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Archive 2006

At each event we ask for a volunteer to record their experiences and impressions. These articles are published in our newsletter and eventually appear here as an archive of our activities. Note that these are only provided for reference, and are not endorsed by Event Leaders. Interpretations can change often and rapidly in Geology, any information stated should be verified from appropriate sources before being quoted.

The Art Bronze Foundry  Ardingly Sandstone and Tombstones at Fletching  Teaching the Dynamic Earth  NHM Earth Science Library  The Vale of Wardour, The SW Wiltshire Anticline  Shetland  Harwich and Wrabness, Essex

Visit to The Art Bronze Foundry (London) Limited

16 February 2006

Round by the gas works in Michael Road near Fulham Broadway is a restaurant ornately decorated with bronzes. The Art Bronze Foundry is next door. One or two people turned up and we waited, only to find that everybody else was inside in the warm. The shop was just big enough for our party.

Sculptures come in all sorts of material – from papier maché to ornately carved wood. This means that the first stage has to be quite gentle. So the lost wax method is eminently suitable. First a negative is made. Macwax, a releasing agent is sprayed on the model and then silicone rubber is applied with a spatula. This method gives good, accurate coverage. Three-dimensional models need a join. For example the join on heads passes vertically through the ears. Locating pegs help the two parts fit neatly together. Any errors here will give a wonky bronze. Silicone rubber is flexible, which helps separation, but its flexibility is also a disadvantage. It has to be supported by a plaster cast for the next stage to make an accurate representation. The cast is made of casting plaster possibly supported with scrim.

Now a positive is made. Molten wax is painted on to a thickness of 1/16th – 1/8th of an inch. This is an ideal thickness for bronze casting. Thicker bronze would distort too much as it shrinks on cooling and solid bronze could never be used. More wax is swilled in to join the pieces. The wax model is then separated from the silicone rubber mould. It is finely inspected and cleaned up to perfection. To help inspection the wax is coloured red colour and lighting is soft to prevent reflections. We only saw Paul use a blunt spatula and old green kitchen scourer. Obviously the finished casting cannot be better than the wax model so this stage is painstaking. Paul was working on a life-sized torso. The wax was sturdy and did not need support but there were peculiar ragged holes in the back walls.

One of the beauties of the lost wax method lies in the ease of connecting cut outs by wax sprues as well as attaching larger wax pathways which the molten bronze will follow during casting. The wax model looks like a peculiar insect with random appendages at this stage.

We went out to the casting room, which was delightfully warm on such a cold day. However on a hot day temperatures may reach well over 45°C.

Now a ceramic shell (investment) is made which completely surrounds the wax (inside and out). We were told that this involves at least eight dips whilst the model is spun. Now we could see the reason for the holes cut in the wax mould as they allow the ceramic core to form. Nails hold the inner and outer shells together. These shells are light and strong but, even so, vulnerable places are reinforced with chicken wire. There is a cup like structure at the top connected to several larger diameter wax sprues where molten bronze will be poured in and fed to all parts of the model. Normally the foundry only casts once a week but they did a special one for us.

First the inverted assembly is heated in a gas-fired kiln. During this time the wax mould and sprues melt and run out. The temperature is carefully controlled so that the molten wax can be collected for re-use rather than burn away. The hot assembly was put the right way up into a metal tank and propped up using stones ready to receive the molten alloy. The foundry prefers to use gun-metal (85% copper; 5% tin; 5% lead; 5% trace elements) but can make casting in other alloys (e.g. spelter, a lead/zinc alloy) if asked. Bronze ingots, which weigh about 20lb, are melted in a plumbago (graphite) crucible. Graphite is particularly suitable, as it gets stronger as it gets hotter. The temperature of the casting metal has to be between 1000°C and 1200°C. Although optical pyrometry could be used to measure the temperature a quick rough and ready cheap alternative is to stick a red-hot iron rod in and if the bronze does not stick the alloy is hot enough.

