Category Archives: Anthropology

Cool Pots

Pots (Photo:Tim Jones)
Historic olla water cooling pots made by Native Americans in San Diego County. Possibly Kumeyaay or Diegueno origin. (Photo:Tim Jones)

Maybe it was the furnace heat of California last month, or the topicality of NASA’s Curiosity landing, but here I am having my first – and almost certainly last – von Däniken moment. 




How else though, aside from some ancient Martian visitation, could Native Americans of centuries past, without the benefit of telescopes or interplanetary probes, design water pots so closely matching the Red Planet?

Well, on reflection, I guess a mixture of clay and cactus juice might just bake out that way in the sun.

Which brings us to the real science behind these earthenware pots.  Because although they may well be over two hundred years old, discovered in 1926 by my wife’s geologist great-grandfather in the desert of San Diego County, these water carrying olla represent nothing less than the world’s first refrigerator.

The larger olla in-situ, San Diego County, 1926, complete with geological hammer for scale. Ollas were not truly ‘fired’, but hardened by baking in the sun (Scan of original photo belonging to Tim Jones)

The water inside the olla reaches a temperature substantially below that of the surroundings thanks to the principle of evaporative cooling – something you can demonstrate to yourself just by licking a finger and waving it around.  The skin feels cooler because the heat needed to turn liquid water molecules in your spit into vapourised water molecules leaving your hand is taken from your skin.  The amount of heat, or energy, needed to change from a liquid to a gas is called the latent heat of evaporation, which for water is 2257 kJoules per kilogram.

Kumeyaay (Wikipedia)

The sun-baked porous clay of the olla acts like a wick, delivering a constant flow of evaporating water to the surface where it quickly evaporates, cooling first the surface and in turn the water inside the pot.

Wondering how effective ollas really are, but with live tests on our delicate pots off the agenda, I turned to theoretical musings and some (not entirely successful) experimentation.


The Theory

The temperature on the pot’s surface, or wet-bulb temperature, is easy enough to calculate if we know the ambient air temperature, relative humidity (how much water is already in it), and local air pressure – as that affects the dew-point temperature at which water changes from liquid to gas.  I got all that info from my local weather station online, and plugged it into one of the many online calculators – like this one at the National Oceanic and Atmospheric Administration (NOAA) – to find the wet bulb temperature. (The exact calculation is complex and explained on the NOAA website, but essentially the drier the air, the lower the wet-bulb temperature; water molecules already in the air decrease the net evaporation rate.)

The day I looked at this, the values were: temperature 34 C, relative humidity 20%, and air pressure 1014.9 millibars, for which the NOAA calculator returned a wet-bulb temperature of about 19 C.  That’s a whole 15 degrees below ambient temperature; modern electric fridges don’t do much better than that (okay – granted they can get to lower absolute temperatures).

Wet-bulb thermometer (Photo:Tim Jones)

The wet-bulb temperature I verified experimentally using a cooking thermometer modified with wet paper-towel stuffed in around the sensor tip (a mercury thermometer would have a bulb of mercury at the end – hence wet-bulb; but this was all I had and works well enough).  Swinging the thing fast round my head on the end of a shoelace simulated wind and, lo and behold, I recorded a wet-bulb temperature of 21C.  Not quite the predicted 19C, but in the right area.

Good ventilation of the olla is necessary as it influences the evaporation rate, and the area for evaporation should be large (the olla’s spherical design is fantastic in this regard as it maximises the area).  The area of non-wetted contact should be small to minimise absorption of heat by conduction from the surroundings – here again, the point contact of the spherical olla is perfect.  Ollas also work better in the shade, to minimise heating by solar radiation.

Calculating how long the contents take to cool is more tricky, requiring an estimate of the evaporation rate from a porous surface.  But we can get some handle on it using an assumed rate of 7kg/ (based on some data I found for swimming pool evaporation rates in Australia of all things), latent heat of evaporation of water 2257 kJ/kg, and heat capacity of water 4.18 kJ/kg.K.  From which I reckon the 0.3m diameter olla, holding 14 litres (=14 kg) of water, needs to lose 878 kJ of heat to fall in temperature by 15 degrees, equal to evaporating 0.4 litres (0.4kg or 3% of the contents) from the 0.28 m2 surface over a 5 hour period.

The numbers aren’t perfect, but suggest in its heyday our olla was up and usefully cooling in a couple of hours.

The Practise

Now for the not-totally-successful experimentation part of the post.

