Vitreous Phase Research Articles

Phase-dependent mechanical properties of two-dimensional silica films: A molecular dynamics study

The recently discovered two-dimensional (2D) silica films can be grown in both crystalline phase and vitreous phase. Using molecular dynamics simulations, we show that these two phases of 2D silica possess vastly different mechanical properties. It is shown that the Young’s modulus of crystalline 2D silica is 54% greater than that of its vitreous counterpart. The vitreous 2D silica fails via the ductile fracture with serrated stress-strain curves, while its crystalline counterpart exhibits the abrupt brittle rupture. Both the fracture strength and fracture strain of crystalline 2D silica are significantly larger than those of its vitreous counterpart. Fracture mechanics theory is utilized to give some explanations to the different fracture properties of these two phases. Moreover, 2D silica mixed by crystalline phase and vitreous phase is also studied. Owing to the different mechanical properties of crystalline and vitreous phases, the elastic and fracture properties of this mixed-phase 2D silica are strongly dependent on its crystallinity. Specifically, we find that the Young’s modulus of the mixed-phase 2D silica, ranging between the values of its crystalline and vitreous counterparts, increases as the degree of crystallinity increases. Meanwhile, with the increase of the degree of crystallinity the fracture strain of the mixed-phase 2D silica is found to decrease, making its value smaller than that of the purely crystalline and vitreous 2D silica.

Physical and mechanical properties improvement of a porous clay ceramic

This work deals with the chemical, physical, thermal and mechanical analyses of porous clay and non-plastic clays mixtures from Tunisia. Five mixtures which are M1 (70 mass% clay +30 mass% waste brick), M2 (70 mass% clay +15 mass% waste brick +15 mass% sand), M3 (70 mass% clay +30 mass% sand), M4 (70 mass% clay +30 mass% chamotte) and M5 (70 mass% clay +15 mass% chamotte +15 mass% sand) were sintered between 900 and 1100°C and were evaluated for their possible ceramic applications. Thermal analysis (DTA/TG), dilatometry, X-ray diffraction (XRD), scanning electron microscopy (SEM) and physical-mechanical analyses (tensile strength, water absorption and bulk density) were used to assess the thermal behavior, phase evolution and microstructure of the fired mixtures. The formations of both the anorthite phase and the vitreous phase have improved the required ceramic product performance. M1 has the highest values of tensile strength and bulk density at 1100°C. According to the European Norm EN 14411, all the mixtures belong to the BIII group which are suitable for the production of earthenware and unglazed stoneware.

Effect of phase and size characteristics of fly ash from power station on properties of bitumen composites

Fly ash from power stations is one of the effective fillers in bitumen binders due to its high specific surface area in origin state. This paper is focused on study of effect of phase and size characteristics of fly ashes on properties of bitumen binder. In this work a high-calcium (FA1) and low-calcium (FA2) fly ashes are used. According to chemical and mineral composition of FA1 is reactive in water medium because it contains hydraulic minerals. FA2 is hydraulically inert mainly consisting of aluminosilicate vitreous phase. FA1 is characterized by higher specific surface vs. FA2 according to Blain method data but lower specific surface measured with BET method. It can be associated with high porosity of FA2 that significantly influent on bitumen content in binding system. Lower packing density of FA2 particles leads to reducing of strength characteristics in bitumen composite. Low adsorption of bitumen to surface of FA1 particles initiates hydration process of reactive minerals as well as formation of ettringite phase that leads to extension of the binder system and structure decompaction. Bitumen composites based on FA2 with higher adsorptive capacity is characterized by more stable structure.

Measuring the Densities of Aqueous Glasses at Cryogenic Temperatures.

