Inquiry about DNA recovery by silica binding

[Wilson]

 Dear all,
 	 I have a few questions about the procedures of silica binding..
	 I have prepare the silica (for binding DNA)
 and resuspended in guanidium thiocyanate.  After adding the silica to
 solution which contain dissolved agarose, DNA and guanidium thiocyanate
 salt,  I tap the solution to mix them.  Then should I incubate the mixture
 in ice or at room temp?  Which temperature can give better DNA recovery?
 Should I use 50% ethanol with 100mM NaCl, Tris-HCl and EDTA or only 70%
 ethanol as the washing solution?  Finally,  which incubation temperature
 45, 55 or 65oC can elute the DNA more effective?
 	One more question is about the safety of guanidium thiocyanate. I
 would like to use silica binding to recovery the DNA in mini-prep.  I know
 that guanidium thiocyanate can release very toxic gas in acidic solution.
 Is it possible that toxic gas released  in the procedure (e.g solution
 III) of miniprep?   
 
 Thanks a lot
 
 Wilson