EP2496120A2

EP2496120A2 - Ceramic cooking utensil

Info

Publication number: EP2496120A2
Application number: EP10787821A
Authority: EP
Inventors: Christophe Amprimo
Original assignee: PILLIVUYT
Current assignee: PILLIVUYT
Priority date: 2009-11-06
Filing date: 2010-11-08
Publication date: 2012-09-12
Also published as: WO2011055043A3; WO2011055043A2; FR2952290A1; US20120273482A1; FR2952289A1; FR2952290B1; FR2952289B1

Abstract

The invention relates to a ceramic cooking utensil (10), especially optimised for using on an induction hob (14). Said utensil comprises a ceramic recipient (11) and an outer base (13) to which a layer (12) of a coating based on silver powder is applied. The invention is characterised in that the layer is deposited in a geometric shape configured in such a way as to optimise the magnetic properties of the silver powder by homogeneously distributing a heating power over the recipient, supplied by the induction hob, a thickness (18) of the layer being defined according to the maximum heating power to be reached by the base of the recipient.

Description

Ceramic cooking utensil

Field of the invention

The present invention relates to a ceramic cooking utensil. More particularly, the present invention relates to a ceramic vessel for induction heating. The field of the invention is that of ceramic cookware.

The object of the invention is to provide a ceramic cooking utensil capable of being heated by induction, and having improved resistance to expansion and / or deformation, while maintaining a good continuity of the currents induced in the bottom.

State of the art

Today, domestic induction hobs are becoming increasingly important in the cooktop market. However, ceramic containers constituting a significant proportion of kitchen utensils are not suitable for use in induction heating.

In the state of the art, it is known from EP 0 695 282 which discloses a solution for using the ceramic containers on the induction plates. To do this, this document proposes to deposit on the bottom of a ceramic container by painting, screen printing or decalcomania, a thin layer of material having electrically conductive properties and / or magnetic properties leading to the production of heat by exposure to an electromagnetic field.

This document has drawbacks. Indeed, currently, it is not easy to deposit a metal layer at the bottom of a ceramic container, because of the difficulty of establishing a permanent connection. This difficulty is mainly due to problems including expansion, oxidation, fixation and weight of this metal layer. The oxidation of the metal layers, when bonded to the container (in an oven of more than 800 degrees Celsius) considerably reduces the effectiveness of such a container.

EP 0 695 282 proposes to use a metal layer based on silver powder, knowing that silver is known for its resistance to hot oxidation. However, using silver as the base of the metal layer to produce heat through exposure to an electromagnetic field, is not obvious. Indeed, silver like copper is known as "diamagnetic". Although theoretically any electrically conductive body can be heated by induction, it is widely known that sufficient efficiency can only be achieved in metals with high magnetic susceptibility. The term "yield" is a ratio between the power delivered by the induction plate and the heating power obtained. Ferromagnetic metals, such as iron, nickel, cobalt or ferritic steel, have in particular a sufficiently high magnetic susceptibility to obtain a satisfactory yield.

EP 0 695 282 does not describe the yield obtained with a metal powder-based silver layer. However, the following publications are known that deal with the magnetic properties of silver:

"PHYSICAL REVIEW LETTERS, volume 80, number 21" by R. Konig et al on the Magnetization of Ag Sinters Made of Compressed Particles of Submicron Grain Size and their Coupling to Liquid ³ He. », And

- "JOURNAL OF LOW TEMPERATURE PHYSICS by Li R. Konig et al on Magnetic Properties of Ag sinters and their Possible Impact on the Coupling to Liquid ³ He at very low temperature."

These two documents show that silver in the form of fine powder

"Sintered" can acquire unexpected magnetic properties, close to those of ferromagnetic metals, when subjected to an intense magnetic field, substantially greater than 1 Tesla.

The fact of using a layer based on metallic silver can therefore make it possible to obtain induction heating. However, in the absence of understanding of the physical phenomena governing induction heating, and in the absence of control of the design of the induced part, it has become apparent that the heating power obtained with such a layer is low. Indeed, it is difficult to create, from the silver powder, via the current cooking plates, the magnetic field necessary for cooking on high fire which can be obtained from substantially 3 Watts / cm 2.

