Abstract:
An article of cookware in the form of a cooking vessel exposes only a portion of a copper inner layer on the upper portion of the exterior sidewall. The layer does not change color or tarnish as it sufficiently distal from the bottom of the cooking vessel that it is not subject to direct heat and oxidation during normal use. The unique bright color of the native copper is further protected from tarnish caused by fingerprint by a protective coating that can withstand moderate heating.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a Division of and claims priority to the U.S. non-provisional patent application having Ser. No. 11/550,205, which was filed on Oct. 17, 2006, having the same title, which is incorporated herein by reference. 
     
    
     BACKGROUND OF INVENTION 
       [0002]    The present invention relates to article of cookware, and in particular to cookware vessels fabricated at least in part from copper 
         [0003]    Copper cookware is appreciated for its high thermally conductivity as well as the reddish-gold appearance. This reddish-gold appearance is unique from other metals with the exception of gold and copper alloys, such as brass and bronze, and thus readily distinguishes such cookware. Although copper cookware is both beautiful and considered to offer superior cooking performance, due to the high thermal conductivity, it tarnishes readily on exposure to heat as well as liquids (including the moisture residue on the fingers to create finger prints). Accordingly, because of the extra effort required to clean such tarnish, copper cookware is not popular with some consumers despite its other benefits. 
         [0004]    Prior methods of minimizing the tarnishing of copper cookware include burying the copper layers in at least one of interior and exterior metal cladding so it is not visible, as for example in U.S. Pat. No. 6,267,830 (issued Jul. 31, 2001 to Groll). While this provides most of the thermal performance benefits of copper it eliminates the secondary benefits of its unique and elegant appearance. 
         [0005]    Another partial attempt to solve this problem is provided by U.S. Pat. No. D474,936 (issued on May 27, 2003 to Groll) wherein a indented golden copper colored band formed around the outer periphery on a lower portion of a cooking vessel as decorative feature. Although this design exposes some copper metal, it still does not prevent tarnishing. Further, such a groove tends to collect dirt and debris and is more difficult to clean out than a largely straight sided cooking vessel. 
         [0006]    Although protective coatings have been applied to copper to prevent tarnishing, they suffer from two deficiencies. A variety of inorganic coatings have been suggested to apply in place of or over copper, such as transition metal nitrides, oxy-nitrides and carbides as they can achieve different shades of reflective golden metal appearances. However, such inorganic coatings are both expensive and not necessary thermally and chemically stable in cooking at high temperature, where they can be exposed to direct flames. It has been found that such coatings, when applied as a thin layer over copper merely temporally minimize the development of a tarnished appearance, which eventually occurs as the underlying copper oxidizes at high temperatures. Eventually this oxidized copper becomes visible, starting first at the bottom of the cookware, which undergoes multiple cycles of exposure to high temperature. 
         [0007]    Organic lacquers, while capable of preventing tarnishing on decorative copper items do not withstand the high temperatures that are reached by functional cookware. This is particularly a problem at the bottom of the cookware article that is exposed either directly to the flame, but also of issue somewhat more distal from the heater element, as the copper is a good conductor of heat. 
         [0008]    It is therefore a first object of the present invention to provide clad cookware that exposes copper layers that are easy to clean and do not trap dirt. 
         [0009]    It is a further object of the invention to provide exposed copper layers that do not tarnish under normal use conditions, maintaining the beauty of copper cookware, yet without the high maintenance of cleaning tarnished copper. 
         [0010]    Yet another object of the present invention is to provide a simpler and lower cost fabrication method for creating clad cookware that exposes such copper layers. 
       SUMMARY OF INVENTION 
       [0011]    In the present invention, the first object is achieved by forming a cooking vessel having an inner lining with a bottom surface and surrounding sidewalls extending upward there from to a rim to form an inner fluid containing vessel, the rim of lining defining the upper extent of the rim of the article of cookware. An outer cladding layer covers the exterior of the bottom surface of the inner lining and extends upward there from to cover a bottom portion of the exterior of the surrounding sidewalls of the inner lining. A copper cladding covers at least the portion of the surrounding sidewalls of the inner lining not covered by the outer cladding to form a portion of the visible external surface of the cookware article by extending upward to join the rim of the inner lining. 
