Abstract:
A method for making a non-stick ceramic cooking vessel for a slow-cooker, the vessel having glazing defining a seal over a cooking surface of the ceramic vessel, the method comprising the steps of removing at least a portion of the glazing while preferably maintaining at least a portion of the seal of the vessel and applying a non-stick coating over the cooking surface. The step of removing the glazing is preferably achieved by grit-blasting at least a portion of the glazing with aluminum oxide. Additionally, the cooking surface preferably includes a bottom surface and a wall surface wherein the glazing is preferably removed from the bottom surface such that the seal is substantially removed, and a portion of the glazing is preferably removed from the wall surface such that the seal there remains substantially intact. The method preferably also includes the steps of applying a first layer of a non-stick coating over the cooking surface, drying the first layer, cooling the first layer and applying a second layer of a non-stick coating over the first layer. Additionally, the method preferably includes the steps of applying a third layer of non-stick coating over the first and second layers, drying the second and third layers, and curing the layers of non-stick coating.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to household cooking appliances, and more particularly to ceramic slow-cookers.  
         BACKGROUND OF THE INVENTION  
         [0002]    Modem cookware is often coated with non-stick material to release food quickly and easily, making serving and clean-up less troublesome. Coating metal cookware has been relatively successful where multi-layered coating techniques have been used to improve the bond between the cooking surface and the non-stick coating.  
           [0003]    Applying non-stick coatings to ceramic cookware has proven more difficult. The ceramic substrate of conventional ceramic cookware is usually glazed to seal the otherwise porous surfaces, but the glazing does not have the characteristics needed to provide the required non-stick surfaces. The glazing commonly hinders or prevents the application of non-stick coatings to the cooking surfaces. As such, food typically becomes baked onto conventional ceramic cookware, making serving and clean-up difficult. While ceramic cookware with a reliable and long-lasting non-stick coating would be extremely useful, it is currently unavailable on ceramic cookware such as baking dishes, casserole dishes, slow-cookers, etc.  
           [0004]    Another problem with applying non-stick coatings to ceramic cookware is the temperature gradients common in the ceramic substrate during cooking or baking. The temperature gradients that exist near the heat source and between the surface of the food and the air provide a harsh environment that tends to degrade and break down non-stick coatings. These temperature gradients add to the above-described difficulty in bonding the non-stick coating to the glazed surfaces and further increase the difficulty of producing ceramic cookware with a reliable non-stick coating.  
           [0005]    Different types of ceramic cookware experience different cooking conditions that directly affect the intensity of the temperature gradients. Ceramic cooking appliances such as slow-cookers typically experience more extreme temperature gradients than baking dishes or casserole dishes due in part to the type and location of the heat source and the long cook times associated with slow-cooker applications. Unlike baking or casserole dishes used in the microwave or conventional oven for relatively short cook times, slow-cookers have heating elements in direct or near-direct contact with the ceramic cooking vessel, thereby generating localized and longer-lasting intense heat that creates sharper temperature gradients over the cooking vessel. Also, slow-cookers are often exposed to cool ambient temperatures at the food/air interface. As it is being cooked, the hot food is in contact with a portion of the ceramic cooking vessel while ambient air contacts the vessel just above the surface of the food, This interface experiences another temperature gradient that can be sharper than those associated with baking or casserole dishes used in warmer environments such as ovens or microwaves, As any slow-cooker user knows, it is these areas (the area adjacent the heating element and the interface between the top of the food and the air), that experience the most stuck-on, baked-on and caked-on food remains.  
           [0006]    In addition to harsher cooking applications, ceramic slow-cookers are also more difficult to clean than common ceramic cookware. Even assuming that the ceramic cooking vessel of a slow-cooker is removable, it is typically bulky and burdensome to clean. It likely does not fit in a dishwasher and often does not even fit in the sink. If the ceramic cooking vessel is not removable from the slow-cooker housing, clean-up is further complicated as the cooling vessel cannot be submerged due to the electronics inside the housing. Maneuvering the slow-cooker to achieve proper cleaning is difficult and troublesome, This cleaning burden is greatly increased when food is baked on to the ceramic cooling vessel. The excessive scrubbing required to remove the stuck-on food is complicated by the bulky and unmanageable shape and size of the typical slow-cooker.  
