Yoram Kirsh

Thermal behaviour of limestone and monocrystalline calcite tempers during firing and their use in ancient vessels

Limestone and monocrystalline calcite tempers (grains) are abundant in ancient pottery. In pottery from the Canaan area the former is common in Iron Age storage and table-ware vessels and the latter is present in cooking pots. Limestone is much more widespread than monocrystalline calcite and the potters used it often as tempers when manufacturing pottery vessels, but usually not for cooking pots. While defects appear frequently around limestone tempers, they do not appear around monocrystalline calcite ones. This study examines the reason for using the latter tempers rather than the former ones.Raw materials of carbonate tempers in a clay matrix were fired and the decarbonation process was followed by quantitative IR thermospectrometry. The results indicate that the monocrystalline calcite tempers prevent formation of defects in the cooking pots during firing or during use. The reasons for this are higher thermostability at elevated temperatures, lower intensity of decarbonation, and retention of grain shape, as compared to limestone tempers.ZusammenfassungIn alten Töpferwaren werden reichlich Kalkstein und monokristalline Kalzitmischungen gefunden. Bei Töpferwaren aus dem Gebiet Canaan ist ersteres bei Vorrats- und Tischgeschirrgefäßen, letzteres bei Kochtöpfen üblich. Kalkstein ist weitverbreiteter als monokristallines Kalzit und es wurde von den Töpfern bei der Fertigung von Töpfereigefäßen oft als Mischung eingesetzt, jedoch nicht für Kochgeschirr. Während Defekte oft an Kalkstein-mischungen auftreten, kommen sie an monokristallinen Kalzitmischungen nicht vor. In dieser Arbeit wird der Grund dafür gesucht, daß die letzteren Mischungen den ersteren vorgezogen wurden.Rohmaterialien aus Karbonatmischungen wurden in einer Tonmatrix gebrannt und der Dekarbonierungsprozeß mittels IR Thermospektrometrie verfolgt. Die Resultate zeigen, daß monokristalline Kalzitmischungen beim Brennen oder bei der Benutzung die Bildung von Defekten bei Kochgeschirr verhindern. Die Gründe dafür liegen—verglichen mit Kalksteinmischungen—in der höherem Thermostabilität bei erhöhten Temperaturen, in der weniger intensiven Dekarbonierung und in der Beibehaltung der Gefügestruktur.

Spectral measurement during thermoluminescence—an essential requirement

Abstract Emission spectra recorded during thermoluminescence provide a fuller understanding of defect properties. Also, as detailed changes in spectra occur with dose, dose rate and thermal treatments, knowledge of emission spectra will make even simple radiation dosimetry, as well as applications such as archaeological and geological dating, more reliable.

Speculations on the blue and red bands in the TL emission spectrum of albite and microcline

Abstract TL spectra of the feldspars albite (NaAlSi3O8) and microcline (KAlSi3O8) were recorded following X-irradiation at room temperature, and presented as isometric and contour plots. Bands at about 425 and 450 nm in both samples are ascribed to electron-hole recombinations in two types of hole centres. An intense band at the red can be attributed either to the excitation of Fe3+ ions by the blue emission, or to valence state conversion of the iron. The kinetic analysis of the glow curves enabled us to interpret the experimental results in terms of a discrete set of traps, and to suggest a model for some of the recombination centres. The analysis of the contour plots implies that at least part of red emission is independent of the blue band.

Rehydroxyiation of clay minerals and hydration in ancient pottery from the ‘Land of Geshur’

