Title:
Drying of porous masses
United States Patent 2320474


Abstract:
This invention relates to the drying of liquidcontaining porous masses. It is of general applicability, and may be applied in the drying of such masses ps liquid-containing glue, gelatin, meat, and other food products. I have made practical application of it in the drying of articles of ceramic...



Inventors:
Ross, Donald W.
Application Number:
US41018141A
Publication Date:
06/01/1943
Filing Date:
09/09/1941
Assignee:
Ross, Donald W.
Primary Class:
Other Classes:
28/285, 34/246, 73/866.2, 236/44R
International Classes:
F26B3/34
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Description:

This invention relates to the drying of liquidcontaining porous masses. It is of general applicability, and may be applied in the drying of such masses ps liquid-containing glue, gelatin, meat, and other food products. I have made practical application of it in the drying of articles of ceramic ware, including refractories, after they have been shaped and before they have been fired.

In the accompanying drawings Fig. I is a diagrammatic view that illustrates the drying of a ceramic body according to the invention. The view shows partly in vertical section and partly in side elevation a block of ceramic material within a drying chamber, together with electrical means for releasing heat within the block in accordance with the relative humidity of the atmosphere within the chamber.

Fig. II is a diagrammatic view, illustrating a modification in the electrical circuit of the blockheating means; Fig. II is a view of a block of ceramic material in end elevation, positioned between electrodes for drying in accordance with a modified procedure of the invention; Fig. IV is a diagrammatic view, illustrative of still another modification in the procedure of the invention.

Fig. V is a diagrammatic view, in which drying by electro-osmosis, as hereinafter set forth, is illustrated.

It is commonly known that, in the drying of porous material, the air first penetrates the coarser pores, and that the water continues longer in the finer pores. I have perceived another principle that expresses itself in waterfilled capillaries, and upon this other principle my present invention proceeds. When a capillary is partially filled with water, menisci are formed at the ends of the column of water; if then there be a difference in temperature at the ends of the water column (having equal menisci), the cooler meniscus will exert a greater surface tension, and the column will tend to flow in the capillary, in a direction from the warmer to the cooler end of the column. The tendency expresses itself in exosmosis. Surface tension varies with temperature, hence control of the temperature of the interior of a porous mass and of the temperature drop between the interior and the surface of the mass controls the rate of water flow from the interior to the surface.

When articles of ceramic ware are taken from the molds in which they have been formed they still are burdened with water, and. before their introduction into the kilns, they have yet to be dried. It is usual practice to introduce the watercontaining articles as they come from the molds into a drying chamber: the articles in relatively cold state are brought into a relatively warm atmosphere. Under such conditions the surfacetension action that I have described tends to carry the water, not outward, toward the surface, where it may evaporate, but inward; furthermore, superficial drying tends to bring about a casehardened condition, that hinders and delays thorough drying.

My invention consists in supplying interiorly of the mass the energy required for drying a porous mass. This supplying of energy may take various specific forms; it preferably is accomplished in establishing, as a condition of drying, a heat gradient that descends, not from the surface of the article inwardly, but from the interior outwardly; in consequence, I achieve drying more rapidly, and have a more uniformly dried and accordingly more satisfactory article for introduction into the kiln.

It will be understood that many substances shrink as they dry, and that if the shrinkage be too great in one part of a mass of the substance (as at and near its surface) as compared to another part of the mass (as at and near its center) the dried mass will be strained, cracked, and otherwise disrupted. In accordance with my invention, I control the heat gradient from the interior of the mass to the surface, and by virtue of such control I am able to regulate the distribution of the water remaining in the mass at any stage of the drying, and thus avoid undue strain and disruption of the mass.

Drying by heating and chilling I may establish the condition that character40 izes my invention by bringing the article preliminarily (and, preferably, under conditions that forbid evaporation) to, or perhaps slightly beyond, the boiling-point of water, and then allow cooling to proceed in a cool, suitably dry atmosphere. In the cooling, surface tension aids in the escape of the water, case-hardening does not occur, and there is no superficial cracking of the ware. The procedure of so heating and cooling may be'repeated, lmtil the article is relieved of substantially all moisture.

