Proceedings of the 1st Iberic Conference on Theoretical and Experimental Mechanics and Materials /

11th National Congress on Experimental Mechanics. Porto/Portugal 4-7 November 2018.

Ed. J.F. Silva Gomes. INEGI/FEUP (2018); ISBN: 978-989-20-8771-9; pp. 1043-1054.

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PAPER REF: 7456

OVALIZATION AND OTHER PORCELAIN FLAWS WHEN FIRED

USING MICROWAVE TECHNOLOGY

Tiago Santos1,2,3(*)

, Vítor F. Costa3, Luís C. Costa

1

1Department of Physics and I3N, University of Aveiro, 3810-193 Aveiro, Portugal

2Porcelanas da Costa Verde S.A., 3844-909, Vagos, Portugal

3Centre of Mechanical Technology and Automation, Department of Mechanical Engineering, University of

Aveiro, 3810-193 Aveiro, Portugal (*)

Email: tiago.santos@ua.pt

ABSTRACT

Research was developed on the ovalization of tableware porcelain cups sintered using

microwave radiation. The firing process control was performed through a calibrated

pyrometer whose set point was varied from 1100 ºC up to 1410 ºC. The dependence of

ovalization on the firing temperature, firing time period, soaking time, number of pieces in the

furnace, their shape and their relative positions inside the furnace are analysed. Results show

that the degree of ovalization of the porcelain cups, induced by the microwave heating

process, depends on all the above mentioned variables except for the soaking time. Superior

shape uniformity corresponding to lower ovalization was achieved when there are a high

number of cups inside the microwave furnace and for moderate heating rates. To achieve

acceptable dimensions, the minimum number of porcelain cups to sinter per batch is around

12.

Keywords: Tableware porcelain, microwave firing, defects, ovalization.

INTRODUCTION

In the ware industry quality of the manufactured products is fundamental. Porcelain is a

manufactured product obtained when the greenware is subjected to firing temperatures close

to 1400 °C. The crystallochemical transformations that occur during the firing, give to

porcelain the impermeable (absence of open porosity) characteristics, high mechanical

strength, chemical and scratch resistance, aesthetical properties such as high brilliance,

translucence and white colour (if not decorated), and a very distinctive sonority [1], [2]. Some

stages during porcelain manufacturing are of major importance as some flaws can be

attributed to them, such as throughout the shaping/conformation and handling of the piece,

scratches, marks, blisters, lamination and capping being some possible defects. After

conformation, the greenware must be dried to remove interstitial water from its porous

structure; a process that needs to be carefully controlled otherwise can be subjected to tensile

stresses that alongside with the pressure created in the ware body due to the vapour released,

can give rise to some defects such as warping. This occurs because the surface will dry more

rapidly than the body’s interior. In extreme cases, if the drying process is uncontrolled, it can

cause the crack or even the shattering of the ware during firing [3], [4]. Some defects can

occur during this process, especially in bodies with low plasticity constituents. Usually quartz,

feldspar and kaolin are the main raw constituents. Kaolin is the raw clay material that

provides cohesion during the drying process, and the plasticity and the necessary strength for

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the piece conformation during its manufacturing. Quartz, or other filler-like constituent,

provides resistance to cracking during the drying process and reduces its deformation due to

shrinkage during the firing process, providing structural stability to the ware body. The

feldspar is, comparatively to the two referred constituents, the low temperature melting

mineral, and it is the main responsible for the development of the vitreous, amorphous, phase,

enabling an open porosity lower than 0.5%[1], [5]-[8].

