Presentation on theme: "British Science Festival 2009 Fire and Water Dating A new method for the archaeological dating of ancient pottery Dr Moira Wilson, The University of Manchester."— Presentation transcript:
British Science Festival 2009 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 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 FILM OF BRICK GAINING MASS
British Science Festival 2009 How big are these effects? EXPANSION: 1 km of wall will expand by ~ 1 m over 200 y
British Science Festival 2009 MASS GAIN: A terracotta warrior will have gained about 4 kg over its lifetime. (And will have grown~10mm) Im on a diet – honest!
British Science Festival 2009 Moisture expansion was found to be REVERSIBLE (but no agreement on the temperature required to do this) Of most interest 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 Discovery of the (time) 1/4 law: The Manchester and Edinburgh work Fresh brick 1900 year old brick 20 year old brick 120 year old brick time ¼ law (2003)
British Science Festival 2009 TIME TO THE WHAT???? The t 1/4 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 1 4, 2 4, 3 4, 4 4 etc seconds / minutes/ years. If we plot mass gain or expansion against t 1/4 we get a nice straight line.
British Science Festival 2009 The older the brick, the bigger and heavier it is. Could the new rate law be exploited to produce a dating method for fired clay ceramics?? It gets bigger and heavier at a precisely defined rate EUREKA!
British Science Festival 2009 Discovery of the 2 stage process: 1 day Expansion versus t 1/4 16 day s Mass gain versus t 1/4 (2005)
British Science Festival 2009 Principle of the dating method 1 Measure the initial length or mass of the sample (L or m) 2. Measure early time mass or expansion following reheating 3. Extrapolate stage II data 4. Intersection of extrapolated Stage II data with initial length or mass gives age of the sample t 1/4 Expansion L, m o L, m t a 1/4
British Science Festival 2009 The first dating attempt using expansion measurements: Linear fits to Stage 2 data: (F) ε = 0.93×10 -5 t 1/4 + 2.77×10 -5 (R1) ε = 0.45×10 -5 t 1/4 + 3.67×10 -5 (R2) ε = 0.33×10 -5 t 1/4 + 2.45×10 -5 Systematic reduction in Stage II gradient on repeated reheating Fresh Reheat 1 Reheat 2 1 day
British Science Festival 2009 FORM of data same as expansion data - scattery. Same 2 stage process observed 16 days Why? Started looking at mass gain again:
British Science Festival 2009 Is the mass gain reversible? Freshly fired brick y = 0.0039x + 0.0205 270 days Same brick reheated y = 0.004x + 0.0248 7 days
British Science Festival 2009 Predicted age of 49 weeks. 510152025 1279.6 1280.0 1280.2 1280.4 1280.6 1279.8 1281.0 1281.4 1280.8 1281.2 1281.6 Initial mass of 39 week old brick Extrapolated Stage II data Stage II data Time 1/4 (min 1/4 ) Mass (g) 14 days The first dating experiment
British Science Festival 2009 A × 1.33 = B (mean over all trials) 1,957 Years AB The first dating trial
British Science Festival 2009 Known age Ratio of predicted to known age (in t 1/4 ) Predicted age with average multiplier (1.33) (a) 150 ±101.36144 (a) 150 ±101.31166 (a) 150 ±101.26192 (b) 367±1601.37298 (b) 367±1601.37303 (c) 1932±751.341968 (d) 1957±501.302123 Mean= 1.33 The first dating trial All dates came out wrong- BY THE SAME AMOUNT
British Science Festival 2009 1,957 Years AB The DATA
British Science Festival 2009 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 Lancashire Hotpots!
British Science Festival 2009 Can collect enough data to define the stage 2 gradient quite quickly. Whole brick Time 1/4 / min 1/4 100m/ m 0 % RH 2 hours RH 100 m/m0 5 g piece of brick Microbalance results
British Science Festival 2009 10 days The data Vastly improved quality of data. Speed of data acquisition. Absolute confirmation of the t 1/4 law NO SCATTER!!!!!! (WHY?)
British Science Festival 2009 Putting the microbalance through its paces
British Science Festival 2009 The stage II gradient increases with temperature!!!
British Science Festival 2009 This shows that we have a chemical reaction going on ……… and that its TEMPERATURE DEPENDENT! Arrhenius plot
British Science Festival 2009 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 O C) AND All samples in 1 st dating trial were measured at ~25 O C! 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 (2008)
British Science Festival 2009 Second dating trial Carried out at 11 O C – mean lifetime temperature of brick
British Science Festival 2009 Started to get some REALLY good results AND THEN … yippee!
British Science Festival 2009 We dated a Roman brick to March 2008! (i.e. 8 months old) AND THEN … EEK!
British Science Festival 2009 We dated a MEDIEVAL brick to 1942! (i.e. 66 years old) AND THEN … – the Canterbury Tale… NOW WHAT?
British Science Festival 2009 Eventually…the line of knowns 50 person-years of effort for 6 data points!
British Science Festival 2009 Well see….. Thank you!
British Science Festival 2009 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 6. Effect of original firing temperature
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). What does RH mean?????
British Science Festival 2009 Cairo (hot and dry?): 32.2 O C (90 O F) and 46% RH (i.e. 46% of a large amount of water vapour) Reykjavik (cold and wet) 12.8 O C (55 O F) 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