Drying by heated cores I bring the still wet body within an envelope of water-dissipating character. Preferably, I 55 heat the article interiorly while maintaining it in an envelope of relatively cool and suitably dry atmosphere, and continue such conditions until the moisture has been driven off with substantial completeness. To accomplish the ends described the articles may be formed with cores of metal to which heat may be imparted by conduction; or the cores may be of electric conducting material and electric current of such strength may be caused to flow through" as to generate heat within the article in desired quantity and at desired rate. In such case the temperature may be controlled by means of thermocouples imbedded in the ware. In fact, in certain arrangements, one portion of the resistor leading to the interior of the ware can be made to serve as one leg of the thermocouple. The current is shut off the resistor while the temperature is being read. With the resistance of the ware and the resistor known, the temperature of drying may be controlled by the amount of current flowing. Since the ware has a resistance greater than most metal resistors, the current will flow almost entirely in such resistors. I find it desirable not to overheat the ware at the points of entry and exit of the current. To prevent such overheating I prefer to have low-resistance leads join a higher resistance resistor within the ware. An example of this is, copper leads connected within the ware to a resistor of iron or of carbon or of carborundum. I have found that such resistor may be a single resistor, a number of resistors arranged in series with low resistance connections, or a number of resistors arranged in parallel. The voltage should not greatly exceed that required to produce the necessary heat.

Since such a resistor is completely imbedded in the material it is to heat, a large proportion of the heat produced will do useful drying work, and efficiency of this device is, hence, high. In the case of many large ceramic shapes, I find that the trace left in the ware from the eventual burning out of the resistor is not greatly detrimental. This is frequently true in the case of large refractory shapes. I further find that, in certain cases, the products of combustion of the resistor may not be particularly harmful, as, for instance, aluminum or silicon in fire clay refractories, and iron in magnesia and chrome refractories. In the case of some resistors, for example, aluminum and silicon in clay refractories and magnesium in magnesite and chrome refractories, the products of combustion of the resistor fill the space as completely as did the original resistor.

In Fig. I of the accompanying drawings I have represented a block I which may be understood to be a newly molded block of ceramic material to be dried in preparation for firing.

The block is shown to be arranged within a 6 chamber 2. Through the center of the block passes a resistor wire 3 of a suitable material such as iron or aluminum, which is used for heating purposes when an alternating current is passed through it. The ends of the wire are separably attached inside the block to insulated copper leads 4. Within the chamber and exteriorly of block I two thermometer bulbs 5 and 6 are arranged, and tubes 7 and 8 establish communication between the bulbs severally and chambers 9 and 10 on opposite sides of a flexible diaphragm II in a pressure chest 12. A plungerrod 13 extends from the diaphragm to the upper end of an arm 14 that is pivotally supported at 15, and the lower end of this arm is articulated to the upper end of the brush 16 of a rheostat 17 included in the energizing circuit of the resistor 3. The brush 16 turns on a pivot 18 in response to the swing of arm 14. The bulbs and 6, tubes 7 and 8, and chambers 9 and 10 are filled with the vapor of a volatile liquid, a liquid having a boiling point of 50° F. The bulb 6 is arranged within a jacket or wick 6G of absorbent material that is in known way saturated with water from a water pan 6b.

In the drying of the block, current from power supply lines flows through the resistor 3, and thus heat is generated and released at the center of the block, such release of heat effecting the desired movement of moisture from the center of the block to the surface. The drying atmosphere within the room 2 evaporates moisture from the surface of the block, and from the saturated wick 6a on the bulb 6 as well. The evaporation of moisture from the wet bulb 6 lowers the temperature of such bulb below that of the bulb 5 whose external surface is dry, with the effect that the vapor pressure within bulb 5 exceeds the vapor pressure within bulb 6. In consequence the pressure of the vapor in cham-. ber 9 exceeds the pressure of the vapor in chamber 10. But the effect of this difference in pres-. sure is opposed, by means of a spring 19 acting through the arm 14 and rod 13 upon the diaphragm.