It is also known from the state of the art that the inductors used in the induction plates generate a magnetic flux, such as the induced currents, which are predominant over a part of the layer. metallic and not important elsewhere. Having a nonhomogeneous heat flow can cause thermal shock between the hot spots and the cold spots of the ceramic container, which can cause it to break. Indeed, ceramic materials are quite sensitive to thermal shock and mechanical stress due to their low elasticity.

Today, despite the demand, there are no ceramic containers on the market capable of withstanding a high heating power delivered by an induction hob to serve as a cooking utensil in the same way as metal utensils.

So currently, is really felt the need to provide a ceramic container suitable for both preheating and cooking on induction plates.

Presentation of the invention

The invention is precisely intended to meet this need while overcoming the disadvantages of the techniques described above. For this, the invention provides a cooking utensil for use on an induction plate, having a very good thermal conductivity for optimum use and being relatively simple to achieve.

For this purpose, the invention provides a cooking utensil comprising a ceramic container and a bottom obtained from a paste containing silver powder. Said bottom is deposited outside the base of the container. In the realization of the invention, it appeared that a mastery of the design and sizing of the bottom allows to obtain, with the current cooking plates, a heating power suitable for cooking over high heat.

Thus, in the invention, the geometry of the deposit is determined so as to optimize the magnetic properties of the silver deposit by homogeneously distributing the heating power delivered by the induction plate. The thickness of the deposit is defined according to the desired heating power. In addition, to avoid possible thermal shocks, the coefficient of expansion of the container is very low.

Thus, the utensil of the invention has a silver powder base for use on induction systems and good heat conduction.

In addition, the variation of the deposit width disposed on the set of the outer base of the container allows rapid heat spread to the entire container.

The invention thus relates to a cooking utensil comprising a ceramic container, said utensil comprising on an outer base of the container a coating layer based on silver powder,

characterized in that

the layer is deposited in a geometrical shape configured so as to optimize the magnetic properties of the silver powder by homogeneously distributing on the container a heating power delivered by the induction plate,

a thickness of the layer is defined as a function of the maximum heating power to be reached by the base of the container.

According to a first preferred embodiment of the invention, the geometric shape comprises a succession of protuberances of the layer based on silver powder. A local width of said protuberances of the layer is defined as a function of the local magnetic field. In this way, it is possible to locally modulate the heating power.

More preferably, the geometric shape further comprises a succession of air channels. Said air channels are delimited by two consecutive protuberances. More preferably, a bottom of said channels is formed by the outer base of the ceramic container. The alternation protuberances / air channels is configured so that the current induced by the induction plate circulates within the protuberances comprising the silver powder.

Even more preferably, the protuberances and the air channels have a shape of concentric rings or spirals around a center of the coating layer, said rings or said spirals being circular or elliptical.

According to one variant, the coating layer comprises a succession of projecting point protuberances separated by the air channels.

According to a second preferred embodiment of the invention, the outer base of the container has on its periphery a capillary barrier, said barrier comprising at least two projections in relief by relative to the coating layer, said at least two projections delimiting at least one groove.

Such projections form a support foot which makes it possible to prevent any thermal shocks on the bottom of the container, following overflow or liquid runoff. This liquid, especially water, is trapped in the foot by capillarity and does not flow to the coating layer being heated.

This foot also makes it possible to slightly elevate the utensil relative to the plate. This elevation of the utensil, for example a few millimeters, prevents marking of the induction plate. In fact, the ceramic is not very conducive to heat and the induction plate locally produces a very intense heat flow. This temperature may exceed 900 ° C in some areas. Such a temperature may cause a softening of the protective vitreous layer of the induction plate, which may be deformed when in direct contact with the cookware.

Preferably, the shape of the projections and the groove adapts to the shape of the periphery of the base of the ceramic container. More preferably, the projections and the groove have a concentric shape. Even more preferentially, this shape is circular or elliptical, depending on the shape of the base of the container.

Preferably, the barrier comprises three projections delimiting two grooves. The embodiment implementing protuberances of the coating layer may advantageously be combined with the embodiment implementing the capillary barrier. These two embodiments can also be made independently.

Advantageously, the thickness of the coating layer is greater than or equal to approximately 10 μm.

Advantageously, the coefficient of thermal expansion of said container is ultra-low in particular of the order of 2.10 ^"6 K ^{" 1} from 20 to 200 ° C.

Advantageously, the maximum heating power on the base of the container for cooking over high heat is greater than about 3 Watts / cm ² .