The process, once the crucible leaves the furnace, has to be quick. Two people controlled the crucible using handles at either end of an iron harness but an overhead gantry did the lifting controlled by a third who worked pulleys – somewhat akin to rubbing your tummy whilst patting your head. The bronze must not be allowed to cool too much as, if it becomes too sticky, it will not flow into all the cavities in the mould.

First slag was scraped from the top into a pit on the floor. Then each ceramic assembly was quickly filled. Finally the almost empty crucible was laid gently at an angle on the floor to cool. Cooling was quite rapid. Bits of ceramic seemed to pop around the tank. This is inevitable as bronze shrinks on cooling. After about half an hour cooling the ceramic shell was knocked away with hammer and chisel. We saw a flattish model (of a women’s back) appearing. It turns out that flat shapes are difficult to cast, as the mould tends to be slightly flexible. Philip pointed out imperfections running across but was not concerned. Separating the casting from the investment is a good job for an apprentice as it is carried out in the yard whatever the weather. It might be unskilled but it is very important to remove every last bit of ceramic from the core as subsequent treatments include acid etching.

Back inside again we went to see the next stage in tidying up the casting called chasing. All sprues are removed and any traces of them obliterated. The pieces are welded together and the joins filled. Full advantage is taken of the excellent malleability of bronze to fill small holes simply by hammering with a steel tool to slide metal about. Sometimes the bronze is a little thin where the moulds have flexed so more can be welded in. The judgement of the craftsman is all-important at this stage. For instance we saw specific areas (knee and elbow of a prone body) being worked on to highlight them. At this stage the casting is a beautiful golden colour but it is vulnerable to corrosion. If left in London the casting would turn black and, indeed, some bronze statues have been painted black to avoid extra labour. At the seaside the copper content leads to green corrosion.

The idea of the patination process is to accelerate corrosion according to the sculptor’s wishes. There are various treatments: potassium sulfate leads to a black finish, ammonium chloride with potash gives a green finish and iron III nitrate gives an attractive brown. Treatment with a bismuth salt leads to a rather curious white finish. However once again the craftsman’s touch is all-important. For instance none of the finished artefacts we saw were a bland flat colour but all had highlighted areas.

Many famous sculptors brought their sculptures to the Art Bronze Foundry. If you have been to Kings Lynn you may have seen Vancouver’s statue and in Westerham there is Winston Churchill. Epstein and Finck are two of the more famous sculptors who brought their models to the Foundry. There was a Bafta bronze on the table. It is quite heavy so it is likely that a less dense material has replaced the original bronze judging by the ease with which recipients wave their masks around at the awards ceremony.

The Art Bronze Foundry is eighty years old. The Foundry, whilst using traditional methods, readily uses modern materials. Silicon rubber moulds, which can be reused, have replaced gelatine negatives. The investment used to be made from bulky grog and plaster but, in the same way that plaster casts protecting broken bones have become thinner whilst retaining strength, these new shells are thin, light and strong. The turn round from original model to finished article is less than a week. This brings it own problems; the Foundry is dependent on a regular supply of customers to keep the plant operating.

Bill Hayter carefully explained legal aspects. The sculptor owns the original and may specify that the moulds can be used a certain number of times, say six. However it is unlikely that all six bronzes will be made at the same time. The foundry stores the moulds and when the sculptor has found a buyer he requests another casting. When the specified number has been made the mould is destroyed. Apparently this has not stopped a third party (an institution that should have known better) from requesting their own casting on the side but this is unacceptable.

When bronzes are sold the dimensions are carefully recorded. This cuts out the possibility of forgeries by making a new mould from a bronze. There are three stages at which shrinkage occurs so any forgeries made from that positive will be too small.

It is interesting to speculate when a bronze becomes a work of art. We all know that a painting by Claude Lorraine for instance is worth much more than a copy made by a pupil. But casting a bronze involves input by craftsmen as well as the designer and their work is an integral part of the finished article. The intellectual property holder has the original idea and produces the original, which is unlikely to have the nuances such as depth of colour of the finished bronze casting but they will have been consulted at each stage and signed the bronze. (Although it has been known for this to be done ‘in house’ when the sculptor has sufficient confidence in the foundry.)

