How Firing of Clay Affects it Microstructures

How Firing of Clay Affects it Microstructures

Of the several interesting physical properties of clay which have claimed the attention of investigators in recent years, none is more important than the behavior of the material when heated to temperatures above those of dehydration.

Some of the problems to be solved are:

  1. The mineralogical changes that take place in firing.
  2. The relation between temperature and change in constitution, shrinkage, porosity, etc.
  3. The importance of the time factor, as duration of firing, either in reaching a given temperature, or of exposure to a constant temperature.

Mineralogical Changes

As a result of fusion, reactions take place between the constituents of a clay, which results in a reorganization of the mass, and the formation, of new compounds. Of the new minerals developed, sillimanite is the one usually recorded, and has been referred to by a number of observers. There appears to be a difference of opinion, however, regarding the temperature at which it is formed. Thus Klein notes that sillimanite forms only above 1300° C., while Mellor claims that it can develop below 1200° C., the difference in results being conditioned by the time factor. In Klein’s experiments the test pieces were fired in 12 hr., while in Mellor’s, the temperature rose from 800° to 1180° in 24 hrs., and occupied the same time in cooling back to 800°.

Mellor and Holdcroft state that when clays are fired at 600°, the molecule probably breaks down in free alumina, and free silica, while ferruginous clays also form Fe2O3. Hussak found that low-burned porcelains show all the constituents doubly refracting and clearly distinguishable from each other, but that higher heating gave an amorphous groundmass, with a few large quartz grains, and needles that were apparently sillimanite.

Klein found that by heating English kaolin for 5 hr. at 600° C., the kaolinite became entirely isotropic, but that the index of refraction (1.55) was only slightly altered. Further, he says that no profound change was noted in the optical properties until 1200° C. was reached, at which point there began incipient dissociation of the kaolinite, into two isotropic substances, with complete dissociation at 1400° C, and the development of prismatic grains between 1400° C. and 1450° C.

There is thus a difference of 600° C. in the dissociation point of kaolinite as expressed by Mellor and Klein and there may also be some question as to whether the change from an anisotropic to an isotropic condition does not indicate a change in the mineral constitution.

Another, possible change referred to in recent papers is that of quartz to tridymite and cristobalite, but on this point the results obtained by different observers vary, and may be due to the influence of the time factor. McDowell found that upon repeatedly burning silica brick the quartz is transformed to tridymite and cristobalite.

Where we are dealing with impure clays, changes, due to fluxing action will undoubtedly begin at a much lower temperature than they do in the purer clays like kaolin.

Temperature and Time Factors

It is commonly recognized that as the temperature increases, shrinkage and density increase up to a certain point, beyond which they diminish, but that the time factor is an important one. In other words, certain changes produced by heating a clay to a given temperature in a given time, may be reached by taking a longer time to heat the same clay to a lower temperature or by holding it at the latter for a considerable period. This is thought to explain the difference of view regarding the formation of sillimanite expressed by Mellor and Klein, although the latter says that repeated burning of a clay at the same temperature does not produce much change in its constitution.

A criticism sometimes made of laboratory tests on clay, is that they lack the long exposure to a slowly rising temperature, obtained in a regular factory kiln. While this is true, it is also true that the test pieces used in the laboratory may be smaller than those fired in a large kiln, and require less time to heat through.

The tests in the present paper contain some data on this subject.

Scope of Present Paper

Most of the petrographic work done on burned clays has been carried out on high-grade clays which contain relatively small amounts of fluxing impurities, and, moreover, the studies have been confined chiefly to results obtained at temperatures of 1200° C. or over.

The present paper deals with some clays that contain notable amounts of fusible impurities, and were fired at temperatures not exceeding 1150° C.

Method of Procedure

In the present investigation, which is to be regarded as a preliminary one, the clays tested were molded in 1-in. (25.4-mm.) cubes, and 3-in. (76.2-mm.) bars. One set was heated to 1000° C. in 1½ hr. (referred to on subsequent pages as the preheating period), and held there for 10 hr.; a second set was heated to 1150° C. in 3 hr., and held at that temperature for 10 hr.

Samples were drawn at the end of the preheating period, and at hourly intervals thereafter, the test pieces after removal from the furnace being buried in hot sand, so as to cause slow cooling.

The pieces so fired were then tested for fire shrinkage, absorption, porosity. Thin sections were also cut for petrographic study.

The tensile strength of both the air-dried and burned clays was also tested.

Clays Selected

The clays selected for investigation were the following: .

