Disappearing Sennelier Paints?

One of the most expensive watercolor sets I’ve ever bought is my Sennelier French Watercolor Paints. While I love it’s creaminess and color vibrancy, because of its price, I use it sparingly and I only use it for final works. However, when I opened my set last week, I noticed this on my set.
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Some of the pans have significant loss of paint. I was scratching my head in wonder and a little bit of panic because I’m pretty sure that I haven’t used that much paint. I was very careful in dotting my brush with just barely a pinprick of pigment everytime I use it. So why does it show so much variance? The last time I looked at it, it was still full, as full as the first time I’ve opened it.
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Now, there are a lot of colors showing so much decrease in pigment content per pan. Are these paints known to evaporate? I mean, when you leave some water on it and then store the set for a time, do the pigments evaporate with the water as it dries up? Has this happened to your set too?
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Sennelier Paints
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 photo cher100_zpsb08016be.jpg   Cher is a practicing Filipino Interior Designer, a full-time blogger and aspiring calligrapher with a passion to discover and learn new things. Follow her on Twitter for more updates and please like her Facebook Page too, you'll find something interesting, inspiring and fun posts there daily. Check out her Mindbox for movies and lifestyle updates. If you're an aspiring calligrapher and an art enthusiast, say hello at Cher's Creative Space.