The relative humidity of the atmosphere within the drying room 2, of course, determines the rate of evaporation of moisture from the wet bulb 6; the rate of evaporation of moisture from the wet bulb determines the difference in temperature between the two bulbs, and such difference in temperature in turn controls the difference in pressure of the liquid on opposite sides of the diaphragm II. It will be manifest that the tension of the spring 19 may be so adjusted that the arm 14 and the rheostat brush 16 will stand in predetermined positions when the relative humidity of the atmosphere within the drying room is of desired value. That is to say, by adjustment of the tension of spring 19 the position of the rheostat brush may be so determined as to provide a current flow of given intensity when the relative humidity of the atmosphere surrounding the block is at optimum value, such determination of current flow establishing a corresponding value of heat generation within the body of the block. If the relative humidity within the drying room falls below optimum value, the rate of absorption of moisture from the wet bulb increases; the temperature of the wet bulb drops; the difference in temperature between the dry bulb and the wet bulb increases, thereby increasing the difference in pressure of the liquid acting on opposite sides of the diaphragm II; the diaphragm flexes, shifting the plunger 13 in left-to-right direction, swinging the arm 14 in clockwise direction, and thereby rotating the rheostat brush 16 counter-clockwise, and increasing the intensity of current flow. The greater the fall of the relative humidity from a given normal value, the greater is the swing of the rheostat brush, and the greater is the increase in current flow. Conversely, when the relative humidity rises, the movement of the parts will be in opposite directions, and the intensity of current flow will be decreased. If the relative humidity rises above normal value, the rheostat brush may swing so far in clockwise direction as to be moved out of contact with the coil of the rheostat, and in such case the energizing circuit of the resistor 3 will open, and the generation of heat within the block will be Intermitted, until the humidity of the atmosphere falls, as it will, to normal value.

As the humidity returns to normal value, or falls below such value, the rheostat brush swings back into contact with the coil I7 and again closes the electric heating circuit. Thus, the release of heat-generating energy within the block I is coordinated with the relative humidity of the surrounding atmosphere, and. an effective heat gradient from the center of the block to the surface is established and maintained.

An ammeter 20 is connected with a thermocouple 21 located at the approximate center of the block. Thus at any time the temperature that exists at the center of the block may be known. The copper leads 4 and the thermocoupe 21 may be so arranged that they may be withdrawn from the block and the bores provided for them may be filled after they have served their useful functions.

In modification of the means described, the circuit through the block-heating resistor may be intermittently opened and closed in response to the swing of the arm 14 of the apparatus that functions in response to changes in humidity within chamber 2. That is to say, the circuit 4, 3, 4 need not be normally closed through a variable resistance device such as the rheostat coil 17, but may, as shown in Fig. II, be closed by means of a relay 17 . More specifically, the arm 14 of the humidity-control apparatus shown in Fig. I may carry a contact 22 adapted to engage a contact 23 in a relay-energizing circuit 24 when the relative humidity of the air in the drying chamber 2 falls below normal value. Such closing of the relay circuit energizes the relay and closes the circuit through the block-heating resistor 3. When the humidity rises to or above normal value, the arm 14 swings counterclockwise and effects the de-energizing of the relay and the interruption of the block-heating circuit.

Thus, the heating circuit may be automatically opened and closed in response to variations in the humidity of the block-drying atmosphere, and the desired temperature gradient may be maintained in the block.

In further refinement the normal value to which the humidity of the block-drying air is held may be varied as the drying operation progresses, and means for automatically controlling such variation of an otherwise constant humidity value will be found in the art. As exemplary of such means I show that the rheostat coil 17 may be mounted upon a disk 25 which is geared to a clockwork 26. As the drying operation progresses, ordinarily over a period of from one to ten days, the disk 25 is slowly turned, carrying with it the rheostat coils 17. Manifestly, the rate of heat release at the normal value of humidity is progressively varied. And it will be seen that, by regulating the speed of operation of the clockwork 26, I may control the rate at which such variation is effected.

Drying by use of the ware as a resistor to alternating current Instead of Including a core of more highly conducting material in the make-up of the article, I prefer to complete an electric circuit through the body of wet ceramic material alone.

Although moist clay is a much poorer conductor than are most metals, I find that electricity can be forced through it. I have found that the conditions of electrical resistance and economical heat production of alternating current passing through moist clay are suitable to serve my ends.

I find that during the warming-up of the wet clay at the start of alternating-current drying the resistance decreases somewhat. In alternating-current drying, I may choose to alter the voltage, the amperage, and the frequency more or less continuously as drying progresses, to maintain optimum conditions for economical electrical heat production and temperature control.

