Aristeidis
Voliotis
a,
Spyros
Bezantakos
ab,
Maria
Giamarelou
a,
Marco
Valenti
b,
Prashant
Kumar
cd and
George
Biskos
*ab
aDepartment of Environment, University of the Aegean, Mytilene 81100, Greece. E-mail: biskos@aegean.gr
bFaculty of Applied Sciences, Delft University of Technology, Delft 2628-BL, The Netherlands. E-mail: g.biskos@tudelft.nl
cDepartment of Civil and Environmental Engineering, Faculty of Engineering and Physical Sciences (FEPS), University of Surrey, Guildford GU2 7XH, UK
dEnvironmental Flow (EnFlo) Research Centre, FEPS, University of Surrey, Guildford GU2 7XH, UK
First published on 19th March 2014
Traditional pottery manufacturing involves firing of the ceramics in kilns, a process that leads to high concentrations of airborne particles that are harmful to human health. In order to assess the associated exposure levels and the involved risks, here, for the first time, we investigate the size, the concentration and the elemental composition of the particles emitted during the different stages of the ceramic firing process. Number size distributions of the emitted particles, having diameters in the range from 10 nm to 20 μm, were measured in a traditional small-sized pottery studio using a Scanning Mobility Particle Sizer (SMPS) and an Optical Particle Counter (OPC). The measurements showed dominance of the nanoparticle mode (i.e., particles smaller than 100 nm) when the kiln reached temperatures above 600 °C. The mean size of the particles ranged from 30 to 70 nm and their peak number concentration was 6.5 × 105 cm−3 during the first stage of the firing process where the ceramics were unpainted and unglazed. During the second stage of the firing process, where the ceramics were painted and glazed, the mean particle size ranged from 15 to 40 nm and their number concentration peaked at 1.2 × 106 cm−3. Elemental analysis of individual particles collected during the two firing stages and studied by Energy-Dispersive X-ray (EDX) spectroscopy showed that the emitted nanoparticles contain significant amounts of lead. These findings provide new information for understanding the health impacts of traditional pottery manufacturing, and underline the need for adopting adequate measures to control nanoparticle emissions at the source.
Environmental impactTraditional manufacturing of ceramic tableware and ornamental ware is a widely dispersed occupation, which in many countries is linked to local tradition. During the manufacturing process, the ceramics are fired before and after paints and glazing are applied on their surface. In both cases, the process can lead to high concentrations of airborne nanoparticles that can be harmful to human health. Here, for the first time, we provide systematic measurements of the size distributions and the elemental composition of the particles emitted during the different stages of the ceramic firing process. |
A wide range of industrial and manufacturing processes involving combustion and/or high temperatures can produce significant concentrations of airborne nanoparticles containing toxic elements and compounds that pose a threat to human health.12–17 For instance, industrial processes such as metal casting or welding emit hazardous particles that contain heavy metals into the breathing air.18–20 A number of manufacturing processes, such as machining of materials, wood processing and asphalt roofing, also have high temperature stages that emit a high number of toxic particles.21–23
Pottery is another industry where high temperature processes are required. Manufacturing of ceramic tableware and ornamental ware is a widely dispersed occupation, which in many countries is linked to local tradition. The turnover of this industry in the EU alone was €1.8 billion in 2006, occupying ∼31000 workers in Small and Medium Enterprises (SMEs).24 The respective figures in the US in 2007 were $2.8 billion and ∼21000 workers.25
Traditional pottery manufacturing involves a two stage firing process for producing the final products. At the first stage, referred to as bisque firing, the ceramics are gradually heated in order to stabilize their shape and structure. At the second stage, referred to as glaze firing, the ceramics are fired again after paints and glazing are applied on their surface. This stage is needed in order to stabilize the surface artwork and to make the pots watertight and durable. In both firing stages, the ceramics are heated at temperatures that reach up to ∼1000 °C for a period of 8 to 16 h.