The forms of silica that may be present in the ware during the heating and cooling show major

changes in thermal expansion/retraction coefficients. Quartz has a continues increase of the

linear thermal expansion (LTE) coefficient up to ∼0.9% until reaching the structural

temperature transition, at approximately 573 °C; it is followed by a sharp increase, of

∼0.55%, reaching a LTE of ∼1.45% at ∼600ºC. Cristobalite has a similar behaviour, the LTE

changing ∼0.2% at a transition temperature between 220 ºC and 280 ºC, increasing

instantaneously up to ∼1.25% at the transition temperature. Transitions temperatures of

cristobalite are not so restrict as that of quartz, as well as that of tridymite, another silica form

which undergoes a series of inversions at different temperatures. These ones have more

significant LTE increase of about ∼0.2% at ∼125 ºC. The amorphous silica presents a

continuous LTE increase, reaching about 0.1% at 1000 ºC [9]. Forms of silica, like

cristobalite can be present in the ware paste or to be formed from quartz during the firing

process. During the heating cycle, due to the ware matrix plasticity, these structural

transformations have little implications; however, the reversible reaction (occurring during

cooling) has greater impact as porcelain is completely densified. If cooled very fast, cracks

can be originated by the mismatches on the thermal expansion coefficients of the vitreous and

the others forms of silica (and also of mullite, another constituent phase of porcelain) [1], [2].

The volume change of the quartz particles during this transformation is, according to [4], of

1% and according to [1] of about 2%.According to [4], cristobalite presents a volume change

of 3% and according to [1] this value goes up to 5%. For tridymite its volume change,

corresponding to the β1 → α structural transition, is of the order of 0.3% [4].

The shape, the wall thicknesses, the contact area with the support base, the thermal

conductivities of the materials to process and its neighbours, as well the volumetric

uniformity on the heat transfers (from and into the material to heat) cannot be neglected. Like

drying time, firing time is also usually dependent on them. Long time cycles are usually

associated with a low surface area to volume ratio, and bad thermal conducting ceramic ware

materials [4].

Appearance of porcelain, just like its colour, which during the firing process can be affected

by the presence of iron particles (impurities) in the ware paste and in the glaze, is crucial for

this industry. Small drake dots/marks on the porcelain are usually due to the remaining

iron/metallic impurities with considerable sizes. Smaller iron oxides, and others, may create

local shadows (attributed to bad degasification), and even affect the colour of the full sintered

porcelain. Not only the size of the particles is important, but the amount, the mixing, the

presence of other impurities and the temperature can influence the colouration of the finished

product [9]. For example, ferrihydrate may confer a brownish red colouration, hematite a red

colouration, maghemite a reddish brown colouration, lepidocrocite an orange colouration, and

goethite a brownish yellow colouration [10]. More information on that can be found in [9].

Residual amounts of 0.4% can be sufficient to provide an ivory/buff colouration to the

porcelain finished product, values higher than 1% giving it a reddish colouration [4], [10].

Usually the iron reduction is achieved with the presence of CO/CO2 inside the kiln/furnace,

accomplished by incomplete gas burning. Small percentages of CO, between 2% and 4% [11],

should be enough to ensure the characteristic porcelain white colour. However, if this reaction

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starts too late and the open pores begin to close before the reaction occurs, a barrier to the

penetration of the CO gases into the ware is created, giving a slightly brown-yellow

colouration to the porcelain. If the piece surface has heterogeneities, it is very probable that

during the firing process the glaze presents superficial defects such as pinhole and bubbles

[4]. The black core [12] is another effect that occurs when the porcelain surface vitrification

takes place before the burning of the organic matter present in the porcelain paste. It is

noteworthy to mention that organic matter is important as it attributes plasticity to the ware

paste. There are several defects and possible causes to them. The coexistence of various

causes and the existence of various defects make difficult a single cause-effect association.

Bloating [9] is one possible defect if porcelain is heated to a temperature higher than its

maximum densification temperature. This defect is characterized by the orange-peel

appearance of the porcelain surface, which arises from the decomposition of oxides and the

expansion of gases that do not escape when the vitrification occurs and the open pores close

before gases are expelled from the ware. Remaining closed porosity, which coalesce with the

temperature increase, deteriorates the bulk porcelain density and its mechanical properties.