           [0007]    In light of the problems and limitations of the prior art described above, a need exists for ceramic cookware (and more preferably for a ceramic slow-cooker) that has a reliable and long-lasting non-stick coating that simplifies use and cleaning. Each preferred embodiment of the present invention achieves one or more of these results.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention alleviates the problems associated with applying non-stick coatings to ceramic cookware and more particularly to ceramic slow-cookers. The present invention provides a slow-cooker that includes a reliable non-stick coating on the ceramic cooking vessel and provides a method for applying non-stick coating to a ceramic substrate suitable for use with a ceramic slow-cooker. The problems discussed above with respect to using and cleaning slow-cookers call for a non-stick surface that can be applied to the ceramic cooking vessel of a slow-cooker and that can stand up to the harsh cooking environment, thereby simplifying serving and clean-up.  
           [0009]    More specifically, the invention provides a slow-cooker comprising a housing, a ceramic cooking vessel received within the housing, the ceramic vessel having interior surfaces defining a cavity and having a non-stick coating covering at least a portion of the interior surfaces and a heating element mounted within the housing for supplying heat to the ceramic vessel.  
           [0010]    Preferably, the slow-cooker has a non-stick coating that is multi-layered and includes a first layer preferably having a dry film thickness (DFT) of approximately 0.3-0.5 mils. As used herein, the term “dryfilm thickness” and “DFT” refers to the thickness of a film after it is dried from a previously liquid state as is commonly understood in the art. The term “mil” refers to a unit of measurement equal to 1/1000 of an inch as is commonly understood in the art. More preferably, the non-stick coating includes a first layer having a DFT of approximately 0.4 mils. Also preferably, the non-stick coating includes a second layer having a DFT of approximately 0.3-0.6 mils, and more preferably a second layer having a DFT of approximately 0.4-0.5 mils. Also preferably, the non-stick coating includes a third layer having a DFT of approximately 0.05-0.5 mils, and more preferably a third layer having a DFT of approximately 0.1-0.2 mils.  
           [0011]    In most highly preferred embodiments of the present invention, the non-stick coating includes a first layer having a DFT of approximately 0.3-0.5 mils, a second layer having a DFT of approximately 0.3-0.6 mils and a third layer having a DFT of approximately 0.05-0.5 mils. Most preferably, the non-stick coating includes a first layer having a DFT of approximately 0.4 mils, a second layer having a DFT of approximately 0.4-0.5 mils and a third layer having a DFT of approximately 0.1-0.2 mils.  
           [0012]    The invention also provides a method for making a non-stick ceramic cooking vessel for a slow-cooker, the method comprising the steps of applying a first layer of a non-stick coating over a ceramic cooking surface of the vessel, drying the first layer, cooling the first layer and applying a second layer of a non-stick coating over at least a portion of the first layer. The method preferably also includes applying a third layer of a non-stick coating over at least a portion of the first and second layers, more preferably before the second layer of non-stick coating is dry. The method is preferably completed by drying the second and third layers and then curing the layers of non-stick coating.  
           [0013]    The present invention finds application on both glazed and unglazed ceramic cooking vessels for slow-cookers. Where the present invention is applied to ceramic cooking vessels having a glazing defining a seal over a cooking surface of the ceramic vessel, the method preferably comprises the steps of removing a portion of the glazing while maintaining at least a portion of the seal of the vessel and applying a non-stick coating over the cooking surface, The step of removing a portion of the glazing is preferably achieved by grit-blasting at least a portion of the glazing with aluminum oxide. Additionally, the cooking surface preferably includes a bottom surface and a wall surface wherein at least eighty-five percent of the glazing is preferably removed from at least a portion of the bottom surface and at least fifty percent of the glazing is preferably removed from at least a portion of the wall surface such that the seal remains substantially intact on the bottom surface and wall surface. More preferably, ninety-five to one hundred percent of the glazing is removed from at least a portion of the bottom surface such that the seal is substantially removed from the bottom surface, and at least 75 percent of the glazing is removed from at least a portion of the wall surface such that the seal remains substantially intact on the wall surface.  
           [0014]    More information and a better understanding of the present invention can be achieved by reference to the following drawings and detailed description.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The present invention is further described with reference to the accompanying drawings, which show a preferred embodiment of the present invention. However, it should be noted that the invention as disclosed in the accompanying drawings is illustrated by way of example only. The various elements and combinations of elements described below and illustrated in the drawings can be arranged and organized differently to result in embodiments which are still within the spirit and scope of the present invention.  