I find that the smaller the dimension of the clay body through which resistance must be overcome, the lower is the economical voltage at any given water content. In the resistor drying of clay by means of an alternating current, I prefer to pass the current through the least dimension of the ware being dried. I find that the more nearly the dimensions of the electrical contacting plates match the size of the sides of the clay body upon which they make contact, the more efficient the electrical heat conversion. I prefer to have such plates of substantially the same area as the sides they contact. In case of poor contact between plates and ware, there is apt to be overheating of the ware at the surface of contact. I prefer to avoid such overheating at the surfaces of contact by painting them with aluminum paint in which the aluminum is not fully suspended In the vehicle, or by coating these surfaces in other manner with electrically conducting coatings, or by pressing the electrodes upon the ware, making the pressure heads of a press as the electrical conducting plates. I find that such plates may be insulated from each other during pressing, by use of nonconducting mold sides. The faces of the plates from their edges inward as far as need be may be formed of insulating material, to insure flow of current through the ware instead of through the side walls of the mold.

In Fig. III a typical apparatus is illustrated.

The block la to be dried is engaged on its opposite sides by electrodes, each of which is formed of a plate 30 of powdered and bonded carbon, together with a porous plate 31 of copper, or other suitable electric conducting material. The contact surfaces may have been prepared in the manner described. Electric leads 32 and 33 connect the electrodes to a suitable source of current. The electrodes may be pressed upon the ware by mounting the assembly in a bracket 34, under the tension of a spring 36, with blocks 35 of insulating material arranged, as shown, to prevent short circuiting.

Drying by direct current It is commonly known that, if direct current GO electricity is passed through clay which contains water, using a relatively insoluble anode, the water content of the clay migrates from the anode toward the cathode (electro-osmosis). In this case also the energy for drying is supplied interiorly of the mass. Electro-osmosis becomes inefficient, however, at a water content of approximately 3 per cent, and at lesser water contents practically ceases to function, unless the electrolyte content of the mass be sufficiently high. (Electrolytes reduce the refractoriness of refractories and are, therefore, substantially absent from the finished refractories.) In the carrying of the operation of drying of refractories, therefore, to the point at which they can safely be fired, it is necessary in any case to find a source of energy other than that of electroosmosis.

I apply this principle by forming an anode as a core through and within the article to be dried, and by placing water-absorbing cathodes against the outer surface of the article. Such cathodes may contain sufficient electrically conducting material to permit current flow in silica-gel, porous carbon, or other porous material. I find that the anode, preferably carbon, may, where the electrolyte content is sufficiently low, be a wire, as, for instance, aluminum wire, that will oxidize on the firing of the article and leave no hole. I find that the anode may be insulated except for a section in the central portion of the article.

This arrangement provides means of flowing moisture away from the center of the material.

In Fig. V of the drawings, Ic may be understood to be a block of water-containing refractory, newly shaped, and ready to be dried. An anode 50 is shown to be embedded in the body of the block, and this anode may be understood to be typically a wire of aluminum. The cathodes 51, here conventionally shown to be two in number, are conventionally shown to be applied superficially to the block. The material of which these are formed may be understood to be a material that is at once conductive of electric current and porous and, because it is porous, water-absorbent. With the flow of direct current through the circuit, the two conditions that have been specified manifest themselves, namely: (1) in consequence of the heating of the anode 50, the interior of the block will be brought to a higher temperature than the surface; and (2) under electric-osmosis, as well as under a difference of temperature, water within the block will flow from the warmer interior to the cool and cathode-faced exterior.

I find that, if the voltage be sufficiently high, the portion of material next the anode soon becomes relatively dry and its temperature rises, and I find that by adjustment of the voltage and by the use of an anode of low resistance the temperature of this region can be controlled. It is manifest that, in recognized manner, the current that flows in the circuit Indicated in Fig. V may be varied as has just been said. I thus have in such procedure two principles at work in removing water from the interior of the article. Towards the end of such drying procedure the resistance to current flow rises. And it is permissible and may be found preferable, when resistance has increased to intermit drying by direct current and to continue the drying by alternating curren., in the manner already described. In this case the conductor that has served as anode is alone brought into the circuit of an alternating current and by such means temperature is maintained and drying is completed.

In this case it will be perceived that the cathodes are, with relation to the ceramic body under treatment, enveloping bodies of water-dissipating character.

Drying by the use of induced current Another procedure by which to heat the interior of the article while the surface remains relatively cold is to pass an induced (or secondary) current through it. I find that, while its moisture content is high, the article itself is a relatively good conductor. The induced current then flows easily, and efficiency is low. But as the moisture content diminishes, resistance increases, and efficiency improves. The induced current at first flows through the wet interior of the mass; but, as the center dries, the current tends automatically to follow the moisture outwardly in a ring-like region of concentration, toward the surface of the article. In this procedure it will ordinarily be found preferable to alter the voltage, amperage, and frequency more or less continuously as required for economical electrical heat production and temperature control.

