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I have used this method to encase many different items. If you want to use dried flowers you are going to have air bubbles trapped in areas which can be a problem. A long skewer can help dislodge bubbles. If you are wanting to do leaves make sure they are as dry as possible. If there is any moisture in them the curing process will turn the moisture to steam and either crack your casting or cause it to fog. I don't know if its been mentioned or not, but the curing process is a chemical reaction and it does get hot enough to burn you, so be careful touching the mold when it starts setting up.