We demonstrate a method for determining the vitreous phase cryogenic temperature densities of aqueous mixtures, and other samples that require rapid cooling, to prepare the desired cryogenic temperature phase. Microliter to picoliter size drops are cooled by projection into a liquid nitrogen-argon (N2-Ar) mixture. The cryogenic temperature phase of the drop is evaluated using a visual assay that correlates with X-ray diffraction measurements. The density of the liquid N2-Ar mixture is adjusted by adding N2 or Ar until the drop becomes neutrally buoyant. The density of this mixture and thus of the drop is determined using a test mass and Archimedes principle. With appropriate care in drop preparation, management of gas above the liquid cryogen mixture to minimize icing, and regular mixing of the cryogenic mixture to prevent density stratification and phase separation, densities accurate to <0.5% of drops as small as 50 pL can readily be determined. Measurements on aqueous cryoprotectant mixtures provide insight into cryoprotectant action, and provide quantitative data to facilitate thermal contraction matching in biological cryopreservation.

Performance enhancement of the ceramic products by adding the sand, chamotte and waste brick to a porous clay from Bir Mcherga (Tunisia)

Investigations were undertaken to assess the chemical, physical and mechanical changes of porous clay and non-plastic clays mixtures fired between 900 and 1150°C. Five mixtures (S1: 70 mass % clay +30 mass % chamotte, S2: 70 mass % clay +15 mass % chamotte +15 mass % sand, S3: 70 mass % clay +30 mass % sand, S4: 70 mass % clay +30 mass % waste brick and S5: 70 mass % clay +15 mass % waste brick +15 mass % sand) from Tunisia were evaluated for their possible ceramic applications. X-ray diffraction (XRD), thermal analysis (DTA/TG), dilatometry, scanning electron microscopy (SEM) and physical-mechanical analyses such as tensile strength, water absorption, linear shrinkage and bulk density were used to assess the phase evolution and microstructure of the sintered mixtures. The formations of both the anorthite and mullite phases and the vitreous phase have improved the required ceramic product performance. S4 and S5 had the highest values of tensile strength at 1150°C. These mixtures, according to the European Norm EN 14411, are suggested for porcelainized and fully vitrified stoneware. S3 sample is suitable for the production of vitrified stoneware. S1 and S2 mixtures are suitable the production of earthenware and unglazed stoneware.

Evaluation of pozzolanic activity of volcanic tuffs from Tibet, China

This paper presents a study of the characteristics and pozzolanic activity of three volcanic tuffs to assess the quality and availability of the tuffs for use as supplementary cementitious materials in cement-based composites. The main parameters of the tuffs (chemical composition, proportion of vitreous phase, mineral species and rock structure) were assessed by chemical composition analysis and x-ray diffraction. The pozzolanic activity component (PAC) of tuff, calculated from chemical composition and amorphous phase content, is proposed as an intrinsic index of pozzolanic activity without consideration of particle size and was used to evaluate the tuffs. The pozzolanic reaction degree (PRD) of the tuff/cement pastes was determined by hydrochloric acid dissolution. The pozzolanic activity strength index (PASI) was evaluated as the ratio of the compressive strength of the blended tuff/cement mortar to that of a control cement mortar. The results showed that the tuffs possess great potential for pozzolanic activity in terms of PAC, PRD and PASI because they have high contents of silicon dioxide and aluminium oxide and plentiful amorphous phases. A correlation between PAC and pozzolanic activity index was revealed and a consistent relationship between PRD and PASI was also found.

White emitting GeO2PbO Bi2O3SrF2 glass and nano-glass ceramics co-doped with Eu3+, Tb3+ and Tm3+ ions