  1. Brown residual clay from Ordovician shale, occurring at Webster, Botetourt County, Va.
  2. Grayish-white residual clay, derived presumably from Cambrian shale, occurring near Lofton, Augusta County, Va.

Brown Clay.—The Webster clay is soft, smooth, yellowish-brown when wet, and contains small fragments of unweathered shale. It grinds easily, and mixes with water to a fairly plastic mass. Under the microscope it shows minute flakes of hydromica and kaolinite, as small particles of iron oxide. Scattered angular fragments of quartz were also noticed. Biotite is rare. The hydromica and kaolinite in the shale fragments were arranged parallel to the bedding planes of the material.

White Clay.—This is very fine-grained, smooth, has a greasy feel, and high plasticity. The constituents, seen under the microscope were light brown aggregates of very minute kaolinite scales, angular fragments of colorless quartz, together with small amounts of red iron oxide and occasional grains of brown biotite, yellow epidote, zircon, zoisite, and tourmaline.

Properties of the Clays

The chemical composition of the two clays is shown by the following analyses:


Effects of Firing

Color.—The brown clay shows a gradual oxidation of the iron oxide, the color becoming darker and darker, though not strongly so, as the time of burning progressed.

The white clay shows no such pronounced change of color, owing to the small amount of iron oxide which it contains.

Hardness.—The longer the heating, the more compact and harder the clays became, due probably to progressive fluxing of kaolinite and hydromica with the cementing material, and with this there appeared to be a decrease in size of the pore spaces.

Loss on Ignition.—The driving off of chemically combined water or other volatile substances seems to have been practically completed, as was to be expected, during the stage of rising temperature (preheating period) and so the amount of loss on ignition of the different, samples drawn from the fire during the periods of constant temperature remains practically the same (Figs. 1 to 4).

brown clay fired at 1000 c

brown clay fired at 1150 c

Absorption and Porosity.—The absorption was determined by soaking for 48 hr., while the porosity was found after a.further boiling for 3 hrs., this giving as good results as immersion in a vacuum. It will be seen by reference to Figs. 1 to 4, that the absorption and porosity curves run practically parallel with each other, the decreasing amount being a

white-clay-fired-at-1000 c


factor of temperature clue to the shrinkage of the pore space in the clays, and fusion.

In the brown clay, both absorption and porosity decreased rapidly until the end of 3 hr., and after that they fell off very slowly.

The white clay behaved somewhat differently from the brown, there being no great change at 1000° C. but a rapid decrease in absorption and porosity at 1150° C. which decreased with the time of burning until near the end when it seemed to rise slightly.

As a matter of fact, both the absorption and the porosity decreased to a certain limit and then began to increase slightly in the tenth hour.

It is a little difficult to explain this increase in the final hour, for this clay does not appear to have reached its vitrification point (Figs. 1 to 4).

Tables showing properties of the burned clays.





Fire Shrinkage.—The figures given in the tables show that in the series heated to 1000° C., the shrinkage remains practically constant after 3 hrs. at that temperature, but that in the 1150° C. series the maximum shrinkage takes a little longer to develop.

Moreover, the fire shrinkage may show little change, even though there is a visible change in porosity; absorption, and hardness (Figs. 1 to 4).

Tensile Strength.—’The tensile strength of the burned clays show’s some interesting changes depending upon the time of burning.

The brown clay (Fig. 5) showed a tensile strength of 370 lb. per square inch after 5 hrs. at 1000° C. and 430 lb. at the end of 10 hrs. In the case



of the briquettes heated to 1150°, the tensile strength was 153 lb. per square inch when this temperature was first reached and then fell off to 80 lb. at the end of 10 hrs.

The white clay shows a maximum tensile strength at the end of 5 hrs. heating at the same temperatures. The specimen heated to 1000° C. gave 360 lb. per square inch, while the one heated to 1150° C. showed 330 lb. per square inch.

Microscopic Examination of the Burned Clays

In order to study the pyrometric changes, a number of thin sections of the burned cubes were examined under the microscope.

Brown Clay

Under the microscope this reveals what at first appeared to be a porphyritic texture (Figs. 7 and 8), but it was found that the phehocryst-like grains were small fragments of shale that had escaped pulverizing, while the groundmass is a brownish mixture of iron oxide with fine particles of kaolinite and hydromica.

These shale fragments are no doubt portions of the original shale, not completely decomposed, and hence they contain less free iron oxide than the other parts of the mass. They are much lighter in color than the groundmass and consist of minute grains of quartz, and small flakes of kaolinite and hydromica, the last two showing a distinct parallel arrangement, which is quite clear when viewed under the microscope, but does not come out very distinctly in the photographs (Fig. 7). The hydromica shows a higher interference color than the normal one and may be a sericite.