I would like to record our thanks to the team at The Art Bronze Foundry for making us so welcome and going to such trouble to give us an insight into the history and development of casting bronzes.

Diana Wrench

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The Crags of Sussex: Ardingly Sandstone led by Gareth George,
followed by Identification of Tombstones at Fletching

1 April 2006

Article to follow

Doug Clements

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Teaching the Dynamic Earth

A demonstration of geomodels for teaching Earth Science.
Set up by Paul Grant and delivered by Nikki Edwards.

18 May 2006

Instead of our usual Thursday evening talk, we invited Paul Grant to come and give us a demonstration of the Earth Science Teaching models developed by ESTA (Earth Science Teachers’ Association) and the Earth Science Education Unit at Keele University. Paul had whetted our appetite by showing us a video clip of his model of mountain-building processes when he talked to us about the Baetics last year.

In the event Paul was called away at short notice to accompany Imperial College students to the Baetics so he sent along Nikki Edwards to guide us through the models.

Paul had brought along the models a few days before – 6 big boxes and a gutter pipe. It was all very exciting and setting it up with my colleague Jill Darrell and Nikki was a bit like opening up a Christmas stocking. Nikki has her own models and unpacking Paul’s produced a few surprises for her: Jill was despatched for some extra bits of kit.

And so the session began. As an ice-breaker we sat in groups and arranged rock specimens at appropriate positions in the rock cycle. Then in our groups we were launched to play with the activities on offer.

Laurie spent most of the time repeatedly applying heat to a small piece of granite and dunking it in cold water and managed to destroy it. Several of us had fun with the gutter. It was half-filled with sand then tilted up so that we could pour water from the top which overflowed into a bucket at the base. We each selected a monopoly house and placed it where we thought it would best withstand the ‘melting glacier’ from above. I can’t remember who won but I know mine, strategically placed near the top at one side, was moved down slope almost immediately. I think the winner – about the only one not to move - was situated on a stable island within the braids. So our group moved on to the activity that had sparked off the session: alternating layers of flour and sand in a Ferrero Rocher box. An exact-fitting piece of wood acted as a ‘slab-push’ force and low and behold our ‘mountain’ acquired exotic folds and even a fault. I think it was Wilf we watched blow down a straw into a complicated arrangement of chemistry equipment to cause an oil-well blow-out, although Nikki was disappointed that no-one got completely soaked! Apparently this activity is very popular with kids. Finally Nikki brought out and demonstrated the piece de resistance: a wax volcano. It worked spectacularly! A layer of red wax was placed under a layer of sand in a glass beaker and then both were covered by water. The wax was gently heated from the base. Low and behold, the sand rose up and red ‘lava’ flowed down the sides at the top and also produced ‘feeder tubes’ to the ‘ground surface’ where it spread out evenly. It was even slightly explosive at one stage.

Unfortunately we were all having such a good time that no-one had the presence of mind to take photographs but details of the models are on their website: www.earthscienceeducation.com with moving images of the wax lava. The teachers amongst us were all enthusiastic about taking the ideas back to the classroom and newcomers to S260 will have gained insight into the basic principles of several geo-processes. For those who were unable to come but could also find the models useful in the classroom, an INSET sessions is offered to schools free of charge, except for travel and refreshment for the team member who facilitates the course, thanks to sponsorship of the UK Offshore Operators Associations. Details are on the website.

Many thanks to Paul Grand and Nikki Edwards for such an exciting evening which will additionally be very useful to quite a few of us. Also thanks to Jill Darrell for her muscle power enabling the models to be safely installed.

Diana Clements

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Visit to the Natural History Museum Earth Science Library

15 June 2006

The Earth Science library were kind enough to offer two London Branch members the opportunity to take a tour round the library facilities at the Natural History Museum in order to promote the wide range of material and services they have available to outside organisations and members of the public.

Jenny Parry and I were delighted to take them up on their offer to view some fantastic books, maps and works of art, both ancient and modern.

