preferred embodiments of the present invention include ( but not limited to ) the followings : 1 . a metallic article of an orthodontic archwire or dental crown having a color different from its original metallic gloss . 2 . the metallic article as recited in item 1 , which is an orthodontic archwire . 3 . the metallic article as recited in item 2 , wherein the color of said orthodontic archwire is formed by an anodizing treatment . 4 . the metallic article as recited in item 2 , wherein said orthodontic archwire is made of an alloy mainly consisting of β - ti . 5 . the metallic article as recited in item 2 , wherein said orthodontic archwire is made of an alloy mainly consisting of ni and ti . 6 . the metallic article as recited in item 2 , wherein said orthodontic archwire is made of an alloy mainly consisting of stainless steel . 7 . the metallic article as recited in item 2 , wherein the color of said orthodontic archwire is blue , yellow , purple , green , golden , or tawny . 8 . the metallic article as recited in item 1 , which is a dental crown . 9 . the metallic article as recited in item 8 , wherein the color of said orthodontic archwire is formed by an anodizing treatment . 10 . the metallic article as recited in item 8 , wherein said dental crown is made of an alloy mainly consisting of ni and cr . 11 . the metallic article as recited in item 8 , wherein said dental crown is made of an alloy mainly consisting of ni . 12 . the metallic article as recited in item 8 , wherein said dental crown is made of an alloy mainly consisting of stainless steel . 13 . the metallic article as recited in item 8 , wherein the color of said dental crown is blue , yellow , purple , green , golden , or tawny . 14 . a method for preparing a colored metal article of an orthodontic archwire or dental crown , which comprises the following steps : a ) cleaning a metallic article ; and b ) performing an anodizing treatment on the cleaned metallic article from step a ) in order to obtain a color on the surface of said metallic article different from its original metallic gloss , wherein an electrolytic solution used in said anodizing treatment is an acidic aqueous solution . 15 . the method as recited in item 14 , wherein an operation voltage of said anodizing treatment is 5 v - 60 v . 16 . the method as recited in item 14 , wherein said metallic article is made of an alloy mainly consisting of β - ti , and said electrolytic solution is an aqueous solution of a sulfate . 17 . the method as recited in item 14 , wherein said metallic article is made of an alloy mainly consisting of ni and ti , and said electrolytic solution is an aqueous solution of a sulfate . 18 . the method as recited in item 14 , wherein the color on the surface of said metallic article is blue , yellow , purple , green , golden , or tawny . 19 . the method as recited in item 14 , wherein said cleaning comprises immersing said metallic article in an acidic aqueous solution . 20 . the method as recited in item 19 , wherein said acidic aqueous solution is an aqueous solution of hydrochloric acid . according to one of the preferred embodiments of the present invention , a metallic orthodontic archwire with a color different from its original metallic gloss is provided . the biocompatibility of an orthodontic archwire is very important because such an orthodontic archwire is mounted on the teeth of a person . therefore , an ordinary dying or pigment coating technique can not be applied on the coloring of an orthodontic archwire . the coloring method of anodizing treatment according to the present invention uses the optical interference effect of the oxide film on the surface of the material without external addition of a pigment . the products resulting from the anodizing treatment are oxides of the metallic elements of the orthodontic archwire , which are formed on the surface of the orthodontic archwire . for an ordinary metal element , its oxide is often more stable than the metal element , that is the biocompatibility after anodizing treatment will become better . therefore , the anodizing treatment will not introduce an additional biocompatibility problem . furthermore , the anodizing treatment according to the present invention only oxidizes the surface of the material such that the mechanical strength of the orthodontic archwire will not be affected conspicuously . in other words , an anodizing treatment can alter the outlook color of an orthodontic archwire , without significantly changing the biocompatibility and mechanical strength thereof ( wherein the former might be increased slightly , while the later might be decreased or increased slightly ). the anodizing treatment of titanium metal has been widely elaborated in the literature ( g . g . turner , gb1100913 , 1968 ; shreir , gb962904 , 1964 ; kendull , u . s . pat . no . 3 , 663 , 379 , 1972 ; kaminski , u . s . pat . no . 6 , 029 , 390 , 2000 )— with a major emphasis on the titanium structural materials . among the prior arts , there is completely no disclosure of anodizing treatment on the orthodontic archwires , as well as completely no disclosure on the anodizing treatment of ti — ni phase . the anodizing treatment is also applicable to dental crowns for altering the outlook color without significantly changing the biocompatibility and mechanical strength thereof . an orthodontic archwire of ni — ti alloy ( code no . n - 003 , ni — ti orthodontic archwire from dca corp . of u . s . a .) was subjected to an anodizing treatment , wherein the electrolytic solution used was an aqueous solution prepared by dissolving 5 g of na 2 so 4 • 10h 2 o in 400 ml of water , the anode was said orthodontic archwire , the cathode was a platinum plate , and the power supply was a lps 305 from the motech co . said orthodontic archwire was immersed in a 0 . 