Heat Treating Flint

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Heat Treating Flint

Post by grendel on Mon Mar 12, 2012 4:01 am

Heat treament is a technique that was used to improve the knappabilty of rocks and make them flake more easily. Some rocks that are completely unknappable before heat treatment become highly knappable after, whereas other rocks already knappable just improve in quality slightly.

The science behind heat treament is still unknown. One theory is that it fueses the quartz chystals together, another that it creates microscopic cracks in the rock for flakes to follow.

Heat at treament also isn't universally good, with some rocks it doesn't necessarilty improve the quality and also the time and temperature must be correct, overbake a rock you ruin it, undercook it it has little effect.

Rocks that improve with heat treatment include Chert, Jasper, Novaculite, Coral, Petrified Wood, Agate, the jury is still out on whether is has any effect on British Flint. Rocks that must not be heat treated include Dacite, Obsidian, Pitchstone and glass.

There are two main methods of heat treating rock.

The Prehistoric Method
The spalls and flakes are buried in the ground and a campfire is lit above them. The temperature the rock is baked at can be varied by how deep the spalls are buried. At 1" deep the temperature will be roughly 320C and reduce by around 30C for each inch deeper. The fire needs to burn for at least 8 hours and after it has gone out the rocks be left to cool for 24 hours before being dug up.

The Oven Method
Fill a large biscuit tin with sand. Burymthe spalls and flakes in the sand, not touching the tin or each other. Place the tin in the oven and bake. Start baking at the lowest temperature on the oven for the first hour then increase the temperature slowly, max 30-50C per hour up to the required baking temperature, then bake (usually 2-3 hours). When finished turn the oven off and leave overnight to cool.

Danger!
- Rocks can explode.
- Make sure the rocks are dry before you bake them, keep them for a few days in a warm place to dry them if they are wet.
- Never bake a whole nodule
- Never bake a flake thicker than 1"
- Make sure the kitchen window is open, try to avoid breathing the fumes for a prolonged period.


Recipes
On the whole darker rocks cook at lower temperatures than lighter ones.
Agate Brazilian around 250C
Chert around 300C
Coral around 250-320C
Flint around 160C (double the baking time and experiment with higher temperatures for grey and white flint)
Jasper around 250C
Novaculate around 400-450C
Petrified Wood around 150-220C


Last edited by Grendel on Tue Mar 13, 2012 4:40 am; edited 4 times in total

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Re: Heat Treating Flint

Post by grendel on Mon Mar 12, 2012 4:24 pm

An experiment carried out by a Yorkshire archaeology group in heat treating British flint.

http://www.britarch.ac.uk/ba/ba93/feat3.shtml

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Re: Heat Treating Flint

Post by the barnacle on Mon Mar 12, 2012 6:31 pm

i put a few lumps in my log burner in my workshop - rule 1 - rocks explode lol - i think you get the picture.

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Re: Heat Treating Flint

Post by grendel on Tue Mar 13, 2012 5:29 am

Fortunately you did it in your workshop, it's deadly to explode a rock in the kitchen or living room. Your other half will kill you.

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Re: Heat Treating Flint

Post by grendel on Wed Mar 14, 2012 1:21 pm

A scientific test carried out to determine the effectveness of heat treating

http://www.latrobe.edu.au/geosci/Downloads/pdfs/John%20Webb_Jan%2009/Geoarchaeology/Domanski%20et%20al%201994.pdf

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Re: Heat Treating Flint

Post by Lumpendoodle on Fri Mar 16, 2012 7:52 am

Probibly my fault, but couldn't get the link to fully download.

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Re: Heat Treating Flint

Post by Native on Mon Sep 03, 2012 7:30 pm

hello

i did a bit of experimenting a couple of summers ago http://paleoplanet69529.yuku.com/topic/42723/Primitively-heat-treating-flint#.UET2IdZlR9s

i'm hopefully going to be having another go this weekend!

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Re: Heat Treating Flint

Post by grendel on Mon Sep 03, 2012 8:20 pm

That's a nice experiment you have there, I've tried hear treating flint too, the white stuff especially goes a really nice marbled brown colour and improves to knap. However there's mixed opinion if the black stuff improves in anyway, what's do you think?


On another point when I was near Salisbury walking at the bottom of a few hills the day after it rained I noticed several flint nodules that had had just subsided from the hill and rolled down to the bottom.

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Re: Heat Treating Flint

Post by Native on Mon Sep 03, 2012 9:27 pm

Grendal- Is the white flint found in Norfolk?

I wouldn't bother heat treating the black flint. In my opinion it's the best I've every knapped. If you manage to get a good last sharpening pass, I think it's the greatest of knapple stone, both strong and razor sharp! I think we are really blessed with having it hear, many of my US friends believe that too especial as they like to hunt with it!