Advantageously, the thickness of the protuberances is greater than or equal to approximately 10 μm.

Advantageously, the utensil is capable of being subjected to cooking on an induction hob.

Advantageously, said utensil is also able to undergo cooking performed by cooking means other than induction, for example microwave ovens, flame or convection.

The invention also relates to a method of producing a utensil as described above. In said method, the deposition of the coating layer on the base of the ceramic container is carried out by decal.

Brief description of the drawings

The invention will be better understood on reading the description which follows and on examining the figures which accompany it. These are presented as an indication and in no way limitative of the invention.

Figure 1 shows an axial sectional view of a ceramic cooking utensil according to one embodiment of the invention.

Figure 2 shows an axial sectional view of the cooking utensil shown in Figure 1, illustrating more precisely the outer base of this utensil.

Figures 3 and 4 show a bottom view of a cooking utensil according to two embodiments of the invention.

Fig. 5 is a diagram showing the relationship between the radius of a deposit on the outer base of the cookware and the heating power of three induction cookers having different diameters.

Figures 6, 7 and 8 show another embodiment of the invention, wherein the outer base of the utensil comprises a capillary barrier.

In the description which follows, the elements of the same function have identical references from one figure to another.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In the description by cooking means the possibility of cooking over high heat. This cooking over high heat can be achieved by means of a minimum average heat flux of the order of three Watts / cm ² on the base of the container placed on the cooking plate.

Figure 1 shows a cooking utensil 10 heated by an induction plate 14. The utensil 10 comprises a ceramic container 11. The ceramic container 11 may be porcelain, faience, terracotta, vitroceramic, sandstone etc ....

The container 11 may be a hollow utensil for containing, storing or transporting any substance (liquid, gaseous or solid). It can also be a flat plate. This container 11 can take various shapes and sizes.

The container 11 is made according to the traditional process of manufacturing ceramic objects. In order to be used effectively as a cooking utensil, the container 11 must have good resistance to thermal shock. For this purpose, the container 11 preferably has an ultra-low thermal expansion coefficient. In a preferred embodiment, the coefficient of expansion of the container 11 is of the order of 2.10 ^-6 K ^-1 from 20 to 200 ° C.

The utensil 10 has a coating layer 12 on an outer base 13 of the container 11. The base 13 is preferably of planar shape. The layer 12 is for example made of silver powder and a binder comprising a glass powder. The amount of silver powder is much greater than that of the binder. In a preferred embodiment, the layer consists of about 90% silver and about 10% binder. Glass powder is a bonding agent that allows to fix the silver powder.

These materials are mixed to form a pulp in which they are proportionally dispersed. This paste is then affixed to the container 11 according to traditional techniques in the field of ceramics. This layer 12 is deposited on the container 11, preferably by decal. The layer 12 may, in a variant, be applied by screen printing. The container 11 with the layer 12 is then fired at a temperature, for example of the order of 850 to 900 degrees Celsius. Then, the layer 12 is advantageously covered with a uniform protective layer. This protective layer is essentially composed of a glass frit, colored or not.

In a preferred embodiment, the layer 12 has substantially a disc-shaped outline. It may also have an outline having any other geometric shape for carrying out the invention.

As can be seen in FIG. 1, the layer 12 is deposited as a decoration in a geometry intended to optimize the properties magnetic of said layer. The layer 12 comprises a succession of protuberances 15 on a face 19 of contact with the induction plate 14. The protuberances 15 are seen here in section.

The induction heating power is proportional to the area occupied by the layer 12 in which the current is induced. A reduction of the heating power is obtained by leaving voids in the layer 12. To this end, the protuberances 15 are separated by channels 17, where the air can circulate. The shape of the air channels 17 and the protuberances 15 define a crenellated profile of the contact face of the layer 12 with the plate 14.

The presence of these voids 17 furthermore makes it possible, during the production of the decal, to control the thickness of the layer 12. In fact, the heating power is proportional to the thickness of the layer 12, up to the thickness of the layer 12. limit of the depth of penetration of the magnetic field. In one embodiment of the invention, the penetration depth is approximately 20 μm.

The air channels 17 form cavities each having a bottom formed by the ceramic base 13 of the container 1 1. Said air channels 17 are delimited by two successive protuberances. The protuberances 15 form protruding parts of the layer 12.