The porcelain body shape is also a central parameter, whose non-compliance may lead to

rejection of the porcelain piece. Porcelain body shape is usually assessed through visual tests

made by specialized workers. In the present work, body shape, and specially ovalization of

porcelain cups sintered/fired using microwave radiation heating is analysed. Ovalization of

porcelain cups sintered/fired using resistive (electric) heating is also analysed for comparison

purposes.

Low thermal conductivity materials, like the porcelain and its base constituents, usually lead

to high thermal gradients in pieces independently of the heating method/technology used to

process them. If fast heating is implemented, it will confer non-uniform properties to the

material and cracks may appear [13], [14]. Using microwave heating, heat can be released

inside the solid material, and if the process is properly controlled it can be heated more

volumetrically. It depends, however, on several factors, including the materials dielectric

properties. When microwave heating is used, the electromagnetic field (EMF) non-

homogeneity may originate hot spots in the material. The EMF maxima and minima, that in

fact are what allows the heating to be carried out, must be minimized. If not, may induce

deformations due to thermal and densification differences between the hot and the cold zones

in the material. The occurrence of thermal runaway must be avoided as it may damage the

material due to local uncontrolled temperature increase. This is due to stationary hot spots in

the material(s) that occur when their dielectric properties change abruptly with the increase of

temperature. The thermal runaway phenomenon is shown numerically in [15], simulation

studies demonstrating how the ceramic ware may be heated when processed using microwave

radiation, the electromagnetic patterns inside the furnace being dependent (not only, but also)

on the number, shape and position of the pieces [16]. In previous studies [14], [17] the authors

experimentally observed that the porcelain deformation might be dependent on some of the

referred variables. The present work aims to demonstrate that, the ovalization is a relatively

important parameter for porcelain fire using microwave radiation.

EXPERIMENTAL METHODOLOGY

The multi-mode microwave furnace used for the ovalization tests is powered by 6

magnetrons, each one with a nominal power of 1kW attached to a WR-340 rectangular

waveguide. The interplay between power, time and temperature ensures a higher uniformity

of the electromagnetic field inside the furnace cavity, allowing the fire of a ceramic ware

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without major defects, faster heating rates than those achieved in conventional porcelain

firing and with lower energy consumption. A silicon carbide (SiC) plate is used in the furnace

as a base for porcelain pieces and as microwave susceptor [17]-[19]. During the firing

process, when changing one of those parameters the others were maintained constants, i.e.

when the number of cups was changed the time and temperature were maintained constant.

Cups whose ovalization is shown here were fired a second time, starting from porcelain cups

which had been heated previously at 1020 ºC during 460 min in the Porcelanas da Costa

Verde S.A. (Portugal) kiln. The unglazed porcelain refers to unglazed ceramic ware fired to

temperatures above 1020 ºC, the initial temperature of the 2nd

firing stage in the tableware

porcelain manufacture.

Supplementary information on the microwave furnace and on the experimental work can be

found in [17], [20].

TESTS AND RESULTS

DEFECTS WHEN THE MICROWAVE HEATING IS UNCONTROLLED

As mentioned earlier, if the EMF is not controlled it may induce deformations due to thermal

differences in the material. As shown in Figure 1, if the heating process is uncontrolled,

bubbles are formed at the hot spots in the ware during microwave heating, and cracks will

appear. Even multiple cracks may occur, as seen in Figure 2.

Figure 3 shows one case study in which the occurrence of thermal runaway is more than

evident, with disastrous results. This phenomenon occurs when the materials dielectric

properties change abruptly with temperature. In this figure are observed the positions of the

hotspots, with localized vitrification and even melting in spots with an average diameter of 5

mm. The remaining parts of the ceramic ware did not experience firing.