Abstract The study is devoted to a luminescent glass and its devitrified counterpart (nano-glass ceramics) both triply co-doped with Eu 3+ , Tb 3+ and Tm 3+ ions. Besides for comparison glasses singly doped with Eu 3+ , Tb 3+ or Tm 3+ and a glass doubly co-doped with Eu 3+ and Tb 3+ were studied. The singly Tb 3+ doped glass did not exhibit cross-relaxation effect. In the case of the di-doped glass energy transfers Tb 3+ → Eu 3+ was certified. Then, for the tri-doped glass energy transfers Tb 3+ → Eu 3+ and in reverse Eu 3+ → Tb 3+ as well as Tm 3+ → Tb 3+ were observed. In the emission spectrum of the triply co-doped devitrified glass, similarly as in the other Eu 3+ doped glasses, band peaked at 612 nm ascribed to 5 D 0 → 7 F 2 transition in Eu 3+ was dominated above the 5 D 0 → 7 F 2 band. It suggests that the majority of Eu 3+ (and very probably also Tb 3+ and Tm 3+ ) ions were incorporated into the residual vitreous phase of the glass-ceramics. Reduced luminescence lifetimes τ 1 of the Tb 3+ and Tm 3+ dopants in the triply co-doped glass in comparison to τ values of these dopants singly embedded in the glasses additionally confirms presence of the energy transfer effects in the tri-doped material. Devitrification of the tri-doped glassy phosphor did not improve its white color emission.

Behaviour of Fly Ash-Based Geopolymer Panels Under Fire

This paper examines the influence of the type of fly ash and activating solution in the development of fly ash-based geopolymers used to manufacture panels with fire resistance properties. Two fly ashes with different vitreous phase contents, from coal combustion and from coal and pet-coke co-combustion, were used as the aluminosilicate source. Two activating solutions were also used: sodium silicate and potassium silicate. Geopolymers, after 28 days of curing at 20 °C and 45% of relative humidity, were subjected to different tests: physicochemical (porosity, density, volumetric expansion, water absorption and moisture content), mechanical (flexural and compressive strength) and fire resistance tests. Panels prepared with both alkali silicates present a similar insulating capacity. Mechanical properties were improved with sodium silicate. The best mechanical properties and the worst fire resistance capacity were observed in the geopolymer panels manufactured using the fly ash with the highest amorphous content. All geopolymer panels were subjected to the Dutch standard leaching test NEN 7345, a common leaching test for monolith samples. The type of ash is the most important factor in leaching behaviour, while the type of activating solution has no influence on the leachability of the geopolymer panels.

Microcanonical molecular dynamics simulation of the vitreous phase transition of poly(binaphthylphosphazene)

Properties of poly(binaphthoxyphosphazene)s were investigated employing molecular dynamics simulations in the original microcanonical ensemble. In particular we have applied our methodology to the case of isotactic poly-(2,2´-dioxy-1,1´-binaphthylphosphazene) (P-DBNP) in order to estimate the vitreous transition temperature Tg, the energy barrier Δμβ for β-relaxation of the BNP unit, and the thermodynamic entropy of the system. According to our results, the transition is consistent with the Adam-Gibbs model and the specific rotation of α and β-relaxations, which varies significantly with the molecular weight Mw. Our results suggest a rapid interconversion between the different proportions of the chiral R and S repeating units (70% and 30%, respectively) of the non-isotactic copolymer below 523K and a slow atropisomerization of the DBNP units above 523K which, in agreement with recent predictions on glassy polymer matrices, becomes much faster as the temperature approaches Tg~573K.

Middle Eocene clay from Goset Abu Khashier: Geological assessment and utilization with drinking water treatment sludge in brick manufacture