The geometry of the layer 12 is preferably configured so that the induced current circulates, within the protuberances 15 comprising the silver powder, in a substantially tangential direction, in a cylindrical coordinate system oriented perpendicularly to the base 13 of the container 11.

Moreover, for a good distribution of the heating power, it is preferable that the layer 12 has a substantially symmetrical shape of revolution around a center 16 of the base 13, especially in the case where this base has a circular contour or elliptical. Figure 2 shows a perspective view of the axial section of the cooking utensil shown in Figure 1. In Figure 2, we see that the container 11 has a base 13 of substantially circular contour. The protuberances 15 are formed by a relief, continuous or not, which winds in a first spiral around a center 16 of the base 13. The channels 17 are in the form of a second spiral interposed between the turns of the first spiral. In a variant, the base 13 and the spirals may have an elliptical profile rather than circular. Figure 3 shows a bottom view of a utensil 10 according to a variant of the invention. The contact face of the layer 12 with the induction plate 14 may be formed of protuberances 15 and air channels 17 in the form of concentric rings around the center 16 of the layer 12. The protuberances 15 and the channels 17 of air have a circular shape. An elliptical shape can also be realized.

In another variant, as shown in FIG. 4, the contact face of the layer 12 comprises a succession of projecting point protuberances 15 separated by channels 17 of air.

In another variant, the layer 12 is in the form of a disk having spot empty spaces.

The protuberances 15 and the air channels 17 can have all the geometric shapes that make it possible to optimize the magnetic properties of the silver powder.

The air channels 17 also make it possible to control the thickness of the protuberances 15. The air channels 17 have a depth corresponding to a thickness of the protuberances 15.

A distance such that 18 (see FIG. 2) separating two successive protuberances 15 may be variable depending on the zone of interest of the layer 12. In a variant, a distance 18 between two consecutive protuberances 15 is constant on the layer 12. In this case, the width of the air channels 17 is also constant.

The alternation of the protuberances 15 and the air channels 17 makes it possible to form a heating power divider between the coldest points, corresponding to the center and the ends of the layer 12, and the hottest points of the layer 12 This power divider makes it possible to vary the temperature received by the protuberances so as to reduce the thermal gradients generated by the induction. This function will be explained below by the description of FIG.

The thickness of the protuberances 15 is adjusted so as to obtain sufficient heating power. This thickness is determined so that it is substantially greater than the depth of penetration of the magnetic field generated by the induction plate.

After several tests with different power ratings Theoretical induction plate provided by the manufacturer, it appeared that a saturation thickness is reached at about 20pm. From this saturation thickness, the maximum heating power received by the container remains almost constant.

Table 1 below shows an example of the result obtained with these tests. This table shows a maximum heating power to be reached by the container according to the thickness of the silver layer. This maximum power is substantially equal to a percentage of the theoretical nominal power of the induction plate. This percentage depends in particular on the type of material of the container.

Table 1

Table 1 is obtained after several tests, for a theoretical nominal power of the induction plate of about 2800 Watts. For a thickness of layer 12 less than about 20 μm, the heating power obtained through the protuberances 15 is of the order of 70% of the theoretical rated power. Only when the thickness of the protuberances 15 is greater than or equal to substantially 20 μm, is observed an optimum heating power of the layer 12 of the order of 90% of the nominal theoretical power of the plate. induction.

FIG. 5 is a diagram showing the relationship between the surface heating power expressed in Watt / cm ² of three induction plates having different diameters and the radius of the layer 12 expressed in centimeters. The abscissa axis corresponds to the radius of the layer 12 and the ordinate axis to the heating power delivered by the induction plate.

FIG. 5 shows three curves 30, 31 and 32 which are graphical representations of the evolution of the power delivered by an induction plate as a function of the radius of the layer 12. These three curves have been established on the basis of the law of 'Local ohm 3 = ^σ Ε, where σ is the electrical conductivity of silver. The electric field E is calculated by solving the Maxwell-Faraday equation This equation gives the rotation of the electric field as a function of the time derivative of the magnetic field: The magnetic field B is obtained by summation of all the turns of the induction plate, whose respective contributions are given by the law of Biot and Savart.

Curve 30 represents this evolution for an induction plate having a diameter of 16 centimeters and a theoretical rated power of the order of 2000 Watts. Curve 31 represents this evolution for an induction plate having a diameter of 18 centimeters and a theoretical nominal power of the order of 2800 Watts. Curve 32 represents this evolution for an induction plate having a diameter of 21 centimeters and a theoretical nominal power of the order of 3100 Watts.

