Fire and Water Dating

# Fire and Water Dating

## Fire and Water Dating

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##### Presentation Transcript

1. Fire and Water Dating A new method for the archaeological dating of ancient pottery Dr Moira Wilson, The University of Manchester British Science Festival 2009

2. In glazed pottery and tiles the expansion of the ceramic substrate eventually produces cracking in the glaze: Basis of method is “moisture expansion” British Science Festival 2009

3. FILM OF BRICK GAINING MASS British Science Festival 2009

4. How big are these effects? EXPANSION: 1 km of wall will expand by ~ 1 m over 200 y British Science Festival 2009

5. MASS GAIN: A terracotta warrior will have gained about 4 kg over its lifetime. (And will have grown~10mm) I’m on a diet – honest! British Science Festival 2009

6. Of most interest • Moisture expansion was found to be REVERSIBLE • (but no agreement on the temperature • required to do this) • The moisture expansion was accompanied by • an increase in mass (but only 1 set of data • in the literature from 1962). British Science Festival 2009

7. Discovery of the (time)1/4 law: 120 year old brick 1900 year old brick 20 year old brick Fresh brick The Manchester and Edinburgh work time¼ law (2003) British Science Festival 2009

8. TIME TO THE WHAT???? The t1/4 law means that equal amounts of expansion or mass gain occur in the time intervals 1, 16, 81, 256 etc seconds / minutes/ years after firing. These correspond to 14, 24, 34, 44 etc seconds / minutes/ years. If we plot mass gain or expansion against t1/4 we get a nice straight line. British Science Festival 2009

9. EUREKA! The older the brick, the bigger and heavier it is. It gets bigger and heavier at a precisely defined rate Could the new rate law be exploited to produce a dating method for fired clay ceramics?? British Science Festival 2009

10. 1 day 16 days Expansion versus t1/4 Mass gain versus t1/4 Discovery of the 2 stage process: (2005) British Science Festival 2009

11. 1 Measure the initial length or • mass of the sample (L or m) L,m Expansion 4. Intersection of extrapolated Stage II data with initial length or mass gives age of the sample 3. Extrapolate stage II data 2. Measure early time mass or expansion following reheating L,mo ta1/4 t1/4 Principle of the dating method British Science Festival 2009

12. Fresh Reheat 1 Reheat 2 1 day The first dating attempt using expansion measurements: Linear fits to Stage 2 data: (F) ε= 0.93×10-5t1/4 + 2.77×10-5 (R1) ε= 0.45×10-5t1/4 + 3.67×10-5 (R2) ε= 0.33×10-5t1/4 + 2.45×10-5 Systematic reduction in Stage II gradient on repeated reheating British Science Festival 2009

13. Same 2 stage process observed 16 days Started looking at mass gain again: FORM of data same as expansion data - scattery. Why? British Science Festival 2009

14. Freshly fired brick y = 0.0039x + 0.0205 Same brick reheated 270 days y = 0.004x + 0.0248 7 days Is the mass gain reversible? British Science Festival 2009

15. Initial mass of 39 week old brick 1281.6 1281.4 1281.2 Extrapolated Stage II data 1281.0 1280.8 Stage II data Mass (g) 1280.6 14 days 1280.4 1280.2 1280.0 1279.8 1279.6 5 10 15 20 25 Time1/4 (min1/4) The first “dating” experiment Predicted age of 49 weeks. British Science Festival 2009

16. The first dating trial A B A × 1.33 = B (mean over all trials) 1,957 Years British Science Festival 2009

17. The first dating trial All dates came out wrong- BY THE SAME AMOUNT British Science Festival 2009

18. A B 1,957 Years The DATA British Science Festival 2009

19. The microbalance: The next step… Allows us to weigh 5 g pieces of brick under tightly controlled conditions to 0.1µg. (1/10 of a millionth of a gram). British Science Festival 2009

20. Lancashire Hotpots! British Science Festival 2009

21. 100 ∆m/m0 5 g piece of brick Whole brick RH % RH 100 ∆m/m0 2 hours Time 1/4 / min1/4 Microbalance results Can collect enough data to define the stage 2 gradient quite quickly. British Science Festival 2009

22. Vastly improved quality of data. Speed of data acquisition. Absolute confirmation of the t1/4 law The data 10 days NO SCATTER!!!!!! (WHY?) British Science Festival 2009

23. Putting the microbalance through its paces British Science Festival 2009

24. The stage II gradient increases with temperature!!! British Science Festival 2009

25. Arrhenius plot This shows that we have a chemical reaction going on ……… and that it’s TEMPERATURE DEPENDENT! British Science Festival 2009

26. AND All samples in 1st dating trial were measured at ~25 OC! • → Stage II gradients were too steep (temp too high), • → Extrapolated Stage II data intersected line of initial mass too soon • → Age of sample too young EUREKA! (again) • Scattery data due to temperature fluctuations • over course of measurement period • First dating experiment worked because the brick had been sitting in the lab for 39 weeks – and the mass gain following heating was carried out at same temp (~ 25 OC) (2008) British Science Festival 2009

27. Second dating trial Carried out at 11OC – mean lifetime temperature of brick British Science Festival 2009

28. yippee! AND THEN … Started to get some REALLY good results British Science Festival 2009

29. EEK! AND THEN … We dated a Roman brick to March 2008! (i.e. 8 months old) British Science Festival 2009

30. NOW WHAT? AND THEN … We dated a MEDIEVAL brick to 1942! (i.e. 66 years old) – the “Canterbury Tale”… British Science Festival 2009

31. Eventually…the “line of knowns” 50 person-years of effort for 6 data points! British Science Festival 2009

32. The real deal? British Science Festival 2009

33. We’ll see….. Thank you! British Science Festival 2009

34. Acknowledgements • The Leverhulme Trust • EPSRC • The Museum of London Specialist Services • Centre for Materials Science and Engineering, The University of Edinburgh • A very patient husband! British Science Festival 2009

35. 6. Effect of original firing temperature British Science Festival 2009

36. British Science Festival 2009

37. What does RH mean????? A volume of air that is supporting as much water vapour as it can is said to be saturated and has a RH of 100 %. If it is supporting less than its full capacity of water vapour it is said to be unsaturated and its RH will be < 100%. RH is defined as the ratio of water vapour present in a parcel of air relative to what it can hold at saturation at that particular temperature (and pressure). British Science Festival 2009

38. Cairo (hot and dry?): 32.2 OC (90 OF) and 46% RH (i.e. 46% of a large amount of water vapour) Reykjavik (cold and wet) 12.8 OC (55 OF) and 67% RH (i.e. 67% of a much smaller amount of water vapour). From these data, the water vapour pressure in Cairo is 2.24 kPa compared with 1.00 kPa in Reykjavik. The air in Cairo therefore contains much more water vapour than in Reykjavik British Science Festival 2009