Plant pot evaporative cooler (Photo:Tim Jones)

You can see what I’m trying to do here: my very own plant pot olla.  The physical conditions (temp.,humidity,pressure) were the same as the theoretical calculation; and I’d confirmed a wet-bulb temperature of 22C as described above.  The pot was kind of working too; that dark band in the middle and top is water seeping through the porous terracotta – and it was pretty consistent throughout the experiment.

Plantpot water cooler

Stirring the contents and taking regular measurements indicated a one degree fall over the first two hours.  But then the temperature started to climb again, which suggests the pot was just not porous enough over sufficient area to counter heating by conduction through the non-wetted areas.  A lack of wind won’t have helped – maybe use a fan next time.  At least no one can accuse me of selectively publishing only positive results.

Other Coolers

The Arab Zeer works in a similar fashion to the olla, but consists of two pots separated by wet sand.  Fruit and other perishables can be kept fresh in the central pot.

Pot-in-pot, Zeer type evaporative coolers (Wikipedia)

A modern invention is this Terracooler: an evaporatively cooled terracotta bell-jar placed over food to keep it fresh.

Evaporative cooling in water features is enhanced by an electric pump (Photo:Tim Jones)
Evaporative cooling in water features is enhanced by an electric pump (Photo:Tim Jones)

And keep an eye out for artificial waterfalls used to create a cool atmosphere in public spaces, or the same principle operating in simple garden water features: the water in this one I measured at 24C on a 32C day.

I’m back in the UK now, where typical humidity levels close to 100% (=rain) preclude the extensive program of further evaporative cooling tests this discussion clearly signposts.  If you have more luck with your own ollas though, do let me know.

Thomas Huxley and the Return of the Rattlesnake Bones

The Guardian this week reported on the UK Natural History Museum’s efforts to repatriate a collection of  human bones, acquired by explorers in bygone years, to their original home with islanders in the Torres Straits.

Outrigger sailing canoe alongside "The Rattlesnake" (Fronticepiece to T.H. Huxley's Diary of the Voyagfe of H.M.S.Rattlesnake)
Outrigger sailing canoe alongside "The Rattlesnake" in the Louisiade Archipelago (Fronticepiece to T.H. Huxley's Diary of the Voyage of H.M.S.Rattlesnake)

It’s not a piece I’d linger over save for the mention of H.M.S. Rattlesnake, a 19th century survey ship involved in, among other duties, the collection of anthropological specimens.   Moreover, the Assistant-Surgeon on the 1846-50 voyage was the young Thomas Henry Huxley, very much cutting his teeth in hands-on nature study and ethnography.

Self-Portrait, Thomas Huxley on H.M.S. Rattlesnake (Huxley's Rattlesnake diary)
Self-Portrait, Thomas Huxley on H.M.S. Rattlesnake (Huxley's Rattlesnake diary)

Regular readers will know I’m quite a fan of the man later known as ‘Darwin’s Bulldog’, so any association with what we now recognise as unsavoury cultural violations demands a look-see.

Huxley worked alongside ship’s Surgeon Dr Thompson and Naturalist John MacGillivray, under the overall command of Captain Owen Stanley.

His Rattlesnake Diary, only published in 1935 by grandson Julian, captures thoughts and details of the voyage with a candour absent from more official reports.

The two diary entries that mention human artifacts, in this case a jaw bone bracelet, give some feel for the circumstances in which such pieces were obtained and the way Huxley spoke  about the indigenous peoples.

And as we have Julian Huxley’s thoughts on his grandfather’s behaviour (via his editorial commentary), there’s an opportunity to compare the ethics and cultural norms in anthropology not only between the mid-nineteenth century (when the bones were collected) and the present day (manifest in the Natural History Museum’s repatriation efforts), but also with the norms prevailing in 1935.

Human Jaw Bracelet (MacGillivray, Narrative of the Voyage of the Rattlesnake)
Human Jaw Bracelet (MacGillivray, Narrative of the Voyage of the Rattlesnake)

On to the diary entries.  In June 1849, with the Rattlesnake anchored among the islands of the Louisade Archipelago, Huxley describes an apparent overnight change in the local people’s willingness to barter a jaw bone ornament:

24th. Sunday [June 1849]

Huxley: “We had four or five canoes off to barter with us this morning – such squealing and shouting and laughing and yelling was never heard!  One of the niggers had a human jaw by way of a bracelet.  There was one tooth in the jaw and the circlet was completed by a smal bone apparently of some animal lashed to the coronoid process.

