# Floating lead



## Cody Reeder (Jul 7, 2011)

I don’t know quite where to put this question or if it has been asked before but after doing a hundred or so fire assays I can’t help but wonder what makes the little lead droplets float on top of the assay charge, the lead obviously has a greater density than the motion glass. I have always written it off as the result of high surface tension and incompatibility of the liquids but I have never found a definite explanation. I have seen this phenomenon with gold and silver as well.

It’s probably something obvious that I should already know, but has any one else wondered about this? 

And if you know the answer or detailed explanation I would love to here it.

Cody Don Reeder


----------



## goldsilverpro (Jul 7, 2011)

After running 1000s of fire assays, I don't ever recall that happening.


----------



## Cody Reeder (Jul 8, 2011)

I am shure you have much more experience than me but I observe it almost every time. could I be doing something wrong?

perhaps I should clarify, the lead droplets come to the surface (float) combine with others untill it gets big enugh and then sinks. it suprises me how big the droplets can get befor finaly sinking. ( around a milimeter accross sometimes more)


----------



## goldsilverpro (Jul 8, 2011)

Can you outline your procedure - flux makeup, temperature, types of materials you're assaying, etc.? Also, how many minutes into the fusion do you observe this?

Since I rarely pull the crucibles for observation before the fusion is completed, typically about 45 minutes, the same thing may have happened with me, but I never noticed it. Also, I rarely ran ore. Most samples were electronic components, bullion, pulps, solutions, etc.


----------



## 4metals (Jul 8, 2011)

As Chris has asked there are many variables in your question and without specifics there is not much that can be said which is not pure speculation. 

There is a question I would like to add however. What type of furnace are you using and how are you able to view the surface of the fusions? I know that fushions, unlike cupellations, usually require removal from the muffle furnace to see into the crucible, and keeping the door open to long to do this usually results in problems with excessive temperature drop. 

I suspect, although without fluxing formulations and specifics about your sample it may be moot, you are producing a "matte" which is due to the inability of your flux to neutralize the effect of the sulfur in the ore. Mattes are almost completely insoluble in slag but they are soluble in molten alkaline salts. My guess is it is a fluxing issue solved by pre roasting your sample to drive off the sulfur by oxidation.


----------



## goldsilverpro (Jul 8, 2011)

Good possibility, 4metals. I might mention that the matte will appear as a separate material between the lead slug and the slag after pouring and cooling. Bugbee says that the matte can often be eliminated by decreasing the silica and increasing the soda ash and litharge.

Are there any lead BBs hanging up in the crucible after pouring? This can be caused by too little time in the furnace or too low a temperature.


----------



## Cody Reeder (Jul 9, 2011)

I am using a little jeweler’s furnace that gets to just over 2000 deg and it has a small vent at the top that I can look into.
I also have a coal fired furnace for larger samples that I can keep an eye on the assay as well.

I am doing mostly quick and dirty assays to see if I have precious metals in a suspected ore and to get an idea on how much I have.

My flux varies but usually consists of borax, sodium carbonate, litharge (red), silica sand (fine), and charcoal dust. I don’t always use silica if my sample contains it.

The lead droplets appear floating on top as soon as the mixture is molten and remain until they have all colleted together and sank, taking as long as 10-15 min or more and long after the bubbling stops.

I have also noticed that they sink much faster if the crucible is disturbed (say if it is taken out of the furnace)

I have also seen this phenomenon with gold that is fluxed only with borax.



goldsilverpro said:


> Good possibility, 4metals. I might mention that the matte will appear as a separate material between the lead slug and the slag after pouring and cooling. Bugbee says that the matte can often be eliminated by decreasing the silica and increasing the soda ash and litharge.
> 
> Are there any lead BBs hanging up in the crucible after pouring? This can be caused by too little time in the furnace or too low a temperature.



I have seen bolth of those things but I figured out how to deal with them some time ago


----------



## goldsilverpro (Jul 9, 2011)

A problem I've had with those small jeweler's burn-out furnaces is that the refractory is usually so thin that it cools rapidly. They work great for burning out investment but not that well for fire assaying, in my opinion. So, when you open the door and load the crucibles, it can often take as long as 30-60 minutes (depending on how many crucibles you load) to get the furnace back up to fusion temperature - this is also due to the low power of these small furnaces and the great mass of the crucibles and their contents. I've seen the temperature drop to 800F in just a few minutes with the door open. Then, after getting the temperature back up, it takes another 45 minutes for the fusion. Maybe the slow heat up time of the charge produces segregated melting of the flux ingredients. 