         [0012]    A second aspect of the invention is characterized in that the exposed exterior copper cladding is covered by a protective coating, and more preferably an organic heat resistant transparent lacquer. 
         [0013]    The above and other objects, effects, features, and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1A  is schematic illustration of a sequence of processing steps used to fabricate the article of cookware shown in  FIG. 1B   
           [0015]      FIG. 1B  is an elevation of an article of cookware showing a partial view of the cross section as resulting from the processing steps in  FIG. 1A . 
           [0016]      FIG. 2  is an elevation of another alternative embodiment for the article of cookware showing a partial view of the cross section. 
           [0017]      FIG. 3  is a partial cross section of another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Referring to  FIGS. 1 through 3 , wherein like reference numerals refer to like components in the various views, there is illustrated therein a new and improved article of cookware with a copper exterior, generally denominated  100  herein. 
         [0019]    In accordance with the present invention,  FIG. 1B  illustrates a cookware vessel  100  having a bottom generally denoted by numeral  2 , and substantially upright and surrounding sidewalls  4  connected to the bottom  2  to form a fluid containing vessel. The surrounding sidewall has a lower portion  5  connected to the bottom  2  and an upper portion  6  that is at least externally visible as extending upward from the edge  5   a  of lower portion  5  to the vessel rim  7 . The visible upper portion of the external wall  6  is preferably copper or an alloy thereof. In contrast, the externally exposed lower portion  5  is preferably stainless steel, or another material that unlike copper, is not susceptible to tarnishing when exposed to high temperatures. By visibly exposing the copper closer to rim  7 , and farther from the bottom  2 , the visible copper remains cooler during cooking and is thus less susceptible to tarnishing, as well as capable of being covered with various protective coatings that can withstand the reduced temperatures in thus region. This embodiment, shown in one form in  FIG. 2 , will be further discussed below. 
         [0020]    As shown in  FIG. 1A , the cookware vessel  100  is optionally formed from a fluid containing inner lining  10  that forms the interior bottom surface  21  and interior side surface  41  of the cooking vessel  100 . The inner lining is nested within and attached to an intermediate lining  20 , of which at least the upper portion has a copper appearance or is fabricated entirely from copper or an alloy thereof. The lower portion  5  of the exterior wall is provided by an outer cladding  30  the covers the exterior of the bottom  2  and the lower portions of the fluid containing inner lining  10  and the lower of portion of the intermediate lining  20 . The intermediate lining  20  is preferably copper or an alloy thereof having a thickness of about 1.5 mm. The inner lining  10 , intermediate lining  20  and outer cladding  30  are nested and bonded to each other in the nested arrangement shown in  FIG. 1 . Alternatively, the inner lining is another metal, such as aluminum or stainless steel, which merely has an upper outer band of copper cladding, thinner copper plating or a copper appearing coating. More preferably, both the inner lining  10  and the outer cladding  30  are 0.6 mm thick grade 18/8 stainless steel 
         [0021]    The outer cladding  30  is shorter than the inner lining  10  to expose an intermediate upper portion of the intermediate lining  20  whereby the layer of copper, or copper appearing portion that is exposed is a portion of a continuous cladding layer that is also adjacent to the outer cladding at the bottom  2  and the lower portion, below seam  5   a , of the sidewall  5 . 
         [0022]    In another alternative embodiment of the invention, which is illustrated in  FIG. 2 , the exposed copper layer  6  is covered with a protective coating  61 . Preferably, this protective coating is transparent, and more preferably this transparent protective coating is an organic lacquer 
         [0023]    It should be understood that in other alternative embodiment of the invention additional layers of thermally conductive materials, such as aluminum or non-exposed layers of copper or alloys thereof, are laminated into the sides  5  or bottom  2  of the cooking vessel  100 . 
