Rehydroxylation [RHX]:

Some scientific comments and issues

Christopher Hall

University of EdinburghSchool of Engineering

Centre for Materials Science & Engineering

UoE/National Museums Scotland Research Laboratory for Conservation Science

Moisture dynamics in buildings andmasonry structures

Hall, Hamilton, Viles et al 2010

Delta-Min Marie Curie Network

Mechanisms of Mineral Replacement Reactions 2009-2012

Penn floor tileSt Bride’s ChurchFleet StreetLondon UK

Museum of London WFG62

Assigned date AD 1350-1390

Penn floor tileSt Bride’s ChurchFleet StreetLondon UK

Museum of London WFG62

Assigned date AD 1350-1390

Normalised stage 2 slope ln(α/α25) plotted against 103 /T K-1, where α25 is theinterpolated slope at 25 °C, for three fired clay brick and tile samples. Red: Romanopus spicatum clay paving brick, London; blue: ‘Westminster’ clay tile, MertonPriory; green: clay brick, King Charles II Court, Greenwich

Arrhenius plot: effect of temperature on rehydroxylation mass gain rate.

Samian ware:X-ray diffraction

Stackhouse, Coveney et al.

Stage I

Stage II

Variation of the fractional mass gain (Δm/mo) with the fourth root of time (time)1/4 of kaolin samples fired at different temperatures [Hesham Mesbah]

Open questions

Why (time)1/4?

A Putnis Rev Mineralogy Geochem 2009

Pseudomorphic replacement of calcite and aragonite byHydroxyapatite

Normal diffusion

(time)1/2

Lizana and Ambjörnsson Phys Rev Lett 2008 v100 20601

Lizana and Ambjörnsson Phys Rev Lett 2008 v100 20601

Open questions

Why (time)1/4?

… and, if single-file diffusion, on what structure?

Open questions

Why (time)1/4?

… and, if single-file diffusion, on what structure?

Why rehydroxylation when firing has proceeded to 1050 °C?

Does accurately describe the entire process on 1000 year time scales?

Some conclusions

Incontrovertible evidence that the RHX process occurs more or less universally in all fired clay ceramics

Strong evidence that the (time)1/4 clock is valid

Data acquisition for a single sample is still laborious

Major validation studies needed

Mechanistic and materials fundamentals not fullyunderstood

AcknowledgementsMoira Wilson, Margaret Carter, Ceren Ince, Bill Hoff, Hesham Mesbah (Manchester)

Andrea Hamilton (Edinburgh)