         [0016]    In the drawings:  
         [0017]    [0017]FIG. 1 is a perspective view of a ceramic slow-cooker according to a preferred embodiment of the present invention;  
         [0018]    [0018]FIG. 2 is a sectional view of the slow-cooker shown in FIG. 1; and  
         [0019]    [0019]FIG. 3 is an enlarged view of a portion of the cooking surface of the slow-cooker shown in FIG. 2,  
         [0020]    It is to be understood that the phraseology and terminology used herein is for the purpose of description only and should not be regarded as limiting. The use of “including” and “comprising” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The use of letters to identify steps of a method or process is simply for identification and is not meant to indicate that the steps should be performed in a particular order. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    [0021]FIGS. 1 and 2 illustrates the slow-cooker  10  of the present invention. Although the present invention is shown applied to the particular slow-cooker of FIGS.  1 - 3 , it should be noted that the present invention finds application to any slow-cooker, regardless of shape and proportions. The slow-cooker  10  includes a housing  14 , handles  18 , a ceramic cooking vessel  22  received within the housing  14 , a heating element  26  mounted within the housing  14  for supplying heat to the ceramic vessel  22  and a control member  30 , preferably on the housing  14 , for controlling the temperature of the heating element  26 . Preferably, the slow-cooker  10  also includes legs  20  mounted to the housing  14  and a cover  38  removably located over the ceramic vessel  22 . A power cord  42  preferably supplies power from a power source  34  to the heating element  26 . The power source  34  is most preferably a standard electrical power source such as a 120 or 240 volt source.  
         [0022]    The ceramic cooking vessel  22  includes an interior wall cooking surface  46  having a rim  50 , and an interior bottom cooking surface  54 . Together the wall cooking surface  46  and bottom cooking surface  54  define a cavity  58  and the rim  50  defines an opening  62 . Vessel handles  66  preferably extend from the rim  50  and are preferably substantially parallel to the bottom surface  54 . In the illustrated preferred embodiment, the ceramic cooking vessel  22  is round such that the opening  62  is substantially circular. Alternatively, the vessel  22  can be oval-shaped such that the opening  62  is substantially oval-shaped. Furthermore, the illustrated embodiment depicts a vessel  22  that is removable from the housing  14 , but the invention is also capable of being practiced with slow-cookers  10  having a non-removable ceramic cooking vessel as is well known in the art (not shown).  
         [0023]    The interior cooking surfaces  46 ,  54  of most ceramic cooking vessels for slow-cookers typically originally include a glazing layer  70  that defines a seal over the interior cooking surfaces  46 ,  54 . The glazing layer  70  can also cover the rim  50 , the handles  66  and any other portion of the vessel  22 . The glazing layer  70  usually has a somewhat glossy appearance and substantially seals the otherwise porous ceramic substrate of the cooking surfaces  46 ,  54 , preventing the penetration of liquids into the ceramic substrate. As will be described below, the glazing layer  70  can be partially or completely removed from the interior cooking surfaces  46 ,  54 , the rim  50  or the handles  66 .  
         [0024]    The ceramic vessel  22  also includes a non-stick coating  74  covering at least a portion of the interior cooking surfaces  46 ,  54 . Preferably, the non-stick coating  74  also covers the rim  50  and the handles  66 , and can also cover any other portion of the vessel  22  if desired. The non-stick coating  74  provides a durable and reliable low-friction surface that substantially prevents the food being cooked (not shown) in the slow-cooker  10  from sticking to the surfaces coated. In the illustrated embodiment, the non-stick coating  74  comprises multiple layers, and more preferably, three layers. An example of such non-stick coating material is that available from Akzo Nobel Non-Stick Coatings, LLC of Chicago, Ill.  
         [0025]    The application of the preferred non-stick coating  74  of the present invention will be described in detail below and results in a three-layered non-stick coating  74  (see FIG. 3) over at least a portion of the interior cooking surfaces  46 ,  54  and preferably over at least a portion of the rim  50  and handles  66 . The first layer or basecoat  78  preferably has a dryfilm thickness (DFT) of approximately 0.3-0.5 mils (0.0076-0.013 mm), and more preferably has a DFT of approximately 0.4 mils (0.010 mm). As used herein, the term “dryfilm thickness” and “DFT” refers to the thickness of a film after it is dried from a previously liquid state as is commonly understood in the art. The term “mil” refers to a unit of measurement equal to 1/1000 of an inch as is commonly understood in the art. The second layer or intercoat  82  preferably has a DFT of approximately 0.3-0.6 mils (0.0076-0.015 mm), and more preferably has a DFT of approximately 0.4-0.5 mils (0.010-0.013 mm). The third layer or topcoat  86  preferably has a DFT of approximately 0.05-0.5 mils (0.0013-0.013 mm), and more preferably has a DFT of approximately 0.10-0.2 mils (0.0025-0.005 mm).  