FIG. 5 shows that the power delivered by the three plates around the center 16 of the layer 12 is almost zero. Since the center of the plate does not heat up, it is therefore not necessary to deposit protuberances near the center 16. As one moves further away from the center 16, the power increases.

With the curve 30, the heating power delivered has a bearing where it is at the maximum on the layer 12. This level is around 2.4 to 2.6 centimeters radius. The further away from this plateau, the more the power delivered decreases to zero at about 7 centimeters of radius of the layer 12.

With the curve 31, the heating power delivered has a bearing where it is at the maximum on the layer 12. This level is around 2.5 to 3 centimeters radius. The further away from this plateau, the more the power delivered decreases, until it is nil at about 8 cm of radius of the layer 12.

With the curve 32, the heating power delivered has a bearing where it is at the maximum on the layer 12. This level is approximately 2.5 to 4 centimeters radius. The further away from this plateau, the more the delivered power decreases to zero at about 9 centimeters radius of the layer 12.

From these three curves 30, 31 and 32, it is possible to determine relatively accurately the heating power delivered to each point of coordinates (x, y) of the plane of the layer 12. With the invention, it is possible to modulate locally the heating power received by the container by varying the thickness or width of the protuberances 15 at each point of layer 12.

Thus, in the example illustrated in FIG. 3, the thickness of the protuberances 15 is greater when these protuberances are deposited around the center 16. This thickness is then decreased as we approach the bearing. to be increased as we move away from this plateau.

In the invention, the number and size of the protuberances 15 on the layer 12 are determined as a function of the reduction of the desired heating power. For example, if it is desired to reduce the heating power received by layer 12 by 25%, the silver powder protrusion coverage rate on layer 12 should be about 75%.

In one embodiment, the protuberances 15 have a width of 1 to 10 millimeters and the distance 18 of the air channels is greater than or equal to about 0.5 millimeters.

The alternation of air channels 17 and protuberances 15 having different widths makes it possible to reduce the temperature gradients received by the container by locally modulating the heating power.

The particular shape, distribution and thickness of the protuberances can give rise to many variations, guided essentially by the optimization of the magnetic properties of the layer 12 and by the homogeneity of the heat flow on the container.

In the example of Figure 3, the protuberances 15 and the air channels 17 are respectively twelve in number. The layer 12 deposited on the base 13 has an external diameter of the order of 152 millimeters. A central vacuum located in the center of the layer 12 has a diameter of the order of twenty millimeters.

In summary, the invention provides a silver powder-based layer of a predetermined thickness so that sufficient heating power can be achieved for optimum use in cooking. The silver layer is further deposited in a geometric shape configured both to control the deposited thickness and to limit gradients on the utensil. This limitation of gradients makes it possible to preserve the integrity of the utensil and to limit the risks of charring food.

In one embodiment, the layer 12 may have a diameter greater than about twelve centimeters, which can reach a maximum heating power of the order of 5 to 7 Watts / cm ² . For a layer 12 of diameter less than or equal to about 12 centimeters, the maximum power can reach 10 Watts / cm ² .

Figures 6, 7 and 8 show another embodiment of the invention. According to this embodiment, the outer base 13 of the container 11 comprises a capillary barrier 40, in addition to the layer 12. This capillary barrier 40 provides an improvement to the embodiments in which the outer base 13 of the container 11 is of planar shape . Indeed, in certain circumstances, this flat shape may have disadvantages.

These disadvantages are related in particular to the very high temperature that can reach certain areas of the container 11. In fact, the ceramic is a relatively low heat conducting material compared to metals, with a conductivity of the order of 2 Wm ^-1 .K ^"1 , against for example 30 Wm ^{" 1} .K ^"1 for a steel. The rate of heat transfer towards the contents of the container 11 is reduced. The bottom of said container 11, when heated by induction, may have very high temperature zones, close to 900 ° C.

There may be a runoff of liquid on the wall of the container 11, such as condensation water located under the lid or following an overflow of a liquid contained in said container. The liquid can then creep into the heart of the interstice between the flat bottom of the utensil 10 and the cooking plate 14. Severe heat shock may occur between the liquid, whose maximum temperature is about 100 ° C, and some areas of the utensil 10, whose temperature may approach 900 ° C. Ceramic being by nature a so-called "fragile" material, that is to say non-ductile and little deformable, this shock may possibly lead to damage to the container 11 in the form of a dynamic fracture. The fracture can also be caused by fatigue damage, following a repetition of thermal shocks occurring during different uses of the container.