Rehydroxylation of clay minerals and hydration in Bronze and Iron Age pottery from the ‘Land of Geshur’ (east of the Sea of Galilee), were investigated by IR thermospectrometry. A weak OH band, which resembles those in smectite or illite, exists in most samples, but it is masked by the water bands at lower temperature. Two types of transformation of clay minerals occur during firing of raw materials, reversible and nonreversible dehydroxylation, taking place at a relatively low and high firing temperatures, respectively.Clay minerals rehydroxylate and reconstruct after relatively higher firing temperatures in noncalcareous raw materials than in calcareous ones. These processes take place after higher temperatures in cooking pots made from the former type of raw material as compared with storage and table-ware vessels made from the latter.The absorbed water in the pottery is mainly connected with the dominant X-ray amorphous matter formed by the firing process.ZusammenfassungMittels IR-Thermospektrometrie wurde die Rehydroxylierung von Tonerdemineralien und die Hydratierung in Geschirr aus dem Bronze- und Eisenzeitalter, aus dem “Land der Geshur” (Osten des Galiläischen Meeres) untersucht. Eine schwache OH-Bande, die denen in Smektit oder Illit ähnelt, existiert in den meisten Proben, es wird aber bei niedrigeren Temperaturen durch Wasserbanden verdeckt. Während des Brennens von Rohmaterial finden zweierlei Umwandlungen der Tonerdemineralien statt, bei relativ niedrigen Temperaturen reversible und bei relativ hohen Temperaturen nichtreversible Dehydroxylierung.Tonerdemineralien werden in kalkfreiem Rohmaterial nach relativ höheren Brenntemperaturen rehydroxyliert und wiederaufgebaut als in kalkhaltigen. Diese Prozesse verlaufen nach höheren Temperaturen in Kochtöpfen aus ersterem Rohmaterial im Vergleich zu Vorrats- und Eßgeschirr aus letzterem Material.Das in der Töpferware adsorbierte Wasser ist in erster Linie an dem beim Brennprozeß gebildeten, röntgenmäßig amorphen Material gebunden.

Kinetic analysis of thermal dehydration and hydrolysis of MgCl2•6H2O by DTA and TG

The thermal decomposition of MgCl2·6H2O (non-dried and partly dried) and the kinetics of the process were studied by DTA, TG, DTG, IR, X-ray diffraction and chemical analysis of Mg and Cl. The reactions which occurred in the course of the thermal analysis were identified as dehydration (in steps), thermal hydrolysis of MgCl2·H2O and dehydrochloridization of magnesium hydroxy chlorides. Melting of the phases MgCl2·6H2O, MgCl2·4H2O and MgCl2 was also identified in the thermal curves. Thermal weight loss continued up to 800 °C in flowing air or nitrogen, but only up to 700 °C in static air. MgO was the end-product of thermal treatment in both cases.The kinetic parameters of the reaction, the activation energyE, pre-exponential factorA and apparent order of reactionb, were computed by several methods. The activation energy and the apparent reaction order of dehydration were found to increase with decreasing hydration number of the Mg. The dehydrochloridization process had the highest activation energy.ZusammenfassungDie thermische Zersetzung von MgCl2·6H2O (nicht und teilweise getrocknet) und die Kinetik dieses Prozesses wurden mittels DTA, TG, DTG, IR, Röntgendiffraktometrie und chemischer Analyse von Mg und Cl untersucht. Die im Verlaufe der thermischen Analyse vor sich gehenden Reaktionen sind Dehydratisierung (in Schritten), thermische Hydrolyse von MgCl2·H2O und Chlorwasserstoffaustritt aus Magnesiumhydroxychloriden. Das Schmelzen der Phasen MgCl2·6H2O, MgCl2·4H2O und MgCl2 ist in den thermischen Kurven ebenfalls zu erkennen. Im Luft- und Stickstoffstrom erfolgt ein Gewichtsverlust bis 800 °C, in statischer Luftatmosphäre dagegen nur bis 700 °C. MgO war in beiden Fällen Endprodukt der thermischen Behandlung. Die kinetischen Parameter der Reaktion (AktivierungsenergieE, präexponentieller FaktorA, scheinbare Reaktionsordnungb) wurden nach verschiedenen Methoden berechnet. Aktivierungsenergie und scheinbare Reaktionsordnung der Dehydratisierung nehmen mit abnehmendem Hydratationsgrad des Mg zu. Die Chlorwasserstoffabspaltung erfordert die höchste Aktivierungsenergie.РезюмеМетодом ДТА, ТГ, ДТГ, ИК спектроскопии, рентгеноструктурно го анализа и химического анализа было изучено термиче ское разложение гексагид рата хлористого магн ия (влажного и частично в ысушенного) и кинетик а процесса его разложения. Были и дентифицированы следующие реакции: де гидратация (ступенча тая), термический гидроли з моногидрата хлорида магния и деги дрохлорирование осн овной соли магния. На термически х кривых идентифицированы те мпературы плавления гексагидрата, тетраг идрата хлорида магни я и безводной соли. В дина мической атмосфере в оздуха и азота термическая по теря веса продолжалась до темп ературы 800 °C, тогда как в статической атмосфе ре воздуха-до 700 °C. B обоих случаях конечн ым продуктом разложе ния являлась окись магния. Несколь кими методами были расчит аны энергия активаци и, предэкспоненциальн ый множитель и кажущи йся порядок реакции. Уста новлено, что энергия а ктивации и кажущийся порядок ре акции дегидратации увелич иваются с уменьшение м числа гидратации магния. Пр оцесс дегидрохлорир ования имел наиболее высоко е значение энергии ак тивации.

