The dehydration, especially the activation energy for it, of magnesium hydroxide was investigated as a part of the studies on the sintering behavior of magnesia obtained by the decomposition of magnesium hydroxide.
The magnesium hydroxide used was the chemically purest grade, being fiborous as shown in Photo. 1.
The dehydration curve (Fig. 4) shows that the specimens contain a few per cent of absorbed moisture and about 31% chemically bonded water, which is expelled rapidly at the temperatures 330°-400°C. After the absorbed moisture was expelled completely by heating the specimens in a platinum crucible at 200°C for an hour in an electric furnace (Fig. 5), the isothermal dehydration was carried out by keeping the preheated specimens at several temperatures ranging from 300° to 400°C. The error introduced by the effect of bouyancy was so small that it was neglected upon comparison with the sensitivity of the thermobalance.
From the curves obtained (Fig. 6), it has been shown that the rate of dehydration of magnesium hydroxide may be represented by the first order kinetics law at least in the initial stage. And (v′ -v)s in the equation (3), derived using Guggenheim's method were plotted against soaking time with a result giving a set of straight lines (Fig. 7).
From the slopes of the straight lines the velocity constant k′s were evaluated. Furthermore, the Arrhenius plots of log k against the reciprocal of the absolute temperature of holding were drawn in Fig. 8, showing that the points lie fairly good on straight lines. From the slope of line the activation energy was evaluated with the value 23±1 kcal/mole.

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In the previous paper the authors have reported that addition of a small amount of pulverized lithia-containing glass to the powdered natural petalite increased the firing range of their pressed mixture, improving modulus of rupture and thermal shock resistance of their sintered body (J. Ceram. Assoc. Japan, 70, 8 (1962)).
In the present study the effects of fineness of the Petalite powder on the above behaviors were investigated. 12 kinds of batches comprising 10% by weight of the pulverized glass (finer than 325 mesh; 10.7 Li₂O, 8.9 MgO, 8.9 Al₂O₃, 71.5% SiO₂ by weight) and 90% of each one or two of the powdered petalites of different fineness (40-80, 80-200, 200-325, >325 mesh) (see Table 2) were prepared, pressed into the bars of 5×5×50mm, and fired at 1250-1280°C until their porosity reached almost zero. The batches consisting of no glass frit were also prepared for the comparison.
The results obtained are summarized as follows: (1) With decreasing the grain size of petalite powders, the firing range and the firing shrinkage of the pressed body increased. (2) With decreasing the grain size of petalite powders, the modulus of rupture and the heat shock resistance of the sintered body were improved. (3) For the body comprising petalite powder finer than 200 mesh, water-quenching from 1000°C increased the modulus of rupture of the sintered body. For the body comprising the petalite grains coarser than 80 mesh, however, the reversed effect was observed. (4) With decreasing the grain size of petalite powders, the thermal expansion coefficient of the sintered body decreased, reaching almost zero.
From above results, it was concluded that the garin size of petalite powder for making petalite-ceramics should be below 200 mesh.

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The composition of Aluminium-enamel frits may be classified under two large groups, viz., lead and non-lead systems. The former is good as the enamels for building and other structural materials, although they can not be used for houshold utensils. Especially, the frits of the system PbO-Sb₂O₃ give the enamels having good colour and luster. They also are chemically stable and show marked tendency of oparization.
Studies have been carried out on the crystalline phase developed from the frits with following conclusions:
1) The structure of the crystals is the same as that of 3PbO⋅Sb₂O₅.
2) The existence of PbO⋅Sb₂O₅ was also confirmed, and it has also been revealed that 2PbO⋅Sb₂O₃ is the mixture of PbO⋅Sb₂O₅ and 3PbO⋅Sb₂O₅.
3) The degree of opacity become larger the smaller is the mole ratio (RO excluding PbO)/(Sb₂O₃+V₂O₅). With increasing amount of PbO the colour inclining with yellow was obtained. However, it would be better that Sb₂O₃ and Bi₂O₃+V₂O₅, the additional ingredients helping opasifier, do not exceed 5 and 8%, respectively.
4) The frits having oxygen potential between 1.00 and 1.16 were confirmed to be good for aluminium enamel, since the firing temperature of those having the value near 1.2 was too high, and the frits, whose oxygen potential were less than 1 were quite useless because of their extreme instability.
5) Provided that the composition of the frits do not differ too much, the lattice constants of 3PbO⋅Sb₂O₅ developed in the frits of higher oxygen potential were larger than those developed in the frits of lower oxygen potential.

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To improve the thermal shock resistance of forsterite porcelain, the effect of the addition of BeO to the mix was studied.
Firing shrinkage, water absorption, apparent specific gravity, expansion and shrinkage in firing, thermal conductivity, thermal shock resistance, load bearing property, thermal expansion, electrical resistivity, dielectric loss and dielectric constant were measured using the specimens containing BeO 10, 20, 30, 40 and 50 per cent by wt.
The difference of sintering behavior of the specimens was observed at the temperatures higher than 1000°C. The trends of the change of properties of the specimens containing the different amount of BeO with increasing temperature were different according to the item of test. Two types, one changing monotonously, and the other having maximum or minimum, were observed. It was considered that the latter would substantially be dependent on the microstructure of the specimens, while the former would not be so.
The thermal shock resistance of BeO containing bodys showed a very good results because of their good thermal conductivity.
As the forsterite porcelain containing BeO has the lower sintering temperature, the higher thermal shock resistance, and the same orders of other characteristics in comparison with the ordinary forsterite porcelain, it may be a good insulating material.

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