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I tried flattening my first wine bottle. I made my own devit spray with 1 T Borax and 1 C distilled water. I was careful to decant it without disturbing any undissolved Borax. I sprayed the bottle down with devit solution, let it dry overnight, and fired. Question 1) why do I have spots on the bottle? They look like the spots left on dishes in the dishwasher when the Jetdry needs replaced. I had very carefully removed all the label residue and washed it. Question 2) is it possible to completely flatten the bottom of the bottle, or will that part always be slightly raised? Thanks for any insight or suggestions!

I don't understand all of what you said. But here are some comments.
Borax solution. The cup of water needs to be heated to get the maximum saturation of the solution. It does not have to boil. When cool, pour off from sediment and add a drop or two of washing up liquid.
The spots you observe are devitrification. That is exactly what mild devitrification is like. It indicates either your solution was not strong enough, was not applied evenly, had areas where the solution did not spread over because of surface tension. The washing up liquid is there to reduce the surface tension of the borax solution.
At first I thought you meant the underside of the slumped piece. I presume you mean the original bottom of the bottle where it is typically thicker, and which typically falls over onto the top surface of the slumped bottle. Yes, of course it is possible to get this flat, by applying sufficient time and heat, but the time will be pretty long (maybe 3-5 hours soak) and the temperature pretty hot (maybe 850C). Float and bottle glass is pretty slow to move. But this is the realms of fusing, rather than slumping.

Thanks for the reply, Steve! I will try again

If you have access to a tile saw, it's fairly easy to cut the bottom of the bottle off. Clean it
extremely well with a brass brush and Dawn on the cut edge, use your Borax spray and fire. If you're doing a lot of these, look into purchasing a Solo sprayer ( pump) to apply a fine mist. They are about $30 to $35 and worth every penny.

Thanks, Barbara!

i only use distilled water for anything that will go on the glass.