Evidence accumulated since the late 80s suggests that hazardous airborne contaminants are emitted from the firing of ceramics.26–28 Hirtle et al. (1998)29 have reported that significant amounts of metals are present in the total suspended particulate matter emitted during the firing of ceramics. These measurements can explain the findings of Hibbert et al. (1999)30 and later of Jones et al. (2013)31 who showed that significant amounts of heavy metals are present in the blood of artisanal pottery workers. Albeit the importance of these findings, to the best of our knowledge, no information on the size distribution or the size-resolved composition of the emitted particles is available.
In order to fill this gap, here we present systematic characterisation of the particles emitted during the manufacturing of ceramics in a traditional small-sized pottery studio. The size distributions of the particles (having diameters from 10 nm to 20 μm) emitted by the kiln during the two different firing stages were measured by electrical mobility and optical techniques. In addition, the elemental composition of particles collected during the two firing stages was determined by Energy-Dispersive X-ray (EDX) spectroscopy.
The studio was equipped with an electrical kiln (Skutt, Model BC 1277) that was fired almost once every other day. The kiln had a cylindrical shape with a volume of 0.29 m3, and a 4 cm hole located at the top that served as an exhaust. During the firings, the temperature of the kiln was gradually increased from ambient to 980 °C over a period of 11 h. After this period the kiln was switched off and its door remained closed until it reached room temperature. In all our measurements the kiln was loaded with the same number of ceramics.
The SMPS consisted of a cyclone, a 85Kr bipolar neutralizer,32 a Differential Mobility Analyzer (DMA),33 and a Condensation Particle Counter (CPC).34 The sampled air was first passed through the cyclone that removed particles larger than 500 nm. The smaller particles that remained in the sample stream were passed through the 85Kr neutralizer to bring the particle charge distribution into Boltzmann equilibrium, and then through the DMA where they were classified based on their electrical mobility. The concentration of the monodisperse particles downstream of the DMA was then measured by the CPC. By scanning the operating conditions of the DMA (i.e., the strength of the electric field used to classify the particles), particles having different electrical mobilities, and therefore sizes, were directed to the CPC. The time needed to scan the electric field of the DMA, and thus to measure the size distribution of the particles having diameters from 10 to 487 nm in our experiments was 3 min. The Aerosol Instrument Manager software (AIM, TSI version 6.0) was used to run the SMPS, as well as to record and invert the raw data. The SPMS was calibrated before the measurements, resulting in a precision of 3–3.5% in particle diameter and 10% in number concentration.
The OPC measured the light scattered by the particles in order to determine their size and concentration. The instrument consisted of a laser diode (λ = 780 nm), and a photodetector.35,36 In brief, the sample stream was drawn through a perpendicular condensed light beam emitted by the laser source, and the light scattered by individual particles was measured by a detector. The number concentration of the particles was then estimated by the count rate of the pulses, whereas the pulse height was used to determine their size. The particles were classified into 15 channels according to their optical diameter.37
The EDX spectra were obtained with an Analytical Scanning Electron Microscope (ASEM; JEOL Model JSM-6010LA). The samples were first inspected with a 20 kV beam having a diameter of 20 nm. Subsequently, EDX spectra from several individual nanoparticles were obtained using a beam that was 70 nm in diameter. EDX spectra were also obtained directly from the microgrid surface in order to quantify and subtract the contribution of the grid to the measurements.
Table 1 shows the average and the maximum PNCs measured during bisque and glaze firing processes, in comparison with measurements when the kiln was not operational (i.e., background particle concentration). Compared to the background, the PNCs during bisque firing was ∼8 times higher (1.6 × 105 cm−3), and during glaze firing ∼12 times higher (2.5 × 105 cm−3). The majority of the particles (85–90%) during bisque firing had diameters <100 nm, whereas all the emitted particles during glaze firing had diameters in the sub-100 nm range.