         [0024]    It should be appreciated the methods of laminating and bonding the nested assembly of  FIG. 1A  includes at least one of brazing, impact bonding, ironing, laser welding and like processes. 
         [0025]    The laser welding can be performed at the upper rim  7  or the junction  5   a  between the copper and the lower sidewall material. Such laser welding is preferably carried out after brazing and/or ironing. 
         [0026]    It should be noted that although  FIG. 1B  of the completed vessel shows the copper intermediate lining  20  as flush with the edge of the exterior cladding at seam  5   a , this is merely a preferred embodiment. This flush seam at  5   a  can be formed during the process of bonding the assembly, such as by impact bonding in a matching die set. Alternatively, the inner copper vessel  20  can also be swaged such that it is the same outer diameter above outer cladding  30 , forming a flush seam at  5   a . Such swaging can be done after nesting and bonding, or before, thus resulting in completed vessel  100  shown in  FIG. 1B . 
         [0027]    Alternatively an ironing process can be used to reduce the thickness of copper in the sidewall  5  with respect to the bottom portion  2 , as well as deforming a portion of the inner lining  10  and intermediate lining  20 , to form the flush seam  5   a . The ironing process is simply a sequence of deep drawing type forming process between a set of dies with the clearance between the male and female die members being progressively smaller than the actual combined side wall thickness of the original wall thickness of the inner cladding  10 , intermediate copper cladding  20  and outer cladding  30 . As the intermediate cladding of copper  20  is much softer than stainless steel inner  10  and outer  30  cladding, it will undergo a reduction in thickness during an iron process. 
         [0028]    Further, as it can be difficult in a brazing process to fully reflow liquid flux over the entire interfacial area to be bonded, air and moisture can be trapped within such gaps that remain at these interfaces. Accordingly, it is preferable in forming cooking vessel  100  to perform the ironing process after brazing the nested assembly of  FIG. 1B . The ironing process has another advantage in that it gradually expels air and moisture trapped in such gaps at each interface between nested components. 
         [0029]    The protective coating  61  can be an organic lacquer or an inorganic coating. Inorganic coatings includes metal nitrides that have a reddish reflective color like copper. Examples of inorganic coatings are disclosed in the U.S. Pat. No. 5,447,803 (issued Sep. 5, 1995 to Nagaoka et al,), U.S. Pat. No. 3,669,695 (issued on Jun. 13, 1972 to Iler et al,) and U.S. Pat. No. 5,320,689 (issued on Jun. 14, 1994 to Mahulikar, et al.) and U.S. Pat. No. 6,586,114 issued on Jul. 1, 2003 to Anton), all of which are incorporated herein by reference. A wide variety of transparent organic lacquers can be used, such as alkyd resin varnish or lacquer, as well as others clear organic coatings of suitable heat-resistance. It is particularly preferable to utilize a non-stick transparent organic coating, such as a flouro-polymer coating. 
         [0030]      FIG. 3  illustrates an alternative embodiment of the invention in which on the exposed portion of the inner lining  10  has a copper or copper colored coating  31 , rather than a full intermediate copper cladding or layer  30  as shown in  FIG. 2 . This copper type coating  31  is preferably covered by a clear thermally resistant coating  61 . Alternatively the copper appearing region  31  is alternatively a band of solid copper metal laminated to the outside of the upper portion of the inner liner, and not extending to the bottom  2  of the cooking vessel  100 . 
         [0031]    The inner lining is optionally stainless steel, aluminum, hard anodized aluminum, cast iron, copper or a ceramic high temperature enamel coating. 
         [0032]    Any of such lining may be further covered by a non-stick coating, such as a flouro-polymer coating. 
         [0033]    The outer cladding below the copper or coated copper layer is preferably stainless steel, however it can also be hard anodized aluminum, cast iron, or a ceramic high temperature enamel coating on a metal base and the like. 
         [0034]    While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be within the spirit and scope of the invention as defined by the appended claims.