I find that heating by electro-magnetic induction, as by placing the article Ib (Fig. IV) on a suitable insulator support 40 within the turns of a coil 41 through which current of a frequency of 3667 kc. is flowing, has the advantage that it does not overheat the surface of the article, and that circuit-closing contacts do not have to be made with the body of the article. I may prefer to use conventional means o0 for production and control of such electrical energy and heating, or I may prefer to use the types of equipment developed for such production and control in connection with radio. I find that with ceramic materials, in case the resistance would otherwise become too high toward the end of the drying operation, materials that increase the conductivity may in sufficient quantity be incorporated in the ceramic mass. Such conductivity-increasing materials are, for instance, finely divided metallic powder, wire, or finely divided carbon, preferably in the form of graphite. In the case of -aluminum-silicate refractories, the conductivity-increasing material may be aluminum or silicon; in the case of magnesia refractories the material may be magnesium or iron; and in the case of chrome refractories it may be chromium, magnesium, or iron.

If wire be used it may be made to follow any desired path within the ceramic mass. I find that the fluidifying agents used in slip casting aid the conductivity during the latter part of drying. This may possibly be due to their concentration on the surfaces of the particles, so that they form, with the residual water, rather continuous paths for the passage of current through the mass.

From known chemical facts, if certain soluble salts tend to remain in the interior of ceramic ware as the water recedes toward the surface, G5 and if these salts be electrical conductors, they tend to permit continued electric flow in the interior of the ware, even after the water is driven Qut. For this and other reasons, I may prefer to keep the soluble salt content of my ware low. On the other hand, in drying by means of methods in which current is made to flow in the ware as the resistor, as with alternating current, direct current, and induced current including electro-magnetic induction, I may prefer to have, in the moist or partially dry ware, a substance that will serve to permit electric flow until substantially all the water is driven off, and which will itself be driven off at high temperatures. I may prefer to use with such purpose and effect a volatile salt, such as ammonium chloride or a substance that is converted into volatile constituents by hydrolysis and decomposition. From known chemical facts and my experiments, chlorides of non-harmful bstances may be used for this purpose; in magnesia and chrome refractories, magnesium chloride; in chrome refractories, chromium and iron chlorides.

The general principles of melting metal masses 76 in induction furnaces are commonly known, including means of production of suitable electrical energy. I apply these in the removal of water from porous masses, as, for instance, ceramic masses and, more particularly, refractories.

In drying by heating inductively I may prefer to use conventional methods; I may prefer to produce suitable electrical energy by use of a variable speed generator in conjunction with suitable resistances, condensers, and transformers, so that voltage, amperage, and cycles per 1I second can be varied and controlled at will; or I may prefer to employ the teachings of United States Patents 1,286,394 and 1,286,395, and variations thereof and aids thereto and improvements thereon, both patented and of common knowl- 1i edge. Following the principles worked out for induction furnaces, I prefer so to design the equipment, including the primary, that, in the masses to be dried, the region of maximum heating can be controlled. For instance, I may 2( prefer to have the region of maximum heating at the center of the ware at one stage of the drying and elsewhere at another stage.

Reasoning from principles worked out for induction furnaces, increase of electrical resistance of many porous masses with decrease of water content indicates that the induced electrical effect is acting through the water, and that the porous material merely serves as an inert framework. Hence, by suitable design and current density, it is possible to develop the "pinch effect," in degree desired, in such contained water, and at temperatures suited to the stage of the drying. It is possible to place the mass to be dried in such position with respect to source of electric induction that the resulting moisture flow will be in desired degree and direction. In some cases it may be advantageous to have the maximum intensity of the effect at the center of the ware and flow the water to the nearest surfaces.

I find that the voltage necessary to overcome the resistance of the mass increases as the mass dries, and that the closer the voltage at any time be to that which is just sufficient to overcome the resistance of the mass and to produce the necessary heating, the higher is the heating efficiency of the apparatus.

I find that another way of drying by heating and chilling is to dry by internally generated heat while maintaining the ware in a relatively cool atmosphere as, for instance, a temperature near or even below 32* F. (freezing). This permits and possibly accelerates flow of moisture to the surface without rapid drying of the outer 53 portion of the ware, such as would cause the ware to crack. Such cooling atmosphere, together with the generating of heat within the ware, serves as a cheap, exact means of control of ceramic drying operations. This also permits GO drying of gelatin, meats, and other perishable porous materials in semi-cold storage under which conditions they do not deteriorate appreciably.

