# How to clean a crucible or melting dish.



## catfish

All:

I have a number of used melting dishes, and crucibles, that I would like very much to clean up and reuse them. They are in good physical shape but have a heavy accumulation of slag (Borax) on them. I think I read somewhere on the forum a post by Harold how to clean a crucible or melting dish. I can’t find the post. If some one could either direct me to it or just maybe tell me how to accomplish this I would appreciate it very much. The crucibles get expensive, especially the good ones.


Catfish


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## lazersteve

Catfish,

Soda ash is the flux needed to clean the dish. It's hard on the dish, but it works great. I've done several using Harold's technique and I'm satisfied with the results. You'll end up with a clean dish with a rough texture on the bottom. The spent borax stuck to your dishes will yield a small amount of tiny BB's as an added bonus. If you had silver in the dish be very careful because the fumes can be deadly with repeated exposure. GSP recommends a full breather whenever silver is heated with a torch. The process takes some time to get the dish clean but the results are obvious. I had to repeatedly add soda ash to the dish in small amounts to get these results:







The top dish was coated with about a 1/4 deep layer of borax as seen in the lower dish in the photo when I started. The top dish could use a little more cleaning, but my torch ran out of gas near the end.

Steve


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## catfish

Thanks Steve, I will try this. The silver is not a problem. I did some silver a long time ago and quickly decided it was not worth it. (too much sugar for a dime).

All my crucibles and melting dishes only have had gold ran through them. Steve I will be using my 2300 degree electric oven to do this. Would this be any different than using a torch. I do have a mapp gas torch that I used to use, if it is required.

Catfish


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## lazersteve

Catfish said:


> I will be using my 2300 degree electric oven to do this. Would this be any different than using a torch.



This is where you need Harold, I haven't got an oven *yet*. I imagine it should work, but I do recall Harold mentioning the torch specificly. I'm not sure MAPP gas will do without O2 or a special tip to focus the heat. Hopefully some of the other guys will jump in and help me out on this one. :wink: 


Steve


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## catfish

Steve:

I should have made my self more clearer on the mapp torch. It is a mapp and oxygen torch. I just don't use the torch any more since I got my oven.


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## Harold_V

Using an oven may not be in your best interest. Fumes from the fluxes may destroy the elements. A gas fired assay type furnace is a different matter, and would work very well aside from the fact that it's difficult to stir the dish as it is being cleaned. That's an important part of successful cleaning. 

I'll give you instructions on how I used to clean dishes with a Hoke torch. I used a Hoke torch and natural gas, but a rosebud with Oxy/acet or other means will work as well. A small tip is not in your best interest----the larger, the better. 

Heat your melting dish until it's red, then sprinkle soda ash on the hot surface. You do that so the soda ash, which tends to blow around easily, melts and fuses to the dish rapidly. Continue to heat and add soda ash, stirring the sticky mess with a carbon or quartz rod. The soda ash dissolves the contaminated borax, reducing the oxides of metals that are absorbed in the dirty borax. That's the reason for the prills that will magically appear in the flux as you heat and stir. The prills will contain values, so save them for future refining. 

When you have the entire mess liquid, add some clean borax, heat it until it is quite fluid, then pour the flux into a small cone mold. Do not attempt to pour unless the flux is very fluid. The values will be found in the tip of the flux from the cone mold, and are very easy to separate from the resulting cone. 

Repeat the above until you're satisfied that the dish is clean enough for your purpose. You'll find a second cleaning is very effective, and is usually adequate for your intended purpose. You can, by repeating the process, clean a dish to new condition, but soda ash dissolves the dish as it cleans, so clean only as much as you feel is necessary to prolong the useful life of the dish. I used to be able to clean a dish maybe three times before it was too thin to use. Spent dishes were crushed with a mortar and pestle and placed in my waste material for evential firing in a furnace to recover values. With the proper flux recipe, the crushed dishes are totally dissolved in the furnace. 

If you have the luxury of having some fluorspar in your possession, it will do a wonderful job of thinning dirty flux----makes it almost water-like, but I'm of the opinion that it is not great for your lungs, so if you choose to use it, be certain you have adequate ventilation, or, better yet, you do the work in a fume hood. Like soda ash, fluorspar is correspondingly hard on the dish, dissolves it at an even faster rate than soda ash. 

I can't stress enough the importance of having a hood for refining-----it is as essential as anything you'll ever accumulate---but you won't know that until you've used one. I can't imagine refining without one, nor without a hot plate. Working without them is too limiting. 