Electron and hole centres produced in zircon by X-irradiation at room temperature

Radiation effects of X-rays in zircon (ZrSiO4) single crystals were studied at RT, by analysing the kinetics and spectrum of the thermoluminescence between 20 degrees and 420 degrees C. The results indicate that some of the electrons produced by the irradiation are captured by Dy3+ impurities apparently substituting for Zr4+ ions, while others are trapped at Si4+ lattice ions. The hole traps include O2- lattice ions near Si3+ or Dy2+ electron centres, as well as other unidentified impurities or point defects. A TL peak at 77.5 degrees C which consists of a broad emission band around 385 nm typical of the SiO44- group is ascribed to the recombination of an electron and a hole trapped at adjacent Si4+ and O2- ions respectively. Its measured activation energy (0.89 eV) is in agreement with the theoretical value (0.87 eV). A series of TL peaks having activation energies of 0.82-1.60 eV and showing the spectral lines of Dy3+ are attributed to electron-hole recombinations at the RE site. The main TL peak at 109 degrees C (E=1.18 eV), which contains both the 385 nm band and the Dy3+ lines, might involve a recombination process which excites both the SiO44- group and the RE ion.

Local transitions and charge transport in thermoluminescence of calcite

Abstract Colourless, natural calcite single crystals exhibit several distinct TL peaks following X- irradiation at room temperature. The emission spectra of all the peaks are attributed to Mn impurities. Kinetic analysis of the main peak at about 80°C discloses a continuous spectrum of traps having the same activation energy, 0.73 eV, and a distribution of frequency factors. Four additional peaks at 152, 179, 254 and 296°C were assigned activation energies of 0.92, 1.01, 1.52 and 1.70 eV, respectively. The kinetic data and the analysis of the emission spectra support Medlins model which ascribes the 80°C peak to local transitions such as tunnelling, and the higher temperature peaks to processes which involve transport of ionic charge-carriers.

TL emission spectra of natural feldspar

Abstract TL emission spectra of feldspars have been recorded over the wavelength range 300–800 nm. The spectra exemplify major differences between plagioclase, orthoclase, albite and microcline as noted by earlier workers. There are changes caused by heat treatments and storage.

THERMOLUMINESCENT PROPERTIES OF MICA

Abstract Natural as well as β-induced thermoluminescence (TL) of mica of the muscovite type KAl 2 (AlSi 3 O 10 )(OH) 2 was investigated. Two main glow peaks were found, as about 150° and 260°C. The 260°C peak is the prominent one in the natural TL curve while the 150°C peak is dominant in the laboratory induced TL. Both curves could be interpreted in terms of two traps with activation energies of 0.75 and 0.97 eV, which are exhausted through a second order process. u.v. phototransfer experiments indicated the existence of additional, deeper traps, undetectable by the regular TL measurements. Dose dependence studies showed a slightly superlinear behaviour. Possible applications to dating in archeology are discussed.

Analysis of the blue phosphorescence of x-irradiated albite using a TL-like presentation

Abstract By analysing the isothermal decay of phosphorescence at various temperatures, the activation energy and pre-exponential factor of the relevant trap, as well as the kinetic order of the process, can be evaluated. Methods of analysis have been suggested, in which t × I(t) is plotted as function of ln( t ), where t is the time and I the intensity of the phosphorescence. For a single trap this is a peak-shaped curve, which resembles the corresponding thermoluminescence (TL) curve. Previous methods used some features of this peak, such as the maximum and the half intensity points. In practice, however, it might be difficult to evaluate these points accurately. An improved method is suggested, which seeks the best fit of the whole curve to the theoretical expression. This method is applied to the blue phosphorescence of X-irradiated albite.