This investigation aims to study the physico-ceramic properties of bricks that were made from the Goset Abu Khashier clay (Qarara Formation, Middle Eocene deposits, El-Minia, Egypt) replaced with 15–60wt% drinking water treatment sludge (DWTS). The Qarara Formation deposits located in the east bank of the Nile River between Beni Suef and El-Minia Governorates, belong to the Middle Eocene deposits. The Qarara Formation deposits in the Goset Abu Khashier area contain several million tons of clay deposits. The Qarara Formation is composed of a grey to green shale (silty clay) grading upward to marl and limestone. The clay contains kaolinite (29wt%), montmorillonite (20wt%), albite (27wt%) and quartz (23wt%). The clay contains a high content of silica (49.41wt%) and a low content of alumina (19.77wt%) with considerable amounts of Fe2O3 (11.37wt%) and fluxing oxides (3.7wt%). Clay bricks containing 15–60wt% DWTS were fired at a temperature of 700–1000°C. The ceramic properties of bricks were determined according to the ASTM specifications. The phase composition of bricks was determined by XRD, FTIR and SEM techniques. It was investigated that the incorporation of DWTS in the body of clay bricks moderates the ceramic properties of fired bricks to be in agreement with the limiting values that were recommended for traditional bricks. XRD results illustrated that dehydration of kaolinite and the formation of metakaolinite facilitates vitrification and contributes to brick densification. FTIR results showed the formation of a wollastonite phase and indicated that increasing DWTS content facilitates the persistence of a vitrified phase. SEM micrographs illustrated that the porosity and densification of brick matrix was affected by the formation of amorphous vitreous phases and decreased with DWTS contents. It is recommended that the replacement of examined clay by 15–30wt% of DWTS is considered as the appropriate percentage for building bricks production and minimizes environmental impacts due to DWTS disposal into water bodies.

Preparation and Characterization of Borosilicate Glass Waste Form for Immobilization of HLW from WWER Spent Nuclear Fuel Reprocessing

ABSTRACTBorosilicate glassy materials for immobilization of HLW from Russian WWER (PWR) spent nuclear fuel reprocessing were designed, synthesized in a resistive furnace, and characterized by XRD, SEM/EDS, and FTIR spectroscopy. Chemical durability was determined by PCT-A procedure and compared to EPA glass and reference data. The glasses with 20 and 25 wt.% waste loading were found to be X-ray amorphous, homogeneous and chemically durable. Glass network formally had a relatively low degree of connectedness that was increased due to embedding of different structural groups thus improving chemical durability. Boron is present primarily in trigonal oxygen coordination. The glasses with 40-45 wt.% waste loading contained minor britholite phase concentrating rare earth elements and as expected trivalent actinides. Glassy product with up to 30 wt.% waste loading was also produced by cold crucible inductive melting at the IPCE RAS lab-scale unit equipped with 56 mm inner diameter copper cold crucible and energized from a 10 kW/5.28 MHz generator. The product was composed of vitreous phase and minor britholite with average composition K0.39Sr1.99Fe0.16Nd5.50Si7.96O26.60.

XAFS Study of Iron and Nickel Speciation in Complex Sodium Aluminophosphate Based Glasses

Iron and nickel oxidation state and coordination in complex sodium aluminophosphate based glasses suggested as potential matrices for immobilization of legacy high level waste currently stored in stainless steel tanks at PA «Mayak» (Ural reg., Russia) were determined by X-ray absorption fine structure spectroscopy (XAFS: XANES/EXAFS). The glasses containing (wt.%) 20-30 Na2O, 6-12 Al2O3, 40-52 P2O5, 2-5 Fe2O3, 1-3 NiO, 0-6 B2O3, 10-15 other waste oxides produced by quenching of their melts were fully amorphous or contained minor Fe and Ni free phases. Fe in the glasses was found to be predominantly trivalent with an average Fe-O distance and a coordination number (CN) in the first shell of 1.94 to 1.97 A and 5.2 to 5.8, respectively, mostly in octahedral oxygen environment. Ni is divalent in all the glasses and has in the first shell an average Ni-O distance and CN of 1.97 to 2.03 A and 4.9 to 5.6, respectively. The first shell of both Fe and Ni is somewhat distorted. The second and further coordination shells are weakly appeared exhibiting no clustering and homogeneous distribution of Fe and Ni ions in the glass network. The data on Fe obtained are in good agreement with those from Mossbauer study of same glasses. After annealing glasses were partly devitrified and interpretation of XAFS data is strongly complicated due to Fe and Ni partitioning among crystalline and vitreous phases.