Harold


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## aflacglobal

Why not coat the dish prior to first use with a sealer. This should protect the dish from the harsh chemicals plus the heat. Use a non wetting sealer and the gold should not be getting asorbed Or the slag sticking to the dish Plus if you use a reflective type it would help the gold retain the heat longer for pouring or what have you. Plus it would reflect the heat to the material more easily. ????

Just a thought.

Ralph

Two-part, 99% alumina-filled Pyro-Paint(TM) 634-AL bonds, coats, and seals refractory fiber boards and other shapes. Product is resistant to wetting by most of all molten metals, optimizing erosion resistance of underlying structure. Mixed in 3:1 ratio, material has solids content of 76.8% by weight and 66.0% by volume, and viscosity range of 5,000-6,000 cps. Water-based formulation dries at room temperature in several hours and cures fully within 2 hours at 200[degrees]F.

Pyro-Paint(TM) 634-AL, a new high temperature, alumina filled refractory coating developed by Aremco Products, Inc., is now used to bond, coat and seal refractory fiber boards and other shapes, providing exceptional resistance to molten metals and open flames to 3200[degrees]F (1760[degrees]C).

Pyro-Paint(TM) 634-AL is a two-part, 99% alumina filled, ceramic coating system used to bond, coat and seal refractory fiber boards and other shapes used in molten metal fluid handling components and structures in foundry applications to 3200[degrees]F (1760[degrees]C). The advanced material helps to prevent wear and oxidation of refractory components and increases heat reflectivity to improve thermal efficiency and enable temperatures to ramp up more rapidly.

Pyro-Paint(TM) 634-AL is rated for temperatures up to 3200[degrees]F (1760[degrees]C) and exhibits exceptional adhesion to refractory fiberboards as well as alumina, graphite and ceramic cloth. Pyro-Paint(TM) is resistant to wetting by most molten metals, increasing the durability, erosion resistance and reliability of the underlying structure.

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Pyro-Paint[c] 634-AL is a simple to use, environmentally friendly, water-based formulation containing no volatile organic compounds. It dries at room temperature in several hours and cures fully within 2 hours at 200[degrees]F.

Pyro-Paint(TM) 634-AL is part of a line of refractory coatings based on ceramic fillers including alumina-silica fibers, boron nitride, carbon, graphite, yttrium oxide, and zirconium oxide.


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## Harold_V

Waste of time and material.

The same problems accompany any surface, be it 94% alumina or fused quartz. 

Clean your melting dishes, or discard them. 

Harold


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## catfish

Thanks Harold.

I dont have a oxygen/acetelene rig, but I do have access to one from my buddy. I also do not have a fume, but plan on doing the cleaning outside with breathing apparatus and goggles, with small fan to move the fumes and gases.

Thanks again to you, Steve and Silverpro for all the help that you continiously share with us Neophytes.

Thanks,

Catfish


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## goldsilverpro

Harold,

Time and time again, you have expressed your reasons for not using plywood for hoods. I disagree but, we've already gone over that. As I understand it, you are recommending using a material that is both heat resistant and chemical resistant. You can't use fiberglass, plastic, or metal (unless it's titanium). 

Your hood was made from the perfect material, Transite. It's about the most chemically inert stuff there is. Unfortunately, it's asbestos and you can no longer buy it, as far as I know. It was used around free standing wood stoves, so I imagine there's still some of it laying around.

What material would you use today?


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## lazersteve

Here's a link to a foundry supply catlogue with many types of materials. I noticed they sell a non-asbestos Transite HT. Do you suppose something here could be used safely for a fume hood ?


Foundry Supplies

Steve


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## goldsilverpro

It doesn't give the chemical resistance. Aqua regia, nitric acid alone, and sodium hydroxide are some of the most powerful chemicals available.

I see no reason to melt in the hood, anyway. Make it a chemical hood with enough heat resistance to use a hotplate.

Flame resistant type fiberglass. Most plastics.


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## Harold_V

lazersteve said:


> Here's a link to a foundry supply catlogue with many types of materials. I noticed they sell a non-asbestos Transite HT. Do you suppose something here could be used safely for a fume hood ?
> 
> 
> Foundry Supplies
> 
> Steve



I'm of the opinion that you could get by with either Transite HT or Minerit. 