I may prefer to control the temperature of the co5 center of porous material in such manner that it will be more or less constantly a suitable amount above that of a variable temperature surrounding atmosphere. This arrangement permits use of drying rooms in which there is little or no control of temperature. It will in particular cases be found preferable to apply this method of drying to ware before the mold is stripped from the ware and in atmospheres controlled as to temperature and humidity. For Instance, moisture can escape through holes and pores in metal and other walls and including plaster walls.

I find that the method of drying porous masses Sby internally generated heat is particularly useful in the draying of large ceramic shapes from the centers of which it is otherwise difficult to remove the last traces of moisture. Such moisture, if left in the ware, may cause the ware to 0 blow up or to crack in the firing. Furthermore, I find that this method of drying by internally generated heat is particularly useful in the drying of silica refractories, especially large silica shapes. In the case of silica refractories, I preSfer to maintain the temperature in the ware and in the surrounding atmosphere relatively low, in order to avoid thermal strain. Thermal strains are commonly set up in silica ware by the chilling of the surface of the hot ware, in moving it from Sthe driers Into cooler atmospheres, as when carrying the ware to the kiln. Although the major thermal expansion of quartz occurs in a region of temperature several hundred degrees above room temperature, its per-degree expansion in Sthe region between room temperature and 212* F. is appreciable, and is relatively great in comparison with that of clay. It is commonly known that relatively dry unfired silica refractories have thermal expansion properties similar to those of Squartz. Such expansion tends to cause "edge" and "mud-pond" cracks in unfired silica refractories if their surfaces are chilled suddenly.

The use of grinding equipment and the grading of the ground material in the matter of partide size according to the teaching of my application filed on even date herewith, Serial No. 239,184, have effect in giving to the molded and dried ceramic article relatively great dry strength. The teaching of that other application together with the teaching of this tend to minimize cracking of dry and partly dry ware. Since strains are usually brought about in unfired silica refractories by relatively sudden chilling of the surface, one way to overcome this trouble is to eliminate the chilling, as by use of low drying temperatures. Such thermal cracking of unfired silica refractories has effectively limited the size of pieces which, in thepast, have been economically produced. By this invention the liability of the ware to such cracking is diminished, and the limit upon size is raised.

The application for this patent comprised a continuation in part of application Serial No. 239,185, filed by me November 5, 1938.

I claim as my invention: 1. The method herein described of preparing a ceramic article for firing which consists in molding upon an electric resistor equipped with low resistance leads a body of wet ceramic material enveloping both the resistor and its leads and passing an electric current through the leads and through the resistor with generation of heat in the resistor, while the molded body is surrounded by a dry and relatively cool atmosphere, and, by regulating the intensity of current flow, controlling the temperature gradient in the molded body from said resistor outward, and thereby effecting an expedited and regulated flow of moisture from within to the exterior surface of the body.

2. The method herein described of drying a newly molded ceramic article, carrying water in capillary spaces with4ni its substance, having embedded in its substance an electric resistor and borne upon superficially by a conductive and porous body, which consists in bringing the said resistor and the said conductive and porous body into an electric circuit in which the said resistor becomes an anode and the conductive and porous body becomes a cathode, and establishing current flow, whereby the said resistor is heated and the water within the ceramic article is subject simultaneously to electro-osmosis, tending to cause the water to flow to the superficial conductive and porous body, and to surface tension in capillary spaces within the article, tending to cause flow from the heated interior toward the surface.

3. The method herein described of preventing thermal and shrinkage injury to water-containing masses of porous material that shrink on drying and expand on heating which consists in imparting heat to an internal region within the article while the article is enveloped in a waterabsorbent and relatively cool atmosphere and increasing the quantity of heat in accordance with decrease in the relative humidity of such enveloping atmosphere.

4. The method herein described of drying a newly molded ceramic article in preparation for firing which consists in releasing energy within the article concentrated in a region remote from the surface while the article is surrounded exteriorly with a water-absorbing medium, and thereby effecting migration of water from interior toward the surface of the article, with absorption of water from the surface of the article by the surrounding medium, and increasing the release of energy in accordance with decrease in the water content of said water-absorbing medium.