Total | d p < 100 nm | d p > 100 nm | ||||
---|---|---|---|---|---|---|
Average (× 105) | Max. (× 105) | Average (× 105) | Max. (× 105) | Average (× 103) | Max. (× 104) | |
Bisque firing | 1.6 | 6.5 | 1.4 | 5.8 | 16.0 | 10.0 |
Glaze firing | 2.5 | 12.0 | 2.5 | 12.0 | 3.0 | 1.8 |
Background | 0.2 | 0.9 | 0.09 | 0.8 | 3.0 | 1.3 |
Fig. 3 shows PNCs of particles having diameters >300 nm as measured by the OPC. For these particles, the concentration was higher during bisque firing (average value of 1.6 × 102 cm−3) than during glaze firing (average value of 70 cm−3). Considering that the average concentration of the background particles in this size range was ∼65 cm−3, the mean increase during glaze firing was in fact negligible. The concentration of the super-300 nm particles started to increase after the 6th hour in both firing processes, exhibiting a peak after the 11th hour (peak value of 4.5 × 102 cm−3) during bisque firing and after the 9th hour (peak value of 1.6 × 102 cm−3) during glaze firing. In both cases the evolution of the PNCs measured by the OPC coincided with that measured by the SMPS (cf.Fig. 2).
Fig. 3 Particle number concentration measured by OPC during bisque (solid line) and glaze firing (dashed line). |
Fig. 4 Evolution of the size distributions of the particles having diameters from 10 to 200 nm emitted by the kiln during (a) bisque and (b) glaze firing. |
The evolution of the particle size distributions during glaze firing (Fig. 4b) exhibits a different pattern. In this case, the first peak in the concentration of particles having diameters from 15 to 30 nm is observed for ∼30 minutes after the 5th hour of the process. The size and concentration of the particles started increasing again after the 7th hour, reaching highest values (∼70 nm and 12 × 105 cm−3) 9–10 hours after the initiation of the process. Interestingly, both the concentration and the size of the particles started decreasing significantly after the 10th hour of the process, reaching background levels, despite the fact that the temperature of the kiln kept increasing. An explanation of this decrease is that most of the material forming the particles during glaze firing is coming from the paints and the glaze applied on the surface of the ceramics, both of which are in small amounts and therefore get depleted before the end of the firing process.
Fig. 5 shows the evolution of the particle size distributions measured by the OPC (i.e., particles >300 nm in diameter) during bisque and glaze firing. In both cases, the concentration and the size of the emitted particles in this size range started to increase after the 8th hour of the firing process. In bisque firing the particles were produced until the end (as also observed in the SMPS measurements; cf.Fig. 2 and 4), whereas in glaze firing until the 10th hour of the firing process. Although in both cases the size of the emitted particles had diameters smaller than 500 nm, the particles emitted by glaze firing were significantly smaller.
Fig. 5 Evolution of the size distributions of the particles having diameters from 300 to 900 nm emitted by the kiln during (a) bisque and (b) glaze firing. |
The differences in the temporal evolution and the individual size distribution observed during the two firing processes can be explained by differences in the composition of the fumes produced in each case. During bisque firing, where only unpainted/unglazed ceramics are inserted into the kiln, the most dominant source of vapours leading to particle formation is the clay. During glaze firing on the other hand, the most dominant sources are the compounds of the glaze and the pigments. Considering that the number of ceramics in the kiln is the same in both firings but the amount of paints/glazing is significantly smaller compared to that of the clay, the systematic difference in the size of the emitted particles between the two stages can also be attributed to the different sources of the vapours.
Fig. 6 EDX spectra on individual particles collected during the two firing stages: (a) bisque and (b) glaze firing. |
The elemental composition of the particles collected during both firing stages showed that they consisted mainly of Si, which is emitted by the clay. Particles collected during glaze firing also contained significant amounts of Pb, which together with a fraction of Cu and possibly C observed in all the samples can be attributed to the materials used in the pigments and the glazing applied on the surface of the ceramics. The results from this study are especially important for understanding the systematic exposure of potters and the incidental exposure of the public to airborne nanoparticles emitted from the traditional manufacturing process of ceramics.
This journal is © The Royal Society of Chemistry 2014 |