Figure 4 shows a cup where massive bubbles appeared without cracking. The porcelain cup

presented was sintered at ∼1350 ºC, using a primary code developed to control the heating

process in a way such that the electromagnetic field inside the furnace was not stationary like

previously. This figure also shows the unglazed porcelain cup wrapping flaw, due to a not so

good control of the electromagnetic field during the heating process.

Fig. 1 - Glazed porcelain coffee cups presenting warping, severe

cracks and splitting of porcelain body parts. Sintered using

microwave radiation without a careful process control.

Fig. 2 - Multiple cracks in a

densified unglazed porcelain tea

cup.

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Fig. 3 - Evidence of thermal runaway in unglazed porcelain ware.

Fig. 4 - a) Unglazed porcelain cup with bubbles of high dimensions; and b) Unglazed porcelain cups

without visible bubbles but that warped during firing.

OVALIZATION METHODOLOGY AND RESULTS The methodology adopted to estimate the porcelain cups ovalization is based on the

measurement of the cup diameter in its edge, along 6 different angles with the magpie as a

reference, and according to the expression below:

Ov ��M � m�

m�� 100 %

where M, m and m� are respectively the maximum, the minimum and the average cup

diameter.

Figure 5 shows three cups used for the ovalization study. Cups A and B are similar in shape.

Besides having a narrower wall, cup A is slightly larger and presents straighter lines than cup

B. Cups B and C have an equal wall thickness, but differ in shape, cup C having a smaller

diameter and is slightly higher than the others. The differences are not just about the cups

shape, as the molding technologies were also different. Cup A was molded by isostatic

pressure and cups B and C by jiggering. Table 1 shows some cups dimensions, particularly

the height, diameter and wall thickness after dried, after sintered and the corresponding

shrinkage. They are referred to as “dried” - “sintered”:”shrinkage”.

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Table 1 - Cups height, diameter and wall thickness dimensions after dried and after sintered. Its

shrinkage is also presented.

Cup A B C

Height (mm) 66.5 - 59.5: 7.0 68.0 - 62.0: 6.0 73.0 - 66.6: 5.2

Diameter (mm) 64.0 - 57.0: 7.0 60.3 - 54.3: 6.0 52.7 - 47.5: 5.2

Wall thickness (mm) 3.8 - 3.4: 0.4 5.3 - 4.5: 0.8 4.3 - 3.5: 0.8

Fig. 5 - Pictures of unglazed porcelain cups fired in the microwave furnace at ∼1335 ºC in ∼70min. a) Cup A, b)

Cup B and c) Cup C.

Figure 6 shows the ovalization of the porcelain cups fired in the microwave furnace at ∼1335

ºC in ∼70 min, for different number of cups and shape. Figure 7 shows the porcelain

ovalization as a function of the temperature when fired in the microwave furnace and in a

common electric furnace. In Figure 8 is shown its variation for different firing cycles, from

room temperature up to its maxima. Figure 9 shows the relationship between ovalization and

the soaking time period, after the cups reaching ∼1335 ºC in ∼70 min, in both microwave and

electric furnaces. Tests were performed at 0, 5, 10, 15 and 20 min. The unglazed references,

references A and C (pictures not shown here) that are presented in Figuress 6 up to 9 were

fired in the conventional gas fired kiln of Porcelanas da Costa Verde S.A. (Portugal) at the

temperature of 1380 °C during 210 minutes in a reductive atmosphere. The atmosphere inside

the microwave and the electric furnaces (which results are shown in Figures 7 and 9), was

oxidant. In Figure 9 points were displaced from their positions, 0, 5, 10, 15 and 20 min, just

for their better visualization. Not all the already mentioned variables were studied for all the

porcelain cups. Whenever some information is missing, the experiment(s) were done at the

temperature of ∼1335 ºC in ∼70 min, and 12 cups of type B were used.

Figure 10 shows the ovalization results when batches of 4 and 6 samples were placed inside

the microwave furnace and in different positions, as schematized in Figure 11 when 6 cups

were used with Figure 11(a) related with the cups distribution corresponding to ovalization

results marked with the letter ‘g’ and Figure 11(b) corresponding to the cups distribution

related with ‘h’.