We started our tour in the main Earth Science Library. If you have visited the Natural History Museum you may have noticed its inconspicuous entrance opposite the Ordnance Survey Shop on the ground floor of the Earth Science building. The staff had displayed some of the fantastic material held in the library. This included twelve 3-dimensional wooden models of geological structures created by Thomas Sopwith (1803 – 1879), which were designed to help geologists and engineers to understand formations. The blocks are approximately ten centimetres square and individual strata are depicted using different colours/types of wood which produces a most fascinating and beautiful finished piece. They were often used in universities and colleges as teaching aids for map-work and structural geology, what a shame we do not have the opportunity to use such wonderful pieces of art as study materials today. We also viewed some unique books by George Cuvier (1769 – 1832), the Rev William Buckland (1784 – 1856), Thomas Burnett (1635 – 1715), and a beautiful illustrated work by John Fullwood (1854 – 1931) entitled ‘Rocks and Minerals, chiefly from the South East Coast of England, an album of 122 pencil drawings and watercolours’, these were just beautiful and unbelievably realistic.

We were then shown up to the fourth floor of the Palaeontological building where we were shown the special collections and the map room. There are three special collections rooms, palaeontology, mineralogy and the John Murray library (John Murray was the naturalist on the HMS Challenger expedition (1872 – 76), this collection was donated with the wishes that it was to be kept together).

Most of the material in these rooms pre-dates 1850 and is available for viewing by special request. Here we were privileged to view some unique pieces; the diaries and letters from the HMS Challenger expedition; A reference book by James Sowerby (1757 – 1822) containing some beautiful drawings of minerals, the staff had been kind enough to also retrieve the original specimen of Sulfur (kept at the museum) that Sowerby used as the source of his illustration; The series of twenty-one sets (355) photomicrographs of snowflakes by Wilson A. Bentley ‘The Snowflake Man’ (1865 – 1931). Bentley was the first person to photograph a single snowflake crystal in 1885 and was the first to make the discovery that no two snowflakes are alike. The pictures were taken during the winters 1885 – 1889 in Vermont, USA. He captured 5000 snowflakes in his life leading to him being affectionately known as ‘Snowflake Bentley’.

On our return to the main library we were allowed more time to view some of the other works on display, this included the ‘Geological evidences of the antiquity of man’ (1873) by Sir Charles Lyell (1797 – 1875). Also some beautiful works of dinosaur art by Neave Parker (1910 – 1961), Henry de la Beche (1796 – 1855), and Thomas Hawkins (1810 – 1889), many of the specimens of reptiles that inspired his artwork were collected in Somerset and Dorset, and a beautiful watercolour by Sir William Hamilton (1730 – 1803) of an eruption at Campi Phlegraei (Naples, 1776).

We also had a chance to see the oldest book in the Earth Sciences library ‘De Re Metallica’ (1556) by Georgius Agrcola (1494 – 1555), the title translates as ‘On the nature of metals’, however this had a much wider meaning at the time and encompassed any mineral. Finally I have to mention ‘The world’s mineral masterpieces’ (2002) by Eberhard Equit (1939 ®). Who is apparently widely regarded as being one of the finest contemporary mineral artists in the world, and you only have to look at this amazing volume to see why. I strongly recommend taking a look. A selection of his work can be viewed online at www.minrec.org follow the art museum link and select the artist from the list.

A big thank you to all the staff of the Earth Science Library who were most helpful and approachable, I hope this brief round-up of a wonderful visit will inspire some of you to take up the opportunity to view some of the wonderful archives available. You can access the library catalogue online at www.nhm.ac.uk or you can visit the library by appointment (48 hours notice required), contact the library staff.

Dee Summers

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Geology of the Vale of Wardour, The SW Wiltshire Anticline

17 June 2006

On a very hot Saturday morning we assembled in the village of Dinton with our guide Lesley Dunlop. Lesley introduced herself disarmingly, to those of us who did not already know her, as primarily a hard-rock geologist. However, we were very much in sedimentary territory and were going to perform a traverse of a section across the SW Wiltshire anticline, the axis of which ran by the river to the south of the village. We would start by walking to the north, and then walk back south (and a detour a little to the west) across the core of the anticline, and then follow the section in reverse order on the other side.