I'm sure you know this, but the idea of fusion is to mix the flux well enough so that, ideally, you have particles of reducing agent (sugar or flour, usually) next to some of the particles of litharge. At some elevated temperature, carbon forms from the sugar or flour and this reduces the litharge particles and produces jillions of tiny lead droplets. These heavy lead droplets "rain" down through the molten charge, collect the PMs, and combine at the bottom. If it takes forever to melt the charge, the lead could form before the flux is very fluid and I can see how this process could be somehow interrupted and some of the lead could collect at the top, rather than at the bottom. Maybe, the lead you see is very thin and surface tension keeps it floating. All this, of course, is just a guess, but I feel I may be on the right track.

The last furnace I had was a #16 Vcella with 4" - 5" thick insulating brick. It took about 2 hours, or more, of preheating, to satisfactorily saturate the brick with the heat. Once it was hot, though, it stayed hot. After loading 6 or 8 crucibles, the air temperature in the furnace might drop to about 1200F. However, due to the still hot brick, the furnace was back up to 1850F-1900F in just a few minutes.

BTW, even if they are excellent furnaces, I would never buy another Vcella. The door swings open to the side and this makes the furnace way too hot to work with comfortably without a lot of hot sweaty safety clothing. The 2 big Cress furnaces I had before featured a counter balanced door that raised upward. Therefore, the extremely hot mass of brick on the inside of the door was not in my face or facing my arm, as with the Vcella, and it was much, much more comfortable to work around. Also, with the Cress, you didn't have to drill a bunch of holes in the door and top in order to get a somewhat even oxygen supply when running a lot of cupellations. With the Cress, I drilled one hole in the back and simply raised the door a crack to let in a little air, evenly across the width of the furnace. My next furnace will definitely be a Cress or something similar The excellent old standard, work-horse, gas fired Johnson 142 furnace, also has a door that raises - I used one of those for about 4 years. These Johnson Gas Appliance furnaces only come with a hearthplate and, therefore, you have to buy a separate muffle for it for cupellation - about $1100. I see these furnaces on occasion, used, pretty cheap.
http://cgi.ebay.com/JOHNSON-GAS-APPLIANCE-FORGE-FURNACE-142L-W-CONSOLE-/140568851609?pt=LH_DefaultDomain_0&hash=item20ba8e7499


----------



## 4metals (Jul 9, 2011)

Cress furnaces are pricey, but they are workhorses. I feel the Vncella's are a good value for the money, but the door swing is an issue. The bigger the furnace, the more of an issue it is. 

One thing to be careful for on small jewelers furnaces is the uniformity of temperature, or should I say lack of it. They are difficult to keep a row of cupels or a pair of crucibles at the same temperature.


----------



## Cody Reeder (Jul 9, 2011)

I would like to have a better furnace, even with my modifications my little furnace is only big enugh for one small crucible or about 3 cupells at a time. it has about 1 1/2 inch of refactory so it stays hot for quite a while when left alown but does take some time to get back to temp when somethin is added. however I can maintain a precise temp for a long time.

I dont think that that is what is cawsing the lead to float though. my homemaide coal furnace can get hot enugh to melt steel (rod placed into coals) in about ten min, yet I get the same thing.

I cant be the only one that has seen this. its proably somthing uninteresting and I am blowing it out of porportion.

I wonder if it could be that the top of my assays are always colder than the bottom, viewing hole for my jelers furnace and the open air for my coal furnace...


----------



## Oz (Jul 9, 2011)

Cress furnaces; http://www.cressmfg.com/
Johnson furnaces; http://www.johnsongas.com/industrial/industrial-furnaces.asp


----------



## Cody Reeder (Jul 9, 2011)

Oz said:


> Cress furnaces; http://www.cressmfg.com/
> Johnson furnaces; http://www.johnsongas.com/industrial/industrial-furnaces.asp




fun to read through  wish I had some more money. I think for now I will stick to trying to make my own furnace. you think that heating elements out of hot plates encased in the walls of a refractory box would do the job? or should I try to find some nicrome wire?

its only a gient light bulb right :lol:


----------



## goldsilverpro (Jul 10, 2011)

Prices from lmine.
http://www.lmine.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=LMS&Category_Code=cress_assay_furnaces
http://www.lmine.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=LMS&Category_Code=vcella_assay_furnace
http://www.lmine.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=LMS&Category_Code=johnson_gas

Here was a great little furnace, the Bico (or, Bico-Braun) #40 - see photo - I think this is the back of the furnace. The crucibles went on the sides and there was a muffle in the center for cupellation. Cheap and very functional. They must have made them for 100 years. I once had plans for making one like it from firebrick. I always though it would be easy to make a muffle from a big melting crucible, laid on it's side, with a floor of castable refractory.
http://www.fireassays.com/forum/viewtopic.php?f=2&t=56


----------