Small amounts of quartz are scattered through the groundmass.

At 1000° C. the specimens indicate no appreciable change in texture, but there seems to be a slight decrease of the scaly minerals and an increase in the amount of amorphous (isotropic) material toward the end of the burning.

The color as a whole became darker and the material seemed to be more tightly cemented in the specimens removed at the end of the firing period.

Vitrification seems to have begun at the end of 3 hrs. exposure to 1150° C. and at this point the color changed to brownish-red and still later to reddish-brown.

The minute flakes of kaolinite and hydromica also seem to be disappearing, due in part to the fluxing of these minerals with the finer- grained ingredients, in which change the hydromica seems to flux more rapidly than the kaolinite. Their disappearance may also be due in part to their becoming isotropic under rising temperature. If so, the change was not complete at 1000° C.

In the shale fragments referred to above, there was noticeably less anisotropic material after 1 hr. at 1150° than there was after 1 hr. at 1000° C. Corrosion of the quartz fragments was also noticed at 1150° C., but it less was noticeable than in the case of the white clay.

When the thin sections are examined with polarized light, it can be seen in passing from the specimens heated for 1 hr. at 1000° to those heated for 10 hrs. at 1150° that there is a gradual increase in the quantity

photomicrograph of thin section of brown clay


of isotropic material. This is somewhat difficult to show in a photograph, as the halation produced by the grains still yielding light colors tends to give the whole field a lighter tone. However, comparison of Figs. 7 and 8 will serve to show the contrast.



As the fusion of hydromica and kaolinite proceeds, it results apparently in the formation of an iron silicate; volume changes also seem to occur resulting in the development of pore space in the clays.

White Clay

Thin sections of the fired material showed a somewhat banded texture which is not an original structure of the clay, but due to the pressure used in molding; they also showed small fragments of quartz scattered through the very fine-grained aggregates of kaolinite and hydromica.

Small grains of iron oxide were also noticed as well as tiny fragments of biotite, epidote, zoisite, zircon and tourmaline.

At 1000° C. the minute flakes of kaolinite and hydromica seem to be altered to a light brown amorphous body and the disappearance of these minerals seems more noticeable as the exposure to the temperature above mentioned continues.

Toward the end of the heating at 1000° C. and 1150° C., corrosive action of the quartz was quite remarkable (Figs. 9 and 10) and the change in color to a darker hue was also evident. At 1150° C., the gradual disappearance of the kaolinite and hydromica after heating 10 hrs. at this temperature was very manifest. The edges of the quartz were more deeply eaten away according to the duration of burning as shown in the figures.



In one sample which had been heated up to 1310° C., the corrosion of the quartz was very prominent, and at this point the clay seemed to show slight vitrification. However, no optical change was noticed in quartz up to this temperature.

In order to obtain some idea of the mineral content of the raw clays and the change in amount of each after heating, an attempt was made to estimate the per cent, of this material present. The figures given in table 5 are a rough quantitative estimate made from a study of the thin sections.

Stages of Firing Clay

From the foregoing data, we see that when fusion begins in the mass the fine mineral grains are first affected and seem to change from a crystalline to an amorphous condition.

  • There is a gradual change in the color as the time of heating at one temperature continues.
  • The hardness gradually increases on longer burning up to steel hardness.
  • Absorption and porosity in each case run practically parallel, decreasing to a certain limit and then increasing again slightly, later, in the case of the clays studied. Absorption and porosity abruptly decrease when the mass approaches vitrification, as shown in the case of the white clay at 1150° C.
  • Loss on ignition is the same at both temperatures, as might be expected.
  • The fire shrinkage in both cases shows no change in the last 5 hrs. of continuous heating, while in the first 5 hrs. of heating atone temperature it shows a marked increase.
  • The tensile strength seems to reach a maximum point after 5 hrs. of continuous heating and decreases beyond that point.
  • Microscopic study shows that the longer the burning the denser the texture up to a certain point and that this is also accompanied in the brown clay by a change in color. There is, furthermore, in both clays, a gradual increase in the amount of isotropic material.
  • Fusion of the hydromica and kaolinite proceed parallel with each other, but the former is more readily fusible.
  • Corrosive action as seen in the quartz increases with the length of burning.
  • Toward the end of the heating at 1150° C., pore spaces begin to develop, especially in the case of the brown clay.