In some categories such as coins, dating is very precise and one can classify them perfectly based on age.
But in many categories of art - especially folk art - where people made these artworks for self-consumption rather than for commercial use, no one thought of tagging the article with the date of production.
So how do we know whether the piece was produced in 1945 or in 1919 or in 1910.
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We don't. And we can't.
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[Carbon dating doesn't help as the margin of error of the process is stated to be +/-150 years to +/- 600 years]
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But what we do know from articles written in those times, is the changes introduced in society at a particular decade.
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For instance the introduction of aniline dyes by Bayer in a particular region is traced to the year 19XX. So we know that prior to that year all dyes were natural.So when we see something with aniline dyes from that region, we can safely say that it was produced afterthat year. But on the other hand if we see a piece with natural dyes from that region, that information alone is not sufficient to conclude the date of production.
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We rely on cultural practices such as introduction of sewing machines.
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And incidence of war or famine or tsunamis that bring about dramatic changes in the social lifestyle of the region and use these to date 'before' or 'after'.
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For example, we date Tampan shipcloths based on the Krakatau volcano that destroyed life (and weaving) in that region. Again, although we know that a particular piece was produced 'before the tsunami' we will never know precisely whether it was produced in 1870 or 1850 unless additional data is available.
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Although the dating of piece affects the value, it important to ask why does age matter.
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The obvious answer is that the older the piece the harder it is for it to have survived for longer - so pieces from the 17th century are rarer than pieces from the 19th century. So older pieces are rarer.
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But not everything that is antique is beautiful. Some have only age going for them and fall into the "why bother" category.
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And conversely not everything that is young is not beautiful. [there is no typographical error in the previous sentence]. There are exceptional pieces being produced even today that are museum worthy and will be worth handing down to the future generations.
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So age is a wonderful feature to be looking for in an old artwork. But age alone is not enough and must be supported by other factors such as motifs, color, craftsmanship, construction etc etc etc.