After appropriate warnings about the need to drink plenty of water in the hot weather, we set off through the picture-postcard village of Dinton, and then climbed a steep hill along a sunken lane, appropriately called "Steep Hollow". To the beginner in geology, a steep climb uphill away from the core of an anticline might seem counter-intuitive, but in fact "reverse topology" is commonly found. The upfolding of the rocks in an anticline causes a weakness, which results in more rapid erosion at the core of the anticline. That same weakness is exploited by rivers, as here, to create a valley.

We paused near the top and viewed the outcrop by Upper Greensand exposed on the sides of the sunken lane. This was characteristically green from the glauconite contained in it and there was also a lot of iron in the rock, which was not particularly well cemented and contained a lot of fine muddy grains. There was much cross-stratification in the lower part of the exposure near a springline. There was also a distinctive shell-bed, and there was some discussion as to the large number of rather broken-up fossils contained in it. Some thought that the fossils looked like Gryphaea be- cause of their shape, but this could not be what they were, as Gryphaea is a Jurassic fossil, and here we were clearly in the Cretaceous. The consensus was that the fossils were terebratulid brachiopods.

However, subsequent research carried out by Di Clements in the Natural History Museum collections established that they were in fact bivalves, of the species Pycnodonte vesiculosus. They are also so identified, specifically in relation to this location, by Isobel Geddes in her book on Wiltshire Geology.

Lesley invited us to measure the dip of the rock (approximately 10° to the North, although the road itself was dipping to the south, following the reverse topology). We then walked up to the crossroads at the top of the ridge from which a chalk landscape was visible to the north. Here Lesley discussed what we had been seeing. The Upper Greensand was approximately 100 million years old. The glauconite and the fossils indicated a shallow marine environment, below the tidemark. Lesley also explained the broader context of the sequence of sedimentary rocks in the anticline, which had been folded and tilted after deposition, by the Alpine Orogeny, and then eroded.

While listening to this the writer was at this point bitten on the finger by a horsefly, which occasioned some discomfort in the following week. Future visitors to the location are advised to bring insect repellent. We then walked back downhill towards Dinton past a quarry at which we paused briefly. Here the rock was paler due to leaching, with iron nodules.

As we came back down towards the village, Lesley pointed out a change in slope as the road levelled off. At the point where the slope changed clay was seen to be exposed in a field by the side of the road. Lesley indicated that the flat level of the village indicated that it was lying on clay at this point. There was then a slight rise again. Re-entering the village and posing a wall on a building to one side we noticed a mixture of harder rocks from the locality, blocks of Greensand with shells, a greyer fossiliferous rock and some whiter carbonate rock. We passed a large new-style country house with an inscription proclaiming it to be "Wright's Manor", built in Chilmark Stone, a creamy variety of Portlandian stone used in Salisbury Cathedral, with a slate roof. It was noted that the stonemasons had inserted blocks of stone upside down into the wall. We then came to St Mary's church. This had some bits of Greensand in its construction, quite a lot of material from the Purbeck beds and a more blocky rock, like Chilmark, probably quarried very locally. Underlying us were now Jurassic beds of the Portland and Purbeck . Lesley said that there was a marked unconformity below the Lower Greensand. Some thirty to forty years of deposition were missing, having been probably eroded away early in the Cretaceous.

Leaving the village and turning a bit to the west, we crossed parkland, passing a lake. The parkland belonged to Philipps House, a National Trust Property built in 1817 on a Greensand ridge to the north- east which commanded a view down to the river Nadder. We crossed a road and then a field and passed through a spinney to a spot near the railway where we paused for lunch.

During lunch Lesley pointed that we were close to the core of the anticline. Here several fault lines ran. There were parallel fault systems running east-west, and the river utilised the weaknesses to lay its course. Lesley also pointed out that many of the anticline features were asymmetrical, the northern part (the side away from the direction of thrust) being steeper.

