British Science Festival 2009 Fire and Water Dating A new method for the archaeological dating of...

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


In glazed pottery and tiles the expansion of the ceramic substrate eventually produces cracking in the glaze:

Basis of method is “moisture expansion”


FILM OF BRICK GAINING MASS


How big are these effects?

EXPANSION: 1 km of wall will expand by ~ 1 m over 200 y


MASS GAIN:

A terracotta warrior will havegained about 4 kg over its lifetime.

(And will have grown~10mm)

I’m on a diet – honest!


• 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).


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

time¼ law

(2003)


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 etcseconds / 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 geta nice straight line.


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!


Discovery of the 2 stage process:

0

0.1

0.2

0 2 4 6Time1/4 (mins1/4)

103 S

trai

n, ε

1 day

Expansion versus t1/4

0

0.1

0.2

0.3

0 5 10Time1/4 (min1/4)

103 ∆

m/m

o

16 days

Mass gain versus t1/4

(2005)


Principle of the dating method

•1 Measure the initial length ormass 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

t1/4

Exp

ansi

on

L,mo

L,m

ta1/4


The first dating attempt usingexpansion 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 reductionin Stage II gradienton repeated reheating

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0 1 2 3 4 5 6

Time1/4 (mins1/4)

10

3 Str

ain

, E

Fresh

Reheat 1

Reheat 2

1 day


FORM of data sameas expansion data- scattery.

Same 2 stage process observed

0

0.1

0.2

0.3

0 5 10Time1/4 (min1/4)

103 ∆m

/mo

16 days

Why?

Started looking at mass gain again:


Is the mass gain reversible?

y = 0.0039x + 0.0285

y = 0.0435x - 0.1174

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 5 10 15 20 25 30

Time^0.25 /mins

Mass

gain

%

Freshly fired brick

y = 0.0039x + 0.0205

270 days y = 0.004x + 0.0248

y = 0.025x - 0.0395

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0 2 4 6 8 10 12

Time^0.25 /mins

Mas

s %

gai

ned

Same brick reheated

y = 0.004x + 0.0248

7 days


Predicted ageof 49 weeks.

5 10 15 20 25

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

Time1/4 (min1/4)

Ma

ss (

g)

14 days

The first “dating” experiment


m = 0.0382t 1/4 + 128.9531

m = 0.0289t 1/4 + 129.02

128

129

130

131

132

133

134

135

0 50 100 150 200

Time1/4 (mins1/4)

Mas

s (g

)

A × 1.33 = B(mean over all trials)

1,957 Years

A B

The first dating trial


Knownage

Ratio of predicted to known age

(in t1/4)

Predicted age with average

multiplier (1.33)

(a) 150 ±10 1.36 144

(a) 150 ±10 1.31 166

(a) 150 ±10 1.26 192

(b) 367±160 1.37 298

(b) 367±160 1.37 303

(c) 1932±75 1.34 1968

(d) 1957±50 1.30 2123

Mean= 1.33

The first dating trial

All dates came outwrong-

BY THE SAME AMOUNT


m = 0.0382t 1/4 + 128.9531

m = 0.0289t 1/4 + 129.02

128

129

130

131

132

133

134

135

0 50 100 150 200

Time1/4 (mins1/4)

Mas

s (g

)

1,957 Years

A B

The DATA


The microbalance:

The next step…

Allows us to weigh 5 gpieces of brick under tightly controlled conditions to 0.1µg. (1/10 of a millionth of a gram).


Lancashire Hotpots!


Can collect enough data to define the stage 2 gradient quite quickly.

0

0.02

0.04

0.06

0.08

0.1

0 5 10 15Time1/4 /mins1/4

100

Δm

/mo 43

days

Whole brick

Time 1/4 / min1/4

100 ∆m/m0

% RH

2 hours

RH

100

∆m

/m0

5 g piece of brick

Microbalance results


0

0.5

1

1.5

2

2.5

3

0 5 10Time1/4 (mins1/4)

103

∆m/m

0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 2 4 6 8 10 12

Time1/4 (mins1/4)

10

3 Δm

/m0

10 days

The data

• Vastly improved quality of data.

• Speed of data acquisition.

• Absolute confirmation of the t1/4 law

NO SCATTER!!!!!!

(WHY?)


Putting the microbalancethrough its paces


0

0.5

1

1.5

2

2.5

3

0 10 20 30 40 50 60

Temperature (°C)

Gra

dien

t (t

-1/4

x10-4

)

The stage II gradient increases with temperature!!!


This shows that we have a chemical reaction going on ……… and that it’s TEMPERATURE DEPENDENT!

Arrhenius plot


EUREKA! (again)

• Scattery data due to temperature fluctuationsover 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)

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 (2008)


Second dating trialCarried out at 11OC – mean lifetime temperature of brick

3941

3942

3943

3944

3945

3946

3947

3948

3949

0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4

Time1/4 (hours1/4)

Mass

(m

g)

m /t 1/4 = -0.00006x + 0.79350

0

5

10

15

0 1000 2000 3000 4000 5000

Data points

Gra

die

nt (m

/t1/

4 )


Started to get some REALLY good results

AND THEN …

yippee!


We dated a Roman brick to March 2008!

(i.e. 8 months old)

AND THEN …

EEK!


We dated a MEDIEVAL brick to 1942!

(i.e. 66 years old)

AND THEN …

– the “Canterbury Tale”…

NOW WHAT?


Eventually…the “line of knowns”

50 person-years of effort for 6 data points!


The real deal?


We’ll see…..

Thank you!


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!


6. Effect of original firing temperature


A volume of air that is supporting as much watervapour 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 airrelative to what it can hold at saturation at that particular temperature (and pressure).

What does RH mean?????


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 Fire and Water Dating A new method for the archaeological dating of...

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