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The material to make a seal was normally bronze , copper , or iron ( generally say , metal materials ) , because metals are not so easily broken , and they were considered as " immortal " , which could represents the authority of the rule or government .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 None