Research Method and Spectral Analysis of Ancient Polychromatic Silicate Artifacts

The spectral properties, chemical compositions and phases of materials constituting the surface of 5 ancient polychromatic silicate artifacts have been analyzed non-invasively with self-built multispectral imaging system, X-ray fluorescence spectrometer (XRF) and laser Raman spectrometer (LRS). Based on spectral response in multispectral images, materials constituting the surface of 5 samples can be divided into different areas, and most of blue, green, purple areas with fluorescence behavior are also mapped. The results of XRF indicate that the chemical compositions of areas are different, but the major compositions of them are SiO(2), PbO, BaO. 5 samples mainly belong to PbO-BaO silicates. The coloring agents of all areas with fluorescence behavior are Cu ions. A variety of mineral phases including vitreous phase, Chinese blue, Chinese purple, quartz, hematite, lead carbonate, amorphous carbon and so on, are also identified by LRS. Chinese blue and Chinese purple can emit infrared radiation when excited by visible LED. The result of LRS is verified by X-ray diffraction (XRD). Combining the multispectral imaging area-measurement technique used to research paintings, and XRF, LRS which are usually used to analyze chemical composition of silicate artifacts, the present research proposes a more efficient and non-invasive research method to analyze ancient polychromatic silicate artifacts. Spectral characteristic and chemical composition of the sample are connected when spectral images, X-ray fluorescence spectra and Laser Raman spectra are combined. It has great significance for increasing efficiency of analysis, enhancing the overall understanding of silicate artifacts and decreasing risk of damage.

Lightweight aggregates from waste materials: Reappraisal of expansion behavior and prediction schemes for bloating

There is a growing concern on lightweight aggregates (LWA) for recycling of waste materials: this manufacturing technology is considered a flexible tool to make it possible the conversion of large amounts of residues into building products. The main challenge for this purpose is the control on the technological behavior during firing of waste-bearing batches. The target is engineering the sintering and expansion of LWA in order to tailor bulk density, mechanical strength and water absorption on various possible applications (e.g., structural and non-structural lightweight concretes, lightweight mortars, filtration substrates, floriculture). The prediction of the aggregate expansion – and hence the batch design – is usually carried out utilizing the Riley’s and/or Cougny’s schemes, which are based on the bulk chemical composition of clay bodies. However, collecting the literature data on waste-based LWA and plotting them in the Riley’s and Cougny’s diagrams, no reliable discrimination turns out between expanding and non-expanding batches. In the same way, the attempt to modify the Riley’s and Cougny’s parameters, including elements present in wastes but not in natural raw materials, was unsuccessful. From this standpoint, a new approach to assess the batch expandability was developed through a deeper comprehension of mechanisms acting on sintering and expansion of LWA. A specific characterization protocol was set up, entailing hot-stage microscopy and determination of phase composition and microstructure. This implies a paradigm shift from batch (chemical composition) to vitreous phase (amount, chemical composition, and viscosity at high temperature). The vitreous phase plays a key-role for bloating, microstructure and physical properties of LWA. A working example is presented about new LWA based on waste glasses, ceramic tile polishing sludge and clay raw materials.

Effects of two mixtures of kaolin-talc-bauxite and firing temperatures on the characteristics of cordierite- based ceramics

