Im always trying to learn more about Shiro Utsuris and always read alot about their development. I was browsing the web and found this intristing article by Mike Snaden (One of my fav resources) and wanted to share these snippets about sumi.

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sumi and water quality
With Shiro Utsuri and Showa there is a popular belief that water hardness affects Sumi development and that soft water will make it go down, and hard water will make it develop. Whilst this is partly true, it is more the case that low quality Sumi is affected in this manner, whereby it is influenced by both water hardness, and water temperature. But, when it comes to high quality Sumi, you will find that it will generally develop irrespective of these factors.

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Another way to help you choose Shiro Utsuri or Showa is to place them into a bowl and watch them for several minutes. If you do this with a number of prospective purchases, you will find that some stay looking good for the duration, but others will have Sumi that will literally turn grey whilst the koi is in the bowl. The latter should be completely dismissed, and returned to the pond.

The above bowl test i experienced myself this last weekend. I have a Shiro Utsuri tosai thats in Qt at cape koi cause it developed a bacterial infection. When christie boweled it for me so i could see its recovering progress the black was amazing but 4 min in the black looked dull gray....so in other words; a nice in the pond koi but not a show fish although it looks as if the quality is there.

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With Showa, you will find any Sumi that lies on the red pattern will always look blacker than the Sumi that is on white ground. The red pattern acts like an ‘undercoat’ that will always make the Sumi look better than it really is, so Sumi on these areas must be completely ignored.

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It is of paramount importance to ignore how the Sumi looks if it falls on top of red pattern, as it will always look better here.

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If, however, a Showa has a lot of Sumi that looks finished where it falls on top of the pattern, yet looks like Kage Sumi on all the white areas, then this kind of koi is best well avoided, as it is most likely that the Sumi on the white ground will never come up!

This is something else i experienced a couple of years ago and did not understand. I bought a rather expensive Hi Showa nissai. It had the most amazing sumi but only some shiro in the nose. 2 months later the koi lost all its sumi, its now 4 years later and this showa represents more a goshiki than a showa.

Ref: http://www.yumekoi.com/images/stories/pdfs/Sumi-not-so-Simple.pdf