In spite of the proclamation by GSP that I used the best available material when I used Transite, I'd like to comment that while it reasonably resists acids, it is not the best of all worlds. It slowly degrades because of exposure to fumes, just as you would expect most anything to degrade unless it is totally inert. Transite is not. It is bonded with cement, which gradually dissolves. All you can hope for is a substance that resists rapid decomposition (and doesn't burn)----and that's what I got when I used the asbestos board. 

Truth be known, I used three different types of board, it was not all Transite. One type was known as Colorlith, which was very similar to Transite, and a third type, name unknown, but is brown in color (looks much like Delrin AF, but is filled with asbestos fibers), is bonded with a plastic of sorts. It resists heat, but degrades with direct heating, such as with a torch. Of the three, Transite was the least stable. The top cover of my fume hood was made of Transite, and was slowly degrading, shedding wet asbestos fibers when I'd clean the hood. That was one of the reasons I cleaned it only infrequently----I wanted to prolong it's useful life. The hood was over ten years old and still functional when it was sold with the business. 

Regardless of you material choice, you'll find it won't have much tensile strength, so long thread engagement is required to resist pulling the threads used in assembling the hood. You can also explore the idea of installing inserts. If construction is proper, the insert will be isolated from exposure, so some made of stainless would likely serve very well. I used all stainless fasteners, primarily 3/8"-16 hex head cap screws, which held up quite well, although those that are exposed to acid fumes eventually get ugly. Again, nothing will survive indefinitely. What you hope for is a few years of useful life, knowing that the fume hood will require eventual replacement. 

Harold


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## Harold_V

goldsilverpro said:


> I see no reason to melt in the hood, anyway. Make it a chemical hood with enough heat resistance to use a hotplate.



I see many benefits from having a fume hood that is fire proof, and absolutely none to having one that is not. Frankly, to suggest one be built of wood is reckless and shows a serious disregard for the safety of others. 

The reasons are wide and varied. Having worked with a fire proof hood for more than fifteen years, I can't imagine not having the ability. 

Here's some things to consider:

A fume hood is not a luxury, it is a necessity if a person is serious about refining. Otherwise you're always at the mercy of weather and other issues that may be better in control of when you can operate. Further, you can operate at greater speed, so your time is much better spent. That may not be an issue if one is simply enjoying playing with a few items, but if the refining operation grows, spending hours on a a ten minute operation is no longer an option. 

Think of handling acids----without the need to wear a respirator. You can dispense all acids with no worries. A good and proper hood will create a negative atmosphere, always pulling fresh air past you. Your wife will love it for peeling onions!  

Fact is, it's not a bad idea to do all torch work in a fume hood so you are not exposed to gasses from melting. There are toxic elements one can encounter on a regular basis (think silver solder with cadmium, for an example). It's not solely about melting-----there are many reasons to work in a hood. 

A hood that is not fireproof is an open invitation to an eventual fire, particularly if it is of wooden construction and has been adequately exposed to nitric acid. A hood that is fire proof is the best insurance you can buy to protect you and your family. 

Assuming an individual decides to serve the jewelry industry, aside from refining items large enough to be inquarted, everything must be incinerated prior to any refining operations. Closing the door on jewelry waste is simply a stupid idea. The effort to extract an ounce of gold from such waste is, perhaps, only 1/100th the amount of effort expended in stripping pins and other low grade wastes. 

Work towards a fume hood with this in mind: 

Building a hood that is not fire proof will cause a person to eventually build a second hood of sorts, for incineration. As I've stated previously, incineration is very much a part of refining and should be practiced on a regular basis. The outcome is far better, often with the benefit of solutions that will filter easily, whereas they were previously difficult. By making a hood fireproof, a second hood is not required, so there is a space savings for those that are working in limited quarters. 

Harold


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## lazersteve

What do you think of using a protective laminate such as this one to line the inside of a fume hood and work bench top?

[img::]http://www.goldrecovery.us/images/adobe.gif[/img]
Chemarmor

Check out the last page for the table of chemical resistances.

I was thinking that the main substructure could be constructed from 1/4" cement board available at any home improvement store. The stuff is frequently used for backing wall tile and is known as Wonderboard. Here's some information on it's installation:

Installing Wonderboard

and the MSDS for it:

[img::]http://www.goldrecovery.us/images/adobe.gif[/img]
Wonderboard MSDS

I'm thinking about framing out the shape of the fume hood and cabinet with 2x4's then coating the structure with Wonderboard as the substructure and surfacing the whole unit with Chemarmor. The design doesn't lend itself very well to disassembly is one flaw I see.

