Hot Spots during Fire Polishing

[Nbolinger]

I'm having trouble with what I call "hot spots" on some of my pieces during fire polishing. Here is what I'm doing:

I construct, full fuse a piece, anneal well, etc
I will then cold work the edges and the new top if I do a flip & fire so I have a smooth surface. I will use the pneumatic angle grinder to level out any deep or high spots and then I hand lap with 220 silicon carbide grit. I will usually sand blast one last time to make sure all surfaces are evenly covered.

I then slump and fire polish, here is my schedule:
300/1000/10
600/1235/10
AFAP/900/ hold depending on thickness
the rest of the schedule is depending on thickness again--I use the BE chart

Some pieces come out with an awesome matt finish and slumped nicely in the mold (plate molds or shallow platters, nothing deep).
Others come out with 97% awesome matt finish, but with spots of gloss here and there. Its quite annoying. I will re-sandblast and fire polish again (at a lower temp), it would work, but then a new piece will have hot spots at that lower temp, while others won't. How can I get a consistent look across firings?

Do I either lower the temp and hold the time more
keep the temp, shorten the time
or both?

For other reference:
the hot spots are not closer to the side elements
I've moved the mold around the kiln looking for a better spot--no joy.

Any help you can give, would be helpful.

(in the image attached, there was as 'hot spot' was to the upper left corner of the red window, everything was great. very frustrating. Also, not the best picture--iphone and all.

)

[Valerie Adams]

In my opinion, trying to fire polish and slump in the same firing are asking for trouble.

You'd be better off doing separate firings and slowing down on your slumping schedule. I use (what I believe to be Brock's schedule) of 250° to 1150° and hold for 30-45 minutes (or until I visually confirm the slump). I think your slumping temp is too high and perhaps that's contributing to what you're calling 'hot spots.'

Or maybe those are areas that were unevenly coldworked and/or blasted?

[JestersBaubles]

1235 really isn't a fire polish temp, so I don't see a slump & fire polish in the same schedule (and isn't the point of a fire polish to get it shiny? )

In my kiln, my experience is a good slump at 1225 (after a 30 minute hold at 1100). I would drop it 20 degrees and see what happens -- your kiln may be firing hot.

Dana W

[Nbolinger]

Thank you for the responses.

I guess I was using 'fire polish' in the wrong way. I've used it as 'cleaning up the cold working', not necessarily meaning a shiny finish. And I've heard well known instructors use it the same way and I was using their schedules, with small tweaks according to my kiln attitude (lol).

So, what temp and how long would folks suggest I use to make my surfaces matte finish? I don't mind breaking it up into two firings, I don't understand why I would have to separate them, but I will.
Would going down to 1150 and holding for longer help me achieve the same thing. I know its all about time and temp, I just haven't found the right balance (consistently) with making these matte finish pieces.

[Brad Walker]

If you want a matte finish, fire to around 1150 and hold for an hour. You can usually do this during the slump.

Firing higher makes it more likely you'll shine up. The variation in finish you're getting is because the coldworking isn't exactly the same everywhere; it's noticeable because of the higher temperature.

[The Hobbyist]

It seems like you're putting in a lot more effort than necessary.

I flip-n-fire nearly all my pieces. I compose the piece (strips and/or whatever) and fire it face down to full fuse. I then sand blast the side that was on the bottom (face), wash well and spray with SuperSpray. This is fired face up to full fuse or maybe 50 degrees less than the first fire. If the edges need cold work it can be done between these firings.

The result is always a beautifully shiny FLAT piece.

If I want a silky matte finish in whole or part I will sandblast with low pressure but as evenly as possible before slumping the piece. I slump at 1250 °F. If the edges were coldworked after the flip-n-fire they will shine up nicely if they were worked to about 400 - 600 grit (even 240 grit will regain the shine but may show some scratches.

Jim "The Hobbyist"

[Joyce Walters]

By the way, that is a good looking plate, and the shiny spots do not show in that photo!
Joyce from MT

[DonMcClennen]

When producing the satin finish you must be aware of the different colours and how they surface melt at diff. temps. Black will be the first to start to get shiney.. The solution has already been mentioned.... slump the sand blasted piece at 1200F max. or less to keep the "shine" away.

[Marian]

that's a great looking piece!