         [0026]    The non-stick coating  74  can be applied to the interior cooking surfaces  46 ,  54 , the rim  50  and the handles  66  regardless of whether only a portion or substantially all of the glazing layer  70  has been removed therefrom. Preferably, substantially all of the glazing layer  70  is removed from the interior bottom cooking surface  54  prior to application of the non-stick coating  74 , but this need not be the case.  
         [0027]    The non-stick coating  74  of the present invention preferably is applied as follows. First, the interior surfaces  46 ,  54 , the rim  50 , the handles  66  and any other desired portion of the ceramic vessel must be clean and free from oil, grease, water or any adhesive from decals or stickers, etc. Once clean, the portions of the vessel  22  desired to be coated with the non-stick coating  74  are preferably grit-blasted to remove at least a portion of the glazing layer  70  thereon. Removing at least a portion of the glazing layer  70  aids in bonding the non-stick coating  74  to the ceramic vessel  22  by providing a roughened surface to which the non-stick coating  74  can properly bond. Removal of the glazing layer  70  is preferably achieved using a 44-64 grit aluminum oxide blast that creates a profile  90  (see FIG. 3) measuring approximately 160-240 micro inches (0.004-0.006 mm). As used herein, the term “profile” refers to the ridges or valleys created during the grit-blasting as is commonly understood in the art. The term “micro inch” refers to a unit of measurement equal to 1/1,000,000 of an inch as is commonly understood in the art. More preferably, a 54-grit aluminum oxide blast is used to create a profile  90  measuring approximately 180-220 micro inches (0.0046-0.0056 mm).  
         [0028]    As noted above, the non-stick coating  74  can be applied to surfaces of the ceramic vessel that have no glazing and surfaces that have had a layer of the glazing removed. Therefore, it is important to note that the profile  90  made by grit-blasting can be created in the glazing layer  70  (as seen on the interior wall cooking surface  46  in FIG. 3) or in the ceramic substrate of the cooking vessel  22  (as seen on the interior bottom cooking surface  54  in FIG. 3).  
         [0029]    Preferably, the grit-blasting removes at least eighty-five percent of the glazing layer  70  over at least a portion of the bottom cooking surface  54  and at least fifty percent of the glazing layer  70  over at least a portion of the wall cooking surface  46 , the rim  50  and the handles  66  such that the reflective or glossy finish of the glazing layer  70  is removed, but the seal remains substantially intact. More preferably, ninety-five to one hundred percent of the glazing is removed from at least a portion of the bottom cooking surface  54  such that the seal is substantially removed from the bottom cooking surface  54 , and at least seventy-five percent of the glazing is removed from at least a portion of the wall cooking surface  46 , the rim  50  and the handles  66  such that the reflective or glossy finish of the glazing layer  70  is removed, but the seal remains substantially intact. In either case, a substantially uniform profile  90  is created that improves the bond between the non-stick coating  74  and the ceramic vessel  22 . While grit-blasting with aluminum oxide is preferred, the invention also contemplates other known methods of removing the glazing layer  70  including, but not limited to, etching and sand-blasting.  
         [0030]    With the grit-blasting complete, the vessel  22  is emptied of any grit or debris. Pressurized air can be used to aid in emptying the grit or debris. It is important that the grit-blasted surfaces do not come into contact with oils (such as from human fingers), as these oils can detrimentally affect the bonding of the non-stick coating  74  and can leave fingerprints that may only become visible after application and curing of the non-stick coating  74 . Clean pressurized water and/or cleaning agents can also be used to remove grit and debris from the vessel  22 .  
         [0031]    Next, the first layer or basecoat  78  is applied over at least a portion of the interior cooking surfaces  46 ,  54 , and preferably the rim  50  and handles  66  as well.  
         [0032]    The first layer  78  is applied using at least one, and preferably up to four spray guns (not shown) as is commonly known in the art. The first layer  78  is preferably applied to obtain a DFT of approximately 0.3-0.5 mils (0.0076-0.013 mm), and more preferably a DFT of approximately 0.4 mils (0.010 mm). The first layer  78  adheres to the profiles  90  and substantially seals the ceramic substrate of the bottom cooking surface  54  which became substantially unsealed when the glazing layer  70  was removed. Furthermore, the first layer  78  improves the existing seal over the wall cooking surface  46  and further protects against penetration of the ceramic substrate by food liquids.  
         [0033]    Preferably, the first layer  78  is then dried until substantially dry to the touch. This is preferably achieved by flash drying the vessel  22  at a temperature of approximately 180-270 degrees Fahrenheit for approximately 2-6 minutes, and more preferably at a temperature of approximately 200-250 degrees Fahrenheit for approximately 3-5 minutes. Other drying techniques known in the art can also be used. After drying, the vessel  22  and first layer  78  are preferably cooled in ambient air while taking care that no dirt or debris sticks to the first layer  78 .  
