To upgrade your Atelier, you need Crystallizers.
With each upgrade, you need more Crystallizers.
Below is a list of possible ways to gain these.
Gems
You can purchase Crystallizers using gems.
1 Crystallizer = 15 Gems
10 Crystallizers = 125 Gems
100 Crystallizers = 1,154 Gems
1000 Crystallizers = 10,000 Gems
Campaign Elite Sweeps
You can sweet elite campaigns to pick up extra Crystallizers.
Elite 6-1
Elite 6-2
Elite 6-3
Elite 6-4
Elite 6-5
Elite 6-6
Chests
Chest from the VIP daily clan gifts (Moonseekers city) gives Crystallizers too.
VIP Daily Clan Gifts
You can collect 5 Crystallizers per gift from daily VIP Clan gifts (Once you have level 25 diplomacy for this city).