Detlef, I would suggest that hot water is not really necessary, and water applied directly to the wood is perhaps not advisable. 
 
Possibly the easiest way to get a glued joint apart is to put it in contact with damp cloth, wrap the cloth around it is what is usually done. It might take a few days, but most joints will usually weaken with this treatment. 
 
Then wait a few weeks until the wood is thoroughly dry.
		 
		
		
		
		
		
		
		
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