Cordierite (2MgO·2Al2O3·5SiO2) is a thermal resistant ceramic presenting both low thermal expansion and electrical conductivity along with good chemical and mechanical properties. Thereby, it is suitable as material for manufacturing electric insulators, catalysis, refractory and porous ceramics. Solid-state reaction is the most common method for cordierite preparation. For this purpose, different mixtures including clay, alumina, talc, sand, diatomite and gibbsite are used. The present study is devoted to the preparation of cordierite-based materials using two mixtures of kaolin, talc and bauxite. Hence, prismatic specimens were shaped and tested to uniaxial compressive strength and heated in the range of 1200–1400°C. The fired specimens were investigated via X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy. Also, physical (firing shrinkage, water absorption, bulk density) and mechanical (bending strength) and dielectric properties of the fired bodies were measured as a function of temperature. The results showed that the fired bodies of the mixture composed of 50% by mass of kaolin, 20% by mass of talc and 30% by of mass of bauxite together with high sintering temperature (1400°C) resulted in the formation of abundant amount of cordierite in conjunction with mullite, cristobalite and pseudorutile. Also, there was increase of firing shrinkage, bulk density and bending strength. These changes were essentially linked to amount of new phases that were formed. Water absorption declined with increase of sintering temperature which was related to the reduction of porosity due to the formation of vitreous phase. The mixtures of kaolin-talc-bauxite which were fired at the range of 1300–1400°C exhibited dielectric constant values varying between 20.3 and 2.8 along with dielectric loss tangent which varied between 0.1316 and 0.0002 at room temperature of the laboratory. The two mixtures can be considered as promising materials for both ceramic and electronic applications.

Methodology for Analysis of the Reactivity of Coal Fly Ash Using Selective Dissolution by Hydrofluoric Acid

The use of some additions as cement replacement has played an important role in the amount of clinker reduction. Due to its huge worldwide availability one of the most important and recognised replace agent is the fly ash, but is also estimated that only 30 to 40% of fly ash is used, although there are some challenges to be overcome, such as the reactive potential quantification. Chemical analysis is typically determined by the oxides content and does not take into account the vitreous and crystalline phase ratio. Therefore, this paper describes in detail one selective dissolution method, based in the hydrofluoric acid attack, to the quantification of the vitreous phase (reactive) and crystalline phases (not reactive). Fly ash from Portuguese Pego thermoelectric power plant was submitted to different attacks. To confirm the method and its effects, chemical and physical analysis were performed, such as STA, XRF, XRD with Rietveld refinement, SEM, EDS and laser diffraction granulometry. The results can be used to quantify the potential reactivity of this type of fly ash. The best results were achieved with 1% hydrofluoric acid attack during 6 hours.

Study on preparation of thermal storage ceramic by using clay shale

Shale was used as main raw material for developing thermal storage ceramics. The samples were fabricated via semi-dry pressing followed by pressureless sintering. The result showed that the sample (75% shale, 10% kaolin, 10% potash feldspar and 5% soda feldspar) fired at 1080°C exhibited the best comprehensive performance. Ocular examination reveals that no cracks were observed after 30 cycle times thermal shock tests (wind cooling from 600°C to room temperature). The results presented that the high bending strength remained after 20 cycle times thermal shock tests but plummeted at the thirtieth time. Other properties were given as follows: bulk density: 2.60g/cm3; thermal conductivity: 2.33W/(m°C); and heat storage density: 578.50mJ/m3. XRD analysis indicated that the quartz and hematite were the main solid phases in the sample. Some isolated pores, quartz crystals, granular hematite crystals and needle-like mullite crystals were observed in the matrix according to the SEM (Scanning Electron Microscope) analysis. More pores were found with temperature rizing according to SEM analysis. The relatively high content of Fe2O3 contributed to the formation of the vitreous phase and favored the densification. Overall, the introduction of shale effectively reduced the firing temperature and performed the better thermal storage properties.

Composition and structure of the shells of aluminosilicate microspheres in fly ash formed on the combustion of Ekibastuz coal

A systematic study of the chemical and phase composition and structure of the shells of narrow fractions of nonmagnetic cenospheres separated from a concentrate of fly ash cenospheres from the combustion of pulverized SS (weakly caking coal) coal from the Ekibastuz Basin was carried out. It was found that the separated narrow fractions were characterized by a high Al2O3 content of 33–38 wt %. The phase composition included 57–73 wt % vitreous phase, 25–40 wt % mullite, and 1.2–2.5 wt % quartz. An increase in the average diameter, thickness, and porosity of the glass-crystalline shell of globules with the concentration of aluminum was observed. Globules of the following two types were identified in the obtained fractions: spherical globules with an annular structure and foamy globules with a network structure formed from different illite and kaolinite mineral precursors, respectively.