In some cases, the temperature reached locally on the outer base 13 of the container 11, during its heating on the induction plate 14, can reach the softening temperature of the fusible mineral materials of the layer 12. In effect, the layer 12 to Silver base comprises vitrifiable mineral materials, fuse at about 900 ° C, to ensure fixation by cooking and protection of silver particles. Therefore, if there is contact between the layer 12 and the induction plate 14, it is then likely to mark said plate.

Similarly, in the case of direct contact between the glass of the cooking plate 14 and the bottom of the ceramic container 11, which has very hot zones, close to 900 ° C., the heat transfer to said cooking plate 14 is not negligible. Local degradation of the internal structures of the hotplate, in very hot areas, can therefore occur after a certain period of time.

Moreover, the cooling of the ceramic is relatively slow. During rapid passage of the container 11 from the induction plate 14, where it is being heated, to a support such as a worktop or a service table, there is a risk of burning the support, or even heat shock potentially damaging to the container 11, particularly if the support is wet.

To overcome these various disadvantages, a capillary barrier 40 can be made. Figure 6 shows an example of a schematic representation of a view of the outer base 13 provided with such a barrier 40. This barrier is shown in more detail in Figures 7 and 8.

The realization of this barrier 40 is conventionally obtained by the shape of the mold, regardless of the technique of shaping the ceramic paste, whether in particular casting or pressing.

The barrier 40 of the invention comprises an outer protrusion 41 in the form of a relief, located near the outer wall of the container 11, delimited by a hollow formed by a first groove 42. The barrier 40 also comprises an intermediate projection 43, in shape of relief, delimited by the first groove 42 and by another hollow formed by a second groove 44. This barrier finally comprises an internal projection 45 in the form of relief, delimited by the second groove 44 and a bottom wall 46 of the container 11 intended for receive the layer 12.

In the example shown in Figures 6 to 8, the outer base comprises a single barrier 40, the three projections 41, 43 and 45 and the two grooves 42 and 44 are annular and concentric. The barrier 40 shown, is made on the entire outer periphery of the base 13 of the container 11, near the periphery. In this example, the barrier 40 is made about three millimeters from the periphery, so as to maximize the surface of the layer 12 and provide better stability of the container 11 on the induction table.

The three projections 41, 43 and 45 have a height above the layer

12 so that the container 11 bears on the plate 14 only through said projections. Said projections (41, 43, 45) form a support foot. In an exemplary embodiment, the height of the protrusions is about 0.5 mm above the layer 12.

In the example illustrated in FIGS. 6 to 8, the presence of two grooves

42 and 44 can be formed on the top of the projections, three capillary pumping areas corresponding to so-called horizontal capillary traps.

Indeed, zones 47 between the induction plate 14 and the top of the projections form the horizontal capillary traps. The trickling liquid falling at the foot of the container 11 placed on the cooking plate 14 is pumped by capillary action between said plate and said apex of the protrusions, possibly passing through the recessed zones constituted by the grooves 42 and 44. The barrier 40 thus allows to trap the liquid by capillarity and constitutes a watertight barrier which prevents any penetration of said liquid to the layer 12.

In addition, the projections 41, 43 and 45 are ceramic. When heated on the induction plate 14, they remain at a moderate temperature, less than 100 ° C. These protrusions are capable of retaining the runoff liquids without undergoing thermal shock which may damage the container 11. The ceramic is indeed capable of withstanding thermal shocks such as soaking in water at 20 ° C. test piece previously heated to 300 ° C.

The furrows 42 and 43 constitute, for their part, so-called vertical capillary traps. Their role is to store liquids between pumping areas. Their presence is useful especially when the container 11 is raised from the cooking plate 14. In their absence, the water which stagnates with horizontal capillary traps could be sucked up during the lifting and then trickle down to the layer 12, the horizontal capillary protection being suppressed by the simple fact of raising the utensil. The grooves 42 and 44 thus constitute permanent capillary traps, taking over the horizontal capillary traps constituted by the interface between the flat tops of the projections 41, 43 and 45 and the cooking plate 14, which only exist in the the container 11 rests on the cooking plate 14.