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Re: Heat Treating Flint

Post by grendel on Mon Sep 03, 2012 9:57 pm

Yes, I got a pure white nodule from Norfolk donkeys years ago. Very chalky, knapped more like chert than flint, but made some great stuff. best flint I've ever knapped I found in the Thames, transparent brown stuff, knapped as good as glass, sadly I only found one small piece. As for Norfolk flint, I have heard said that South Downs flint is the best in Britain. Be interesting to compare the two.

Some Norfolk White and bottom right, the brown stuff from the Thames

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Re: Heat Treating Flint

Post by Native on Tue Sep 04, 2012 6:22 am

The Thames flint looks like nice stuff!

Was the white flint found in chalk deposits or in the gravel... I've been told that there's a theory that the white has been pushed down from Lincolnshire!... I've never gone rock hunting there.

Anyone here live in the south downs?

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Re: Heat Treating Flint

Post by grendel on Tue Sep 04, 2012 12:56 pm

No idea of the source, someone gave it to me.

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Re: Heat Treating Flint

Post by grendel on Sat Sep 07, 2013 2:28 am

Rather a large quote but as page I link to in an above post is gone, I thought I'd take no chances as this write up is so good.

it's from here
http://paleoplanet69529.yuku.com/topic/15098


I wrote a short piece on heat treatment, based on some experiments that were undertaken with a computer controlled kiln and so made exact replication of heating possible. It may be of use to you. I would add that I now believe that the costs in time would not always make the effort worthwhile (I also love in England where I can get plenty of flint!) 
Heat Treatment of Sussex Flint and Other Rocks



Abstract. The desire to improve the quality of the available Sussex flint for flintknapping led to a series of heat-treating experiments.Previous work has shown that flint can be heat-treated, but the heating cycles to use for the Sussex flint and other local siliceous rocks were unclear. Experiments showed that the knapping characteristics of the rocks tested could be improved, some markedly, using commonly available equipment.


Background


There is a considerable body of literature available in the US on the heat treatment of different rock types in order to make them more workable. Indeed, certain types of rock can only realistically be knapped after heating, and it is possible that for other types, the aesthetic changes in colour and texture may have motivated their heat-treatment. It is generally agreed that heating does improve many rock types but exactly how is less certain.


Heat treatment of rocks in the Old World does not appear to have been as common as in the New. Its use in the Old World is "considered a Solutrean invention" (p. 23. Inizian et al. 1999) but they also report longstanding use on the Indo-Pakistani continent. Their Knapping Suitability Tests (p22) indicates that UK flint has the potential to improve when heated.


There are no Sussex quarries working at an appropriate geological level to produce flints for an amateur knapper, so obtaining good rock can be difficult. Sussex flint varies considerably in its knapping properties and though searches in cliff sections have shown that very good flint is found, especially low in the flint succession, it tends to be in smaller nodules rather than larger or tabular form.


Apart from varying quality, surface flint has additional potential problems e.g. difference in moisture content between samples. After consultations with American knappers, a series of experiments were undertaken to try to establish the heat treatment most appropriate to Sussex flint. Though Luedtke (1992) (p93) includes comments on 'flint' as benefiting from heating, it does not make clear the cycles used or the flint type/s. Griffiths' et al (1987) work gave more detailed information on temperatures and the problems they encountered, notably with the brittleness of the flint increasing with the maximum temperatures used.


Methodology


The experiments were undertaken using a computer-controlled pottery kiln with approximately 2 cubic foot capacity. Though not historically accurate, it did allow more exact and reproducible results. The flint for trials was obtained from a number of locations and over a 2-year period a large number of tests were carried out. The flint, varying between about 5-10kg a time and in size from thin flakes to large rough outs (up to 8 inch in length), was placed inside a square metal container, with sand used to fill in voids. As the heating elements were in direct line of sight with the metal container, heating of the flint was by convection and conduction rather than direct radiation.


Drying Time From 6 to 24 hours at 80oC
Heating Increase Rate 20oC per hour
Maximum Temperature and Duration 230oC for 8 hours
Cooling Cycle Naturally for 24 hours minimum


Should others wish to experiment, these temperatures can be reached using a domestic oven, and whilst not as precise as the kiln, tests samples were carried out in ovens to out to confirm suitability.


Discussion


No change is apparent when the flint is first removed from the cooled sand; only once a flake is removed is the much-improved workability of the stone evident, and the characteristic waxy lustre shows on the flake scar and flake's ventral surface. There was no visual evidence of colour change or transparency in flint.


The most common failures were not apparent when first removed from the kiln and as they are totally confined within the stone only show when a piece is being knapped. The failures have a characteristic crazing with many small cracks, in all planes, similar to 'pot-boilers', and may result from the inability of rock to adjust to the expansion/contraction with heating and/or where water has been unable to escape from insufficiently dry stone, though further research would be needed into this. If a piece is thick enough, the 'good' stone, above or below the cracked area, can still be knapped successfully, the failed stone being removed as flakes.