5. The method herein described of preventing thermal and shrinkage injury to water-containing masses of porous material that shrink on drying and expand on heating which consists in releasing energy within the mass and in a region remote from the surface while the mass is envelopedn a water-absorbing medium of varying temperature, with migration of water from the Interior toward the surface of the mass and the interior toward the surface of the mass and absorption of water from the surface by the surrounding medium, and increasing the quantity of energy so released in accordance with decrease in the relative humidity of such enveloping medium.

6. The method herein described of drying uniformly and without case-hardening a ceramic body that in the condition of a water-filled plastic has been molded to shape, which consists in including in the ceramic mix, in addition to the mixing water, another electric-conductivity-increasing ingredient, bringing the still wet body within an envelope of water-dissipating character and causing electric current to flow through an interior portion of the body and in so doing to heat such interior portion to higher temperature than the surface, whereby the flow of water from interior to surface is expedited.

7. The method herein described of drying uniformly and without case-hardening a ceramic body that in the condition of a water-filled plastic has been molded to shape, which consists in including in the ceramic mix an electric-conductivity-increasing volatile salt, bringing the still wet body within an envelope of water-dissipating character and causing electric current to flow through an interior portion of the body and in so doing to heat such interior portion to higher 3,320,474 temperature than the surface, whereby the flow of water from interior to surface is expedited.

8. The method herein described of drying uniformly and without case-hardening a ceramic body that in the condition of a water-filled plastic has been molded to shape, which consists in including in the ceramic mix ammonium chloride as an electric-conductivity-increasing ingredient, bringing the still wet body within an envelope of water-dissipating character and causing electric current to flow through an interior portion of the body and in so doing to heat such interior portion to higher temperature than the surface, whereby the flow of water from interior to surfaceis expedited.

9. The method herein described of drying uniformly and without case-hardening a ceramic body that in the condition of a water-filled plastic has been molded to shape, which consists in bringing the still wet body within an envelope of water-dissipating character, heating electrically to a higher temperature than the surface an interior portion of the body, whereby the flow of water from the interior toward the surface is expedited, and as operation progresses varying the intensity of the electric influence.

10. The method herein described of drying uniformly and without case-hardening a ceramic body that in the condition of a water-filled plastic has been molded to shape, which consists in the maintaining the still wet body in a water-absorbing atmosphere, heating electrically to higher temperature than the surface an interior portion of the body, whereby the flow of water from 33 the interior toward the surface is expedited, and, as under the effect of evaporation at the surface the enveloping atmosphere becomes less readily absorbent, in accordance with increase in the relative humidity of the atmosphere, decreasing the intensity of the electric influence.

11. The method herein described of drying a moist porous mass of ceramic material which consists in effecting by the release of energy within the body of the mass the migration of moisture from the interior to the exterior of the mass while it is enveloped in a moisture-absorbing medium, and increasing the rate of energy release in accordance with decrease in the relative humidity of such moisture-absorbing medium from a given value.

12. The method herein described of drying uni. formly and without case-hardening a ceramic body that in the condition of a water-filled plastic has been molded to shape upon a heat-transs5 mitting core, which consists in bringing the still wet body within an envelope of water-dissipating character and raising the temperature of said core, whereby a heat gradient is established within the body, from a warm interior toward a cold (0 surface, and the water flow from the interior toward the surface is expedited.

13. The method herein described of drying uniformly and without case-hardening a ceramic body that in the condition of a water-filled plastic has been molded to shape upon a resistanceaffording electrical conductor, which consists in bringing the still wet body within an envelope of water-dissipating character, and heating under electric-current flow the said conductor, whereby a heat gradient is established within the body, from a warm interior toward a cold surface, and water flow from the interior toward the surface is expedited.

14. The method herein described of drying uniformly and without case-hardening a ceramic body that in the condition of a water-filled plastic has been molded to shape upon a resistanceaffording electrical conductor, which consists in bringing the still wet body within an envelope of water-dissipating character, and heating under alternating current flow the said conductor, whereby a heat gradient is established within the body, from a warm interior toward a cold surface, and water flow from the interior toward the surface is expedited.

15. The method herein described of drying uniformly and without case-hardening a ceramic body that in the condition of a water-filled plastic has been molded to shape, which consists in bringing the still wet body within an envelope of water-dissipating character, and heating electrically to a higher temperature than the surface an interior portion of the body, whereby the flow of water from interior to surface is expedited.