Pre-mix vs Surface mix

Dragonharper · #1 2011-03-27, 2:03pm

"Many colors of soft glass, mostly opaque colors, when worked in a premix flame, will reduce and become gray. A surface mix torch properly adjusted to a neutral flame will keep the soft glass colors true. If you plan on working only soft glass or both soft and hard glass you should consider a surface mix torch."
-Excerpt from Arrow Springs Web site.

My question is "Why?". Isn't a neutral flame a neutral flame?

shawnette · #2 2011-03-27, 3:33pm

I'm no expert but I have experience with both and have found that pre-mix torch flames are extremely harsh on soft glass. Even if you work your flame cool, your colors won't be as nice and vibrant as with a surface mix. (although you can still work soft glass with a premix.)

Dragonharper · #3 2011-03-28, 8:22am

Thanks Shawnette, Harsh makes more sense. I've got a surface mix, I just want to know "why?".

cheng076 · #4 2011-03-28, 10:23am

To get a neutral flame with a premix torch requires the flame to be more strident or harsh as if the pressures were too high. You'll sometimes notice a hissing sound and the cones will be sharp and pointed. It is possible to get a neutral flame chemistry with a surface mix torch while having a 'soft' flame as the pressures 'in the flame' are often less so the area of the flame in the working part above the candles can be and often are softer. Make sence??

Dragonharper · #5 2011-03-28, 12:31pm

I think that makes sense, let's see if I can rephrase that. In a premix torch the velocity of the flame and the combustion byproducts is (much) higher than a similar volume flame on a surface mix torch. It's the increase on velocity that can cause problems with colors in soft glass.

Correct?

cheng076 · #6 2011-03-29, 9:08am

Said much better than I. Thank you. My brain was pretty much in neutral all day yesterday. I think another factor in the premix torch is that the higher velocity combustion gasses tend to drag air, and thus oxygen, into the flame area making it harder to get a true neutral flame just by appearance.

Dragonharper · #7 2011-03-31, 11:51am

Thanks for all the help, everyone.

laserglass · #8 2011-04-05, 12:34pm

you can get a neutral flame on either a surface mix or a premix torch. and if you get a multi-candle tip for your premix torch, you would have a hard time telling the difference between them. the biggest difference is that with a premix flame, such as you get from a national 3a/b torch with a single hole tip, it is very easy to burn the colors because the sweet spot or hot zone of the flame is longer so you have to work further out on the flame. with a multi-candle flame, the sweet spot or hot zone is wider but shorter so that you can work in closer to the torch. if i was going to buy a torch i would get one that allows for both types of tips like the national, or a torch like the red max that has a premix torch on top.

Dragonharper · #9 2011-04-06, 2:11pm

That makes a lot of sense too. I knew there had to be valid and explainable reasons.

Cosmo · #10 2011-04-16, 9:05pm

I have worked a lot of soft glass on my CC. It works fine if you just turn the flame down.

Paul Stankard works on a CC as well, so that should tell you something...

nate-d · #11 2011-05-01, 8:15am

I have a flame chemistry question. One person told me that the more oxygen rich part of the flame is in the outside since most of the fuel has been combusted already. I was also told that on a premix it is the opposite, that the oxygen rich part of the flame is towards the torch. Can someone shed some light on this for me? I use a natty 3a and two oxycons.

Conrad Hoffman · #12 2011-05-02, 7:05am

I'm also curious about that and the overall differences between pre-mix and surface-mix flames. As far as I can tell, a neutral flame on a pre-mix will have clean cones and no yellow, but achieving the same thing on a surface-mix takes quite a lot of oxygen and has more of the aggressive hiss of too much oxygen. Is there any quick test that involves sticking something in the flame, a piece of metal perhaps, and determining the flame type by the appearance?

nate-d · #13 2011-05-02, 3:19pm

I've read that GA's Amazon Night is a good color to use for neutral. It will not change color in a reducing flame and is very reactive in reducing or oxidizing environment. I haven't tried it yet though.

Dragonharper · #14 2011-05-11, 11:44am

Quote:

Originally Posted by Conrad Hoffman

I'm also curious about that and the overall differences between pre-mix and surface-mix flames. As far as I can tell, a neutral flame on a pre-mix will have clean cones and no yellow, but achieving the same thing on a surface-mix takes quite a lot of oxygen and has more of the aggressive hiss of too much oxygen. Is there any quick test that involves sticking something in the flame, a piece of metal perhaps, and determining the flame type by the appearance?

Yes, a neutral flame on a surface mix may have some yellow in it. For me and my torch I find I have a neutral flame when the candles are crisp but still have a tinge of yellow at the tips. Remember the oxy is mixing after everything leaves the torch, working further back in the flame is a must for surface mix too.

Conrad Hoffman · #15 2011-05-13, 5:16pm

Roy- thanks! That was just the few words I needed to have more confidence in my understanding of things.

Cosmo · #16 2011-05-14, 6:30am

On my CC, my neutral flame doesn't have any yellow in it. How I tell is actually by the outer flame. If the outer flame is not very bright, it's more reducing. If it's brighter blue, it's more oxidizing. I actually have to get it pretty reducing to get any yellow in it.

Take this image for example:

That's a super-soft, super-reducing flame, and there is hardly any yellow in it.

Conrad Hoffman · #17 2011-05-14, 8:31am

Correct me if I'm wrong, but a CC is premix center and surface mix outer? Nice torch, BTW! I don't see any candles on the outer in your photo- is that just a trick of how it photographs, or do the gases mix really close to the surface? Also, are you running propane or something else?

Dragonharper · #18 2011-05-16, 5:40pm

This is why you need to spend quality time with your torch. When I take classes at Corning I take my torch, that way I get instruction not only in glass but the torch as well. And yes the CC center fire is premix, the outer is surface. It looks like he only has the center on in that photo. My first class at CMoG was on a CC.