There were some major failures during the experiments, with some pieces literally blowing apart, as refitting pieces were separate from each other within the sand. Just how much force is released is not clear but the metal container is recommended! Failures did not generally present as a single 'crack' in the rock. When single cracks were found, the crack surfaces showed partly new and shiny and partly old matt, surface. Test pieces with similar but known flaws were heated and produced identical patterns of lustre.


It became clear that though not absolutely necessary, a sand 'bath' was an advantage. The sand provides a physical (safety) barrier to any failures, a convenient method of containing the rock, and it does appear to ensure temperature changes are gradual. Although some early tests were run without the sand, it was used for all later heating cycles.


Smaller flakes, arrowhead and dart-sized pieces, were found to change at lower temperatures than larger pieces and were more tolerant of temperature cycles. The experiments also bore out verbal advice from the USA (Martin, pers comm.) that the maximum thickness should be less than one inch. Once this threshold is exceeded, failures increased very rapidly. The maximum thickness appears to be determined by the ability of the rock to accommodate the expansion during the cycle.


The drying cycle appears crucial to ensure that the moisture content is reduced. Similar larger sized pieces of rock, trialled with and without drying, showed far better results for the dried samples. For tests using only small flakes, no drying cycle was needed, other than the ramping temperatures to the maximum from room temperature. It appears that a sliding scale might be possible for drying time/ rock thickness, though the variables such as moisture content, mean this would be a 'rule of thumb' only. The standard drying cycle now used for all runs, to ensure all pieces treat successfully, is 24 hours.


The relatively low increase in temperature per hour also seems to allow the rock to expand and change slowly; faster rates of temperature rise increased the chance of failure and no attempts were made with temperatures increases of more than 35oC per hour.The duration of holding at the maximum temperature was very important, especially when using a sand bath, allowing the heat to reach all stone, and to effect change. The duration quoted above can be extended but this was found to be unnecessary.


Mention must also be made of the maximum temperature used. Higher temperatures can be reached when heating the flint without causing failures. However, as Griffiths et al (1987) reported, the brittleness does increase with temperature though without a standard brittleness test, the must remain a subjective knappers' assessment. The desired improved workability can be achieved at the temperatures in the cycle above.


Similarly cooling is important and if done incorrectly, will cause the stone to fail. Some American literature, e.g. Waldorf (1993), suggests temperatures be ramped down so that the cooling period is extended. This was tried initially but found to be unnecessary.


One characteristic of Sussex flint are the 'ghosts' within flint, the lighter grey coloured, round or elongated areas. These areas are of a noticeably different texture and flakes tend to 'tear' across them, rather than cleanly flake over them, in untreated flint, possibly due to differing silica content. No cycle of heating improved knapping characteristics or altered the appearance of these ghosts.


In conjunction with those using flint, tests were carried out on a large range of other rock types. Small samples were collected from sources across southern and southwestern England and included carnelian, agates, cherts, jaspers, and chalcedony. These too were all successfully treated using this cycle and many of these also displayed colour change in addition to alteration of lustre, transparency and workability.


Some comment must also be made regarding the effect on the flint itself. It became clear that heat treated flint should or would not be used for items such as axe-heads as the improved flaking properties and increased brittleness would work against the intended tool use; however, for smaller items, such as arrowheads where pressure flaking or detail work is required, and items that are not intended for repeated heavy impact, then there are benefits to be gained from using treated flint and the efforts involved in so treating them.


Conclusion


It became apparent that there cannot be a definitive or exact cycle to heat treat flint as variation between pieces ensures there is no 'standard' rock. However, this heating cycle above has been shown to work on all rocks tested and it is feasible for anyone with access to a domestic oven to produce very workable knapping stone. The balance of improved working characteristics with the temperatures needed to achieve the desired results make this cycle feasible with common equipment and ensure that the rocks' workability increased significantly. A final point of note; the lack of colour change, and the difference between heated flint and flint that has begun to weather/patinate, is negligible to the naked eye; perhaps heat treatment was practiced more widely but has just not been seen.


Bibliography


Griffiths, D.R.C., Bergman, C.A., Clayton, C.J., Ohuma, K., Robins G.V. and Seeley, N.J. Experimental investigation of the heat treatment of flint. In The Human Uses of Flint and Chert. Edited by Sieveking, G. de G. and Newcomer, M.H. Cambridge University press, Cambridge.


Inizian, M.L., Reduron-Ballinger, M., Roche, H, and Tixier, J. (1999). Technology and Terminology of Knapped Stone. CREP, Nanterre.


Luedtke, B.E. (1992). An Archaeologists Guide to Chert and Flint. Institute of Archaeology, University of California, Los Angles.


Waldorf, D.C. & Waldorf, V. (1993). The Art of Flintknapping. 4th Edition. Mound Builder Books, Branson.


NB I have still not seen the original work by Griffiths quoted in Luedtke, as I cannot locate the Human Uses of Flint & Chert Volume she talks about. Griffiths may have said all this already and I am just not aware of it.



Acknowledgements: Grateful thanks to Dane Martin of Knappers Corner, and Rocky and Gem for their help with the kiln.

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