. The question of whether they are capable of this and how they would stand up to prolonged use is unanswered.

I carved several lamps out of chalk pebbles found on my local beach. I used rendered pork fat which had set solid as the fuel and a vegetable fibre wick. I burnt each lamp for 6+ hours a day for a week making several analyses .

Burn Time and Refill
6mm Wick
The 2011 survey suggested they may not be suitable as lamps as the lamps only burn for around an hour. The first experiment I conducted was burn time. I used three sizes of wick and a 30mm x 20mm x 10mm piece of pork fat.  A 2mm wick burnt for around 3 hours, the output of light was that of a weak candle. For the second test I used a 4mm wick which reduced burning time to around 2 hours and produced the flame of a strong candle. Finally I used a 6mm wick which produced a strong flame, easily that of 3 or more strong candles but last no more than an hour.

2mm Wick
With the 2mm wick the pork fat around the flame turned to liquid but the rest stayed solid while it was burning. This was not the case with the thicker wick where all the fuel turned to liquid. This meant with the thin wick is had to be moved several times during the burn to stop it going out. With the other two wicks, the fuel ran to the wick and needed no maintenance once lit.

The lamps with a thick enough wick provides an excellent source of light for 1-2 hours. That the fuel went to liquid made is more difficult to move the lamp but also made it very easy to add fuel to while burning. I must also disagree with the 2011 survey that it burns too shorter time to be a viable source of light as fuel can easily be added while burning..

Wick Position + Refill Test
Char marks from the wick on rim of bowl
In Palaeolithic lamps, wicks were positioned leaning on the side rim of the lamp, this left scorch highly visible scorch marks. The Neolithic chalk bowls show no such burn mark on the rim, which suggests they must have had central wicks. Central wicks are unnecessary as side wicks burn just as well and are more difficult to create as the tallow turns to liquid and they need to be propped in position. . Just why they would have gone to the trouble wick disposable seems strange.

Secondly with the lamps having a short burn time I wondered how easy it would be to add fuel to the lamps while burning. The answer to this is with a central wick it is impossible to add tallow to small burning lamp and extremely difficult to a large one. On the other hand it is relatively easy to refill a lamp with a side wick. This would suggest if used as lamp miners went to the inconvenience of letting lamps burn out and relighting them as the fuel burnt down rather than simply adding fuel. This could be explained by flames being readily available from other lamps. 

Side wicks are easy to create, burn just as well and allow the lamp to be refilled without extinguishing. Central wick are more difficult to create inhibit the refilling of lamps. This test makes it seem unlikely they were lamps. 

Absorption and Leakage and Overheating
Lamp cut in half showing the pork fat absorbed all the way through
Flint is very porous and I did wonder if over prolonged usage the bowl would actually continue to contain all the liquid or it would be absorbed straight through the chalk leak out the bottom. After the first use of the lamp, there was no sign of absorption or leakage however this drastically changed. After the second use a small 15mm round moist patch of pork fat was visible at the bottom of the pot. Overnight this turned into a large patch covering half the bottom area and some of the sides. Within three days the bowl was completely discoloured by the pork fat, which had been absorbed throughout. This fat melted when the bowl warmed while in use. However the was no leakage of fat, the bowl restricted itself to be damp to the tough and leaving a stain on where it stood.

Another potential problem with usage as a lamp was how well does chalk absorb heat and how hot the chalk would get after the lamp had been lit for a while. The answer proved to be connected to fat absorption. The lamp proved cool for the first day's usage, the exterior temperature of the chalk was little effected by burning the lamp for several hours. However once the pork fat had been absorbed completely through the lamp this changed. The heat of the melted pork fat in the lamp was transmitted by the pork fat absorbed into the lamp the the pork fat on the exterior of the lamp. Temperature varied on different burnings, with weaker flames it was mildly warm to pick up, while with strong flames it could get quite hot, occasional it got hot enough that it was uncomfortable to how, but never painful or scalding. 

A Bowl used for a week alongside and unused one to show discolouring
The porousness of the chalk doesn't seem to be a problem and the bowls hold the pork fat adequately enough for a mining environment. The exterior stays cool enough to make the lamps easily movable even after several hours of use. 

The final test was to test how much would prolonged usage would discolour the chalk and could it explain the yellow colouration of the bowls as suggested by the 2011 study. 

After a week's use, the absorption of animal fat in the lamp was so complete it changed the lamp colour from white to murky grey. Also redder heat marks appeared on the the bottom of the lamp which used the centre wick, this did not happen with the side wick. 

The question then has to be what happens to this discolouration over millennia? Tests on lamps in the 2011 survey found no traces of animals fat, this doesn't mean they were not used as lamps as it could simply suggest the fat did not survive, though one was orange perhaps from being exposed to heat perhaps consistent with the redder discolouration appearing at the bottom of the lamp. 

Having no complex equipment I simply took a week long discoloured lamp and left it to soak in a bowl of warm water and washing up liquid. After a few hours the liquid had dissolved the fat completely and the result was the chalk lamp returning to a pure white as if unused. 

The test shows lack of discolouration can't be considered evidence of lack of use as a lamp. It all depends on the unknown factor of whether traces of pork fat can survive in chalk from Neolithic times. 

Bowls unused in test
So far no compelling evidence has emerged to prove if the chalk vessels were lamps of not. The 1980's study found animal fat residue in some of the vessels, but no scorch marks, however as this test shows, not finding marking on the lamps is not problematic. The 2011 test found no animal fat but only tested 1/4 of the vessels, and also it is unknown how well animal fat survives time. However did find evidence of modern day contamination which possibly make any positive result on the lamps invalid. 

This test has shown the bowls could be used as lamps, provide enough light and show little visible signs of being used as a lamp, providing the animal fat doesn't survive. However it also showed usage of lamps in this manner would not be very convenient for mining,  refilling would be awkward and burn times short. While living in the Far East at some animist shrines I saw small similarly sized vessels are available at the entrances. These are filled with tallow, lit and one can be taken in by a worshipper who places it on in front of the main image in the shrine, prays and leaves it there. As these vessels burn out the shrine priest collects the bowls and replaces them at the temple entrance for newly arriving worshippers to use. As these lamps are constantly reused, they go to all the trouble of having central wicks so to leave them pristine for next worshipper and are small as they only need to burn for a short period of time. At the moment research into the usage of chalk vessels is rather hampered by a dichotomy, mining lamps of not lamps. While I'm not suggesting the Far Eastern scenario is true for British chalk vessels it perhaps does show there is a whole world of other possibilities out there.