Form birefringence induced in multicomponent glass by femtosecond laser direct writing.

We demonstrate a new kind of form birefringence in lithium niobium silicate glass induced by femtosecond laser direct writing. By combining electron backscatter diffraction and transmission electron microscopy, we reveal a self-assembled nanostructure consisting of periodic phase change: nonlinear optical nanocrystals embedded in a network of "walls" in a vitreous phase. These "walls" are aligned perpendicular to the laser polarization direction. This self-organized nanostructure may successfully explain the origin of the laser-induced birefringence in this multicomponent glass quite differently from pure silica. These findings highlight a spectacular modification of glass, and enable construction of a high contrast three-dimensional refractive index and birefringent structures at the micrometer scale in multicomponent glasses.

Melting behaviour of feldspar clasts in high-fired Sue ware

This contribution examines the melting behaviour of feldspar clasts in 9 th century CE Sue ware from the Nakadake Sanroku kiln site, Kagoshima prefecture (Kyūshū), southern Japan. Sue ware is stoneware of typical grey to brownish-grey appearance fired at high temperature; its surfaces may be untreated or covered by natural ash glaze. Firing was performed in sloping single-chamber tunnel kilns at reducing conditions and temperatures exceeding 1000 °C. Set in a vitrified matrix (glass, mullite, minor spinel and cristobalite), quartz and feldspar constitute ubiquitous clast components. The varying degree of vitrification (15–60 wt.% glass phase) and reversely correlated modal proportions of quartz and feldspar clasts likely reflect superposed effects of varying exposure time to peak-firing temperature and temperature gradients in the kilns. Sodic plagioclase clasts (An 18-25 ) melted congruently, producing sieve-textured domains of vitreous phase and residual calcic plagioclase, while calcic plagioclase clasts (An 35-63 ) were dissolved incongruently by reaction with matrix melt, generating narrow vitreous rims embedding residual calcic plagioclase laths at the immediate contact with the host plagioclase and acicular mullite towards the ceramic matrix. Elevated Fe, Mg, and K contents of the melt phase with development of concentration gradients indicate bi-directional diffusive element transfer between reaction domains and the ceramic matrix. Melting phenomena of alkali feldspar clasts, depending on the intensity of vitrification, range from few micrometre-thin vitreous margins to completely melted blistered clasts. With advanced melting, pronounced chemical zonations are developed with continuous gradation from potassic core compositions (Or 90-80 Ab 10-20 An ) to sodic compositions (Or 68-51 Ab 30-46 An 2-3 ) in domains adjoining the vitreous domain. The chemically homogeneous vitreous phase retains the 1:3 Al:Si-stoichiometry of the feldspar framework, whereas K and Na are depleted and Ca, Fe and Mg enriched relative to the precursor feldspar. These findings demonstrate that (1) alkali feldspar clasts did not melt through eutectic reaction with silica/quartz nor via the incongruent reaction K-feldspar → leucite + liquid; (2) melting was initiated by diffusive K-Na exchange with the matrix, thereby shifting the pristine clast compositions towards the thermal minimum (Or 40 Ab 60 ) in the Ab-Or-An ternary system; (3) while sodic marginal domains (Or 68-51 Ab 30-46 An 2-3 ) melted congruently, residual core domains (Or 75-66 Ab 24-33 An 1 ) melted incongruently producing micrometre-scale intergrowths of skeletal plagioclase and melt; (4) melting involved diffusive cation exchange through a quasi-stationary aluminosilicate framework, viz . influx of Ca + Fe + Mg to the feldspar melt domains and concomitant release of the fluxing components K + Na to the ceramic matrix. The evaluation of feldspar melting phenomena suggests firing temperatures did not exceed ca . 1150 °C.