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Fig. 6 - Ovalization dependence on the number and

shape of the porcelain cups placed inside the

microwave furnace.

Fig. 7 - Ovalization dependence on the firing

temperature for the electric and microwave different

heating technologies. In each batch 12 cups were

fired.

Fig. 8 - Ovalization as a function of the firing time

(cold-hot) cycle.

Fig. 9 - Ovalization as a function of the soaking

time (showing essentially independence on this

variable).

Fig. 10 - Ovalization dependence on the relative position of batches with 4 and 6 cups fired in the microwave

furnace. Only cups A and B were tested.

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Fig. 11 - Schematic of two batches with 6 cups that were fired according to the results presented earlier in

Fig. 10 and marked as ‘g’ and ‘h’.

DISCUSSION

The ovalization tests are indicative of a potential dependence on the shape/thickness, number

of cups, firing time period and temperature, and on the samples relative positions, relative to

the furnace cavity/microwave generators positions, as shown numerically in [16]. On the

other hand, ovalization is essentially independent on the soaking time (Figure 9). Figure 6

shows that the ovalization of porcelain cups B and C decreases with its number, reaching the

minimum when 12 cups are placed inside the microwave furnace. This can be attributed to the

wall thickness and cups shape, because the difference in the wall thickness between cups B

and C is greater than when compared with cups A. At the same time, cups A and B have

similar shapes; however, results of Figure 6 show an ovalization indifferent to the number of

cups when cups A are considered. Another difference that may be significant for the

ovalization of the product is the way on how the cups were formed. As mentioned earlier,

cups A were formed by isostatic pressure whereas cups B and C were formed by jiggering.

Aspects like the lower water content in the greenware are important as it is responsible for

some of the shrinkage of the ceramic body. Comparing these two forming techniques, the dry

granules raw material used for isostatic pressure can have a water content up to 5% and for

jiggering it is necessary a water content of 18% to 22% [4] to give the optimum plasticity to

shape the greenware piece. According to [2], the water content is found between 15% and

25%. The polymeric binders that are used to bond the powder/granular material, when

isostatic pressure is employed, can have some impact on the ovalization, particularly during

its burning, like the water content release during firing. Not only, but particularly, the higher

structural homogeneity of the greenware body achieved when fabricated by isostatic pressure

[4], surely contributes to the lower deformation during firing. Except the study presented in

Figure 10, in all the others and independently of the type of the porcelain cup, for each batch

case cups were positioned in the same positions. The disposition of the cups inside the

microwave furnace can be seen in [20], for tests with 9 and 12 samples. Figure 7 shows that

from the biscuit temperature (1020 ºC) up to 1410 ºC there is an increase of the ovalization,

varying from 1% to 2%, but even so lower than the acceptable ovalization limite. Due to the

lack of a normative, to the authors knowledge, the ovalization methodology was also visually

assessed throughout the tests that were performed. Ovalizations lower that 2.5% are

considered acceptable as they are visually difficult to detect. Actually, and mainly in cases

where these defects are evaluated by automatic imaging systems, more extensive studies are

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needed to statistically validate these methodology. It also needs to be visually assessed by

experienced technicians. Figure 8 shows an ovalization decrease with the firing time increase

(with the decrease of the heating rate), converging to the reference ovalization value. It should

be mentioned that the referred values are of the same order of magnitude as those for soaking

time, and leading to ovalizations below the acceptable limit of 2.5%. In Figure 10 is visible

that the test cases ‘a’, ‘b’, ‘c’, ‘d’, ‘g’ and ‘g’ (all cups type B) present distinct ovalizations.