5 m naoh solution and subjected to an ultrasonic oscillation for 5 minutes . this alkaline washing treatment removed esters on the surface and activated the surface of the metal in order to achieve a uniform surface property . next , said orthodontic archwire was immersed in a hydrochloric acid solution ( 30 g hcl / 100 ml h 2 o ) and subjected to an ultrasonic oscillation for 10 minutes in order to remove the oxides on the metal surface , followed by washing with a large quantity of distilled water and drying by blowing . a constant voltage was used to perform an anodizing treatment at room temperature , wherein the voltage used was 10 v , and the anodizing treatment time was 120 minutes . next , said orthodontic archwire was removed , washed with distilled water , and dried by blowing , to obtain a blue orthodontic archwire . said orthodontic archwire has a bright metallic gloss of ni — ti alloy prior to receiving the treatment . an orthodontic archwire of ni — ti alloy ( code no . n - 003 , ni — ti orthodontic archwire from dca corp . of u . s . a .) was used as a specimen . the electrolytic solution used was an aqueous solution prepared by dissolving 10 g of na 2 so 4 • 10h 2 o in 400 ml of water . the experiment system and method for pre - treating the specimen were identical to example 1 . the anodizing treatment was carried out at room temperature with a constant voltage . the operation voltage used was 10 v , and the anodizing treatment time was 120 minutes . next , said specimen was removed , washed with distilled water , and dried by blowing , to obtain a yellow orthodontic archwire . an orthodontic archwire of ni — ti alloy ( code no . n - 003 , ni — ti orthodontic archwire from dca corp . of u . s . a .) was used as a specimen . the electrolytic solution used was an aqueous solution prepared by dissolving 20 g of na 2 so 4 • 10h 2 o in 400 ml of water . the experiment system and method for pre - treating the specimen were identical to example 1 . the anodizing treatment was carried out at room temperature with a constant voltage . the operation voltage used was 10 v , and the anodizing treatment time was 120 minutes . next , said specimen was removed , washed with distilled water , and dried by blowing , to obtain a purple orthodontic archwire . an orthodontic archwire of ni — ti alloy ( code no . n - 003 , niti orthodontic archwire from dca corp . of u . s . a .) was used as a specimen . the electrolytic solution used was an aqueous solution prepared by dissolving 5 g of na 2 so 4 • 10h 2 o in 400 ml of water . the experiment system and method for pre - treating the specimen were identical to example 1 . the anodizing treatment was carried out at room temperature with a constant voltage . the operation voltage used was 20 v , and the anodizing treatment time was 120 minutes . next , said specimen was removed , washed with distilled water , and dried by blowing , in order to obtain a blue orthodontic archwire . an orthodontic archwire of ni — ti alloy ( code no . n - 003 , niti orthodontic archwire from dca corp . of u . s . a .) was used as a specimen . the electrolytic solution used was an aqueous solution prepared by dissolving 10 g of na 2 so 4 • 10h 2 o in 400 ml of water . the experiment system and method for pre - treating the specimen were identical to example 1 . the anodizing treatment was carried out at room temperature with a constant voltage . the operation voltage used was 20 v , and the anodizing treatment time was 120 minutes . next , said specimen was removed , washed with distilled water , and dried by blowing , in order to obtain a green orthodontic archwire . a β - ti orthodontic archwire ( code no . 100 - 942 , β - ti orthodontic archwire from inc corp . of u . s . a .) was used as a specimen . the electrolytic solution used was an aqueous solution prepared by dissolving 10 g of na 2 so 4 • 10h 2 o in 400 ml of water . the experiment system and method for pre - treating the specimen were identical to example 1 . the anodizing treatment was carried out at room temperature with a constant voltage . the operation voltage used was 60 v , and the anodizing treatment time was 5 minutes . next , said specimen was removed , washed with distilled water , and dried by blowing , to obtain a purple orthodontic archwire . a β - ti orthodontic archwire ( code no . 100 - 942 , β - ti orthodontic archwire from inc corp . of u . s . a .) was used as a specimen . the electrolytic solution used was an aqueous solution prepared by dissolving 10 g of na 2 so 4 • 10h 2 o in 400 ml of water . the experiment system and method for pre - treating the specimen were identical to example 1 . the anodizing treatment was carried out at room temperature with a constant voltage . the operation voltage used was 40 v , and the anodizing treatment time was 5 minutes . next , said specimen was removed , washed with distilled water , and dried by blowing ,. to obtain a golden orthodontic archwire . a β - ti orthodontic archwire ( code no . 100 - 942 , β - ti orthodontic archwire from inc of u . s . a .) was used as a specimen . the electrolytic solution used was an aqueous solution prepared by dissolving 10 g of na 2 so 4 • 10h 2 o in 400 ml of water . the experiment system and method for pre - treating the specimen were identical to example 1 . the anodizing treatment was carried out at room temperature with a constant voltage . the operation voltage used was 20 v , and the anodizing treatment time was 5 minutes . next , said specimen was removed , washed with distilled water , and dried by blowing , to obtain a blue orthodontic archwire . a β - ti orthodontic archwire ( code no . 100 - 942 , β - ti orthodontic archwire from inc corp . of u . s . a .) was used as a specimen . the electrolytic solution used was an aqueous solution prepared by dissolving 10 g of na 2 so 4 • 10h 2 o in 400 ml of water . the experiment system and method for pre - treating the specimen were identical to example 1 . the anodizing treatment was carried out at room temperature with a constant voltage . the operation voltage used was 10 v , and the anodizing treatment time was 5 minutes . next , said specimen was removed , washed with distilled water , and dried by blowing , to obtain a tawny orthodontic archwire . a nicr dental crown was used as a specimen . the electrolytic solution used was an aqueous solution prepared by dissolving 5 g of na 2 so 4 • 10h 2 o in 400 ml of water . the experiment system and method for pre - treating the specimen were identical to example 1 . the anodizing treatment was carried out at room temperature with a constant voltage . the operation voltage used was 5 v , and the anodizing treatment time was 30 minutes . next , said specimen was removed , washed with distilled water , and dried by blowing , in order to obtain a slight bronze color .
this invention provides metallic archwires or dental crowns with various colors , which can be prepared by subjecting the orthodontic archwires or metallic dental crowns to an anodizing treatment .