         [0034]    When the first layer  78  is cool, the second layer or intercoat  82  is applied over the first layer  78  using at least one, and preferably up to five spray guns (not shown) as is commonly known in the art. The second layer  82  is preferably applied to obtain a DFT of approximately 0.3-0.6 mils (0.0076-0.015 mm), and more preferably a DFT of approximately 0.4-0.5 mils (0.010-0.013 mm). The second layer  82  adheres to the first layer  78 .  
         [0035]    The third layer or topcoat  86  is preferably applied over the second layer  82  while the second layer  82  is still wet. The third layer  86  is applied using at least one spray gun, and preferably only one spray gun (not shown) as is commonly known in the art. The third layer  86  is applied to obtain a DFT of approximately 0.05-0.5 mils (0.0013-0.013 mm), and more preferably a DFT of approximately 0.1-0.2 mils (0.0025-0.005 mm). The third layer  86  preferably adheres to, and partially mixes with, the second layer  82 . The third layer  86  is preferably the final layer of non-stick coating  74  to be applied and provides the non-stick surface characteristics desired for the ceramic vessel  22 .  
         [0036]    The second and third layers  82 ,  86  are then dried until substantially dry to the touch. This is preferably achieved by flash drying the vessel  22  at a temperature of approximately 180-270 degrees Fahrenheit for approximately 2-6 minutes, and more preferably at a temperature of approximately 200-250 degrees Fahrenheit for approximately 3-5 minutes. Other drying techniques known in the art can also be used.  
         [0037]    After the second and third layers  82 ,  86  are dry, the three layers  78 ,  82  and  86  are preferably cured by heating the vessel  22  for approximately 18-33 minutes at a temperature of up to approximately 900 degrees Fahrenheit, and more preferably for approximately 25-30 minutes at a temperature of up to approximately 850 degrees Fahrenheit. The non-stick coating  74  can be cured in any way commonly known in the art, but is preferably cured using a six-zone curing oven (not shown) having the temperature zones set at 400 degrees Fahrenheit, 520 degrees Fahrenheit, 820 degrees Fahrenheit, 810 degrees Fahrenheit, 800 degrees Fahrenheit and 800 degrees Fahrenheit respectively. The vessel  22  preferably moves at a fixed rate through each of the six zones. Alternatively, a four-zone curing oven (not shown) can be used, having the temperature zones set, for example, at 400 degrees Fahrenheit, 650 degrees Fahrenheit, 820 degrees Fahrenheit and 800 degrees Fahrenheit respectively. Again, the vessel  22  preferably moves at a fixed rate through each of the four zones. Other curing techniques known in the art can also be used.  
         [0038]    After curing, the non-stick coating  74  and vessel  22  are preferably cooled in substantially still ambient air. Taking care not to cause surface cracking of the non-stick coating  74  or vessel  22 , forced air from fans can be used to cool the vessels  22  if desired. If the forced air cools the vessel  22  too quickly cracks can form, usually around the handles  66 .  
         [0039]    The above-described application of non-stick coating  74  to the ceramic slow-cooker  10  produces a ceramic vessel  22  having excellent non-stick characteristics. The non-stick coating substantially prevents food from sticking to the vessel  22 , especially in the areas of high temperature gradients, such as directly adjacent the heating element  26  and at the food/air interface. The non-stick coating  74  bonds well to both the glazed layer  70  and the ceramic substrate of the vessel  22  and is therefore durable and long-lasting. Because the food does not stick to the vessel  22 , serving and clean-up are essentially trouble-free. Whether the vessel  22  is removable from the housing  14  or fixed to the housing  14 , cleaning the slow-cooker  10  is simplified due to the fact that little or no food will be stuck to the vessel  22 . No excessive scrubbing, submersing or manipulating is needed.  
         [0040]    The embodiments described above and illustrated in the figures are presented by way of example only and are not intended as a limitation upon the concepts and principles of the present invention. As such, it will be appreciated by one having ordinary skill in the art that various changes in the elements and their configuration and arrangement are possible without departing from the spirit and scope of the present invention as set forth in the appended claims. For example, the amount of glazing removed from any surface of the vessel  22  may vary as desired. Furthermore, the non-stick coating  74  can be applied to any combination of locations on the vessel  22  as desired. For example, the non-stick coating  74  can be applied only to the bottom surface  54 , only to the wall surface  46  or to any other location or combination of locations as desired.