At the same time, tests ‘e’, ‘f’ and ‘i’ (cup type A) show the same ovalization for batches of 4

and 6 cups and placed in different positions than those which results are shown in Figure 6. It

is immediately observable that the reference ovalization is around one-half of the lowest value

achieved for the porcelain cups sintered in the microwave furnace, achieving the value of

∼0.9%.

During the development of the microwave furnace several modifications were made, both

physical and mostly in the LabView control code. The code was developed for the monitoring

and temperature control, and consequently to control the energy and how it is applied to the

samples to fire. The created interlock between power, time and temperature ensures a higher

uniformity of the electromagnetic field inside the furnace, allowing faster heating with lower

flaws in the finished product. Pieces still present some defects, mainly when glazed porcelain

is fired. Apart from the yellow/ivory colour and regardless of the heatwork, the fired

porcelain, when glazed, presents pinholes on its surfaceThe same firing procedure, also in an

oxidizing environment with exactly the same heating curve, was made in an electric furnace.

The porcelain glaze of the latter present more pinholes than those porcelain cups sintered in

the microwave furnace. If part of the surface of the porcelain is not glazed, it is verified that

the fired product presents less pinholes in the surface that was glazed. The pinholes

distribution on the glazed surface tends to decrease with the increase of the porcelain surface

without glaze. This means that the observed pinholes are formed due to gases that are trapped

by the glaze. As the unglazed surface area increase, more gases escape through this more

permeable surface, with less pinholes in the glazed surface. These defects, visible both in

porcelain cups fired in the microwave furnace and in the electric furnace (in an oxidizing

atmosphere), might be due to the formation of secondary porosity associated to the thermal

decomposition of trivalent iron impurities. A trivalent state that is not formed when the

porcelain is fired in a reducing atmosphere [2].

CONCLUSIONS

From the analyses performed on the ovalization of porcelain cups sintered using microwave

heating, it is verified that the soaking time has negligible effect when compared with other

referred parameters. The decrease of the heating rate has a decreasing effect on the

ovalization, like the number of cups in the furnace. The firing temperature has an increasing

effect on the ovalization when 12 cups are fired in a time period of approximately 70 minutes.

Samples relative positions and their shape and thickness effect on ovalization are not linear

and simple to evaluate. More studies are needed, with a greater variety and number of shapes

of different thicknesses for a more complete and conclusive analysis. The same is also valid

for the cups relative positions. As ovalization below 2.5% are visually undetected, except for

firings with few cups, in this case less than 12 cups, all the data present (on average)

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acceptable ovalizations. Even so, exceptions were observed for one or another cup of each

fired batch.

It is observed that a low porcelain ovalization requires a minimum of 12 fired cups per batch,

in the used furnace. It is also shown that the number of cups and their positions can be very

important in the heat release homogenization inside the microwave furnace. From these

observations it is experimentally verified the dependence of the electromagnetic field pattern

that is created inside the microwave furnace on the mentioned parameters, as it was shown

numerically elsewhere.

This conclusion belongs on the fact that ovalization/material physical deformation results

from the thermal gradients in the porcelain material, and that when the heating process is

uncontrolled it can cause cracks and even the complete fracture of the piece. The adequate

interlock between power, time and temperature ensures a higher uniformity of the

electromagnetic field inside the furnace, allowing faster heating with lower flaws in the

finished product.

ACKNOWLEDGMENTS

We thank FEDER funds through the COMPETE 2020 Programme under CerWave:

“Demonstração do processo de cozedura de porcelana por gás-microondas”, project POCI-01-

0247-FEDER-006410 and National Funds through FCT-Portuguese Foundation for Science

and Technology under the project UID/CTM/50025/2013. The authors express their sincere

thanks to Porcelanas da Costa Verde S.A. and to his staff member Eng. Jorge Marinheiro for

his help and technical support, and, not less important, for providing the samples required for

this study. The authors also express their sincere thanks to Luc Hennetier from the

Technological Centre for Ceramic and Glass Industries, Portugal, for his technical support

along this project.

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