There was also some discussion over lunch about a fault running parallel to and on the south side of the anticline axis. This was thought to be a deep Mesozoic tension fault, one of a number in the area associated with the first opening of the Atlantic in the early Jurassic or earlier extension in the late Palaeozoic as an eastward extension of the Variscan Orogeny, now deeply buried. This may have controlled subsequent deposition and folding. Initially these faults were down-throwing to the south, and may have caused much thicker deposition of Jurassic sediments down a fault scarp to the south. The direction of movement or faults such as those would then have been reversed at the time of the compression due to the Alpine orogeny, so as to cause a down-throw to the north. Therefore although the impetus came from elsewhere, much of the movement in this area would have been on the ancient faults, which in this case may have contributed to the asymmetry of the anticline. The movement of the crust during the Alpine orogeny would also have caused a number of minor tension faults to open up which are found here trending in a NW/SE direction.

After lunch, Lesley invited us to examine the loose material lying exposed on a bare patch of the field where we had stopped. This proved to be of varied origin, iron nodules from the Greensand, flints from the chalk, shelly limestones and laminated Purbeck material.

Moving across the railway (and crossing the core of the anticline) we came down to the river, passing an elegant hamlet with an old mill house. We followed along a path bordered by barbed wire and nettles, but then had to leave it passing under the barbed wire, the party losing some of its style and dignity for a moment in the process! We could see that the ground over which we were passing was partly alluvial , partly limestone. We were over the Jurassic. Crossing the river, we came up across a ploughed field containing laminated Purbeck stones, but on the upper and southern side there were numerous flints. Lesley observed that the River Nadder had probably cut down further after the deposition of the flints on the southern side of the field.

Passing along the top of the field there appeared to be significant amounts of clinker, thought to be from a limestone kiln , and then one of the party picked up a piece of what seemed to be carboniferous limestone, rather than the more recent Jurassic limestone we had been seeing. An animated discussion ensued about this, which was cut short when some one consulted the OS map and pointed out that we were walking along the course of a dismantled railway. What we had been looking at was in all probability remnants of the ballast!

Moving south, we skirted a fenced-off wooded area, and passed into a spinney through which ran a track. There we paused, to consider where we were in the succession. It was felt that we had now climbed up beyond the Purbeck and Lower Greensand beds and the ground was suggestive of Greensand, perhaps Upper Greensand. In that case where was the Gault? It was speculated that this was due to a fault. However, in retrospect, given that our passage through Dinton suggested that there was an extensive deposition of Gault in the area, it is probable that our initial assessment may have been mistaken. Passing along the track, we did in fact come upon Bog Pepper, just before a break of slope, which indicated a clay substrate. We reached Ford Lane, walked along it a short way and then most of us followed the steep track up to Fir Hill. A few of the less energetic of us stayed behind.

On the way up, we passed a second exposure of Upper Greensand, which contained a fossil bed very similar to that at the top of Steep Hollow above Dinton. We examined this more closely on the way down, but before proceeding took the dip of the Upper Greensand at that point. It was approximately 5° to the South. We continued up to the top of Fir Hill where there was a reservoir and from this point the Chalkland of Fovant Down was visible further to the south. On the Down, military units which had encamped there in the past had carved out their regimental badges into the Chalk. Some of us had seen those badges that morning on the drive over to Dinton.

It was felt that the structure of the anticline had been convincingly demonstrated. Summing up, Lesley said that the Vale of Wardour provided a good illustration of an Alpine anticline, like that found in the Weald, but in a much smaller area and with a more extensive series of rocks exposed, because the core went down into the Jurassic. It was well worth revisiting if some of us should be again in the area. We then walked back downhill and crossing the river returned to Dinton and the cars. Then, after a pause for appropriate refreshment at a local hostelry, we returned to London.

Richard Trounson

With thanks to Kim Vignitchouk for her photographs

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Shetland led by Allan Fraser

31 July - 7 August 2006

Article to follow

Various Authors

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A Novice Eye View of a Trip to Harwich and Wrabness, Essex

7 October 2006

Article to follow

Author

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