In an exemplary embodiment, the grooves 42 and 44 have a height h of about 2 mm. The dimensions of these grooves 42 and 44 are determined so that the hollow of said grooves can not be clogged, during the enameling operation that takes place during the manufacture of the utensil 10. The internal width Lint of the grooves 42 and 44 is for example of the order of 1.5 mm. The outer width Lext grooves 42 and 44 is for example of the order of 2 mm. In the description, the terms inner and outer mean with respect to the direction of contact of the container 11 on the induction plate 14 and the container position of the container 11.

In a preferred embodiment, the lower width Lint of the grooves 42 and 44 is smaller than the outside width Lext of said grooves. This type of sizing grooves 42 and 44 facilitates the demolding of the container 11, after shaping the dough.

In the embodiment illustrated in FIGS. 6 to 8, the grooves 42 and 44 have a trapezoidal cross section of approximately 3.5 mm ² in area. In other embodiments, the grooves may have a rectangular or hemispherical section.

In general, the dimensioning of the barrier 40 is determined so as to ensure perfect stability of the container 11 without degrading the aesthetic while allowing the formation of a capillary dam whose water retention capacity by the furrows are quite effective.

The present invention is not limited to the embodiments described above. Many alternative embodiments are possible without departing from the scope defined by the appended claims.

In particular, the number of protrusions and furrows of a barrier may be different from that of the embodiment illustrated in FIGS. For example, the barrier 40 may comprise a single groove 42, framed by two projections (41, 43). In a variant, the barrier 40 may comprise more than two grooves. However, the width of the barrier 40 would then be increased, which could excessively and unnecessarily reduce the surface of the bottom wall of the container that should accommodate the layer 12.

In addition, the layer 12 may have any geometric shape and any thickness capable of providing the container with a heating power suitable for cooking. Advantageously, the layer 12 comprises protuberances 15 and air channels 17, as shown in FIGS. 1 to 4.

The cooking utensil obtained according to the invention can also be adapted for any other conventional cooking means than induction. These conventional means can be selected from microwave ovens, gas burners, radiant, oven, barbecue etc ....

Claims

1 - Cooking utensil (10) comprising a ceramic container (11), said utensil comprising on an outer base (13) of the container a layer (12) of coating based on silver powder,

characterized in that

a thickness (18) of the layer is defined as a function of the maximum heating power to be reached by the base of the container.

2 - Utensil according to claim 1, such that the geometric shape comprises a succession of protuberances (15) of the layer based on silver powder.

3 - Utensil according to claim 2, wherein the geometric shape further comprises air channels (17), said air channels being delimited by two consecutive protuberances and having a bottom formed by the outer base of the container (11). ceramic.

4 - Utensil according to claim 3, wherein the protuberances and the air channels have a shape of concentric rings or spirals around a center (16) of the layer, said rings or said spirals being circular or elliptical.

5 - Utensil according to claim 3, wherein the coating layer (12) comprises a succession of protruding point projections, separated by the air channels.

6 - Utensil according to one of the preceding claims, such that:

the outer base has on its periphery a capillary barrier

(40)

- said barrier having at least two projections (41, 43, 45) in relief relative to the coating layer (12), said said at least two projections delimiting at least one groove (42, 44).

7 - Utensil according to claim 6, such that the projections and the groove have a concentric shape.

8 - Utensil according to claim 6 or claim 7, such as the barrier has three projections delimiting two grooves.

9 - Utensil according to one of claims 7 to 8, such that the projections and the groove or grooves have a circular or elliptical.

10 - Utensil according to one of the preceding claims, characterized in that the thickness of the coating layer (12) is greater than or equal to about 10μιτι.

11 - Utensil according to one of the preceding claims, characterized in that the coefficient of thermal expansion of the container (11) is of the order of 2.10 ^"6 K ^" between 20 and 200 ° C12 - Utensil according to one of the claims previous, characterized in that the maximum heating power on the base of the container for cooking over high heat is greater than about 3 Watts / cm ² .

13 - Utensil according to one of the preceding claims, such that it is capable of undergoing baking performed on a plate (14) induction.

14 - Utensil according to claim 13, such that it is capable of undergoing cooking by cooking means selected from microwave ovens, flame or convection.

15 - A method of producing a utensil according to one of the preceding claims, characterized in that the deposition of the layer (12) on the base of the container (11) is carried out by decal.