The old fellow would not part from it for love or money.  Hatchets, looking-glasses, handkerchiefs, all were spurned and he seemed to think our attempts to get it rather absurd, turning to his fellows and jabbering, whereupon they all set up a great clamour, and laughed.  Another jaw was seen soon in one of the canoes, so that it is possibly the custom there to ornament themselves with the memorials of friends or trophies of vanquished foes.” [Entry continues.]

Things have changed by the next day.  Huxley doesn’t mention any additional enticements that might have been used to achieve this, although it’s clear from other parts of the diary that iron and tools were particularly valued:

25th. [Monday, June 1849]

Huxley: “Several canoes came off this morning; one of them brought the figure-head which was so much wanted yesterday, and bartered it immediately.  In one of the canoes was a man with a jaw bracelet.  The jaw was in fine preservation and evidently belonged to a young person, every tooth being entire.  They seemed to have no scruple in selling it.  A jade hatchet was procured from them also.” [Entry continues.]

H.M.S. Rattlesnake

The jaw is also mentioned in a more formal report by MacGillivray in his Narrative of The Voyage of H.M.S.Rattlesnake (2) (And from which the drawing of the jaw bone above is taken.)

MacGillivray: “…But the most curious bracelet, and by no means an uncommon one, is that made of a lower human jaw with one or more collar bones closing the upper side crossing from one angle to another.  Whether these are the jaws of former friends or enemies we had no means of ascertaining; no great value appeared to be attached to them; and it was observed, as a curious circumstance, that none of these jaws had the teeth discoloured by the practice of betel chewing.”

First off, Huxley’s vernacular is alarming to modern ears – and this from a bastion of 19th century intellectual enlightenment.  Likewise, we wouldn’t by present standards in these circumstances take a willingness to hand over cultural artifacts as ethical licence to receive them.

Moving to Julian Huxley’s editorial.  Introducing a chapter titled “Huxley and the Savages”,  J.H. appears to be at pains to rationalise, if not apologise for, certain of T.H.’s behaviours, in doing so revealing his own predudices:

“He had none of the trained anthropologist’s insight into the black man’s mind, little conception of the alien ways of thought and feeling in which a primitive savage is enmeshed.  His reactions were those of a generous-minded young man with plenty of common sense but a strong feeling for justice.  He felt that there was some absolute standard of moral behaviour by which both the explorers and the natives could and should be judged.  On the whole, he censured his white companions more hardly than  the Papuans and Australian blacks.”

Although his views changed radically in later life, there’s a consistency here with Julian Huxley’s advocacy for Eugenic principles, a belief in the genetic basis for differences between human groups, and the concept of genetic inferiority.  I read the passage as an oblique approval of T.H.’s egalitarian sense of justice, but with the suggestion he’s applied it through ignorance and an incorrect assumption that blacks and whites are fundamentally the same.  One wonders what T.H. would say, had he the benefit of a time machine, in 1935?  Would he ask his grandson, politely, to stay off his team?

Thomas Huxley
Julian Huxley

From this example, it does start to look in some important respects like cultural attitudes in 1935 hadn’t progressed as much as one might think from those of Victorian times.  And were museums still accepting human artifacts in 1935? (I suspect they were, but please speak up if you know).  I doubt there was much repatriation of bones going on.

Well, that turned into something of a Huxley-bashing session afterall.   In fairness, isolated diary extracts don’t  give the most rounded impression of a person and, as I actually think the Rattlesnake diary does a particulary good job of that for Huxley, I’ll close by encouraging you to make a full reading (it’s not too long, very readable, and not at all boring).

Update 13.3.11Natural History Museum news release on the Torres Strait repatriation (10.3.11)

Update 6.5.11 Torres Strait Island Community ancestral remains return begins and video

Update 23.11.11 Museum Returns 19 Ancestral Remains to Torres Straits Islanders (Natural History Museum)



(1) T.H.Huxley’s Diary of the Voyage of H.M.S. Rattlesnake. Ed. Julian Huxley, Chatto and Windus, London 1935

(2) Narrative of the voyage of H.M.S. Rattlesnake, commanded by the late Captain Owen Stanley during the years 1846-50

John MacGillivray, George Busk, Robert Gordon Latham, Edward Forbes, Adam White – 1852

(3) The Huxley File, Guide 2, Voyage of the Rattlesnake. Charles Blinderman, Clark University.

(4)  Natural History Museum returns bones of 138 Torres Strait Islanders. Guardian newspaper, 10th March, 2011


Also of interest: Julian Huxley and the Invention of the Public Scientist (BBC Radio 4)


Photographs are taken from the author’s copy of T.H.Huxley’s Rattlesnake Diary and public domain sources.