Si/Pyrex Anodic Bonding

I have been trying to anodically bond pyrex glass to one of my silicon devices, but am having considerable trouble with the system 'tripping
off' all the time. This may be due to a dirty surface (due to the features on the Si surface, it is difficult to do a proper clean before the bond), but I am reasonably confident this is not the case. However, as I am new to using this technique, I was wondering if it is a problem if the glass plate (3 cm^2) is smaller than the wafer (~4 cm^2) to which it is bonded. I'm concerned about sparking between the electrode (which is larger than both teh Si and glass side) and the Si wafer outside the insulating layer. The voltage is around 800V, temp is 300C (necessary for our process), while the current comes in at 0.01mA.
Thanks in advance,
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