Perhaps with a little thought I can design a fume hood that can be readily disassembled for periodic interior cleaning and maintenance.

Steve


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## aflacglobal

Great post Harold.

How about Steves idea, But coat the wonder board with chemical and heat resistant epoxy ?

I would not trust this particular product. http://www.epoxysystems.com/634.htm
http://www.epoxysystems.com/chemical_resistant.htm

I can get you some of the same thing if you have a sherwin-williams or an ICI ( devoe, duluxe, Glidden ) paint store near you.


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## Harold_V

Steve,

The chemical resistance of the Nevamar (sp?) is quite impressive, and would likely serve you quite well for a chemical hood, although I believe they recommend it not be applied to a cement type substrate. 

The only other issue I'd have is that it won't resist the type of heat required to incinerate----so you'd likely have to consider a second unit for that purpose. It also may not be an issue if you have no plans of running wastes from a jeweler's bench. The incineration process for buffing lint and furnace filters used in their polishing machines is quite a long one, so what ever you'd use would be required to resist excessive heat for some time. 

My second hood was entirely made of asbestos, but it was open bottomed, and sat on a piece of 3/8" Transite. Under the Transite was a sheet of 3/4" plywood. I used a three burner hot plate, fired by natural gas. The top of the hot plate was 6" from the Transite (Susan is using an identical hot plate for cooking while we're building our house).. I regularly noticed the slight smell of burning wood when I'd incinerate. Only when we moved to the castle did I discover the source. Directly under the hot plate burners there were circles that had charred almost completely through the plywood. That provided the input I need for construction of my final hood and rest. I used a base of 1¼" asbestos board in place of the 3/8" Transite, and that was supported by additional pieces of the same material, set on edge, 6" tall, so the dead air space under the 1¼" board was great enough to eliminate any chance of combustion. That system worked perfectly. I had two drawers under the hood (below the dead air space), in which I kept all the little tools and aids that I used in melting and handling hot items, so it worked out very well. 

Expect to go through a couple of renditions of what ever you design, each one better than the last----for each time you're likely to miss something. It was my third hood that turned out quite well, but even then I missed the size of the fresh air supply. They were too small, so the hood was quite turbulent unless it was operated without the front cover. 

Work towards a hood that doesn't have to be dismantled to clean it. A couple of removable covers may be all that need be required. If you make the cleaning operation too difficult, you won't do it. I dreaded doing my hood, and it wasn't all that hard to do. It was one hell of a messy job, for I always ended up with gold stains. I got over my ill feelings when I'd recover the values!  

Here's a tip for you to consider. By now you know that gold chloride is photo sensitive, and doesn't wash off. Silver nitrate is, as well. I routinely worked with a few layers of newspaper spread around my counter area. About every second or third day, I'd remove the top one and incinerate it, to recover any values that may have been splashed, plus to keep the paper looking as good as old newspaper can be expected to look. If, by chance, you happen to have a spill, the paper will absorb the spill, making recovery of values a lot easier. It is for that reason that I kept a few layers of papers instead of just a sheet. 

Harold


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## lazersteve

Harold,

Thanks for your advice. I've cleared an area where my bench will go, next I'll begin with a few sets of plans.

If I use a single squirrel cage fan for air flow how large will the intake supply need to be?

Steve


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## Harold_V

Steve,

My first hood used a squirrel cage blower, which was a mistake. Bear in mind, I incinerated with mine, as well as evaporating solutions, so the problems I experienced were not the same that you may experience. If you come close to the methods that are suggested in Hoke's book, could be you'll have the same problems, though. 

A lot of gunk gets sucked up with the fumes. I found it was very eager to accumulate in the squirrel cage, slowly reducing its ability to move air. That is what prompted me to build a hood with a filter. As it got loaded, it would unload bits and pieces on a random basis, which caused the fan to vibrate like crazy. It was very noisy and annoying. Remember, the gunk has value----it should be recovered and refined. Cleaning the squirrel cage is a real chore, and it is virtually impossible to get it to run without vibration. 

They make a blower assembly that is considered clog free. Instead of a squirrel cage, it uses a paddle wheel, which is much easier to clean, and does an equal job. You should be able to find examples in the Grainger catalog. I used one that was 1/4 hp, and moved, as I recall, less than 1,000 cfm. It's not a bad idea to be able to move a fair amount of air, so if you're handling acids in the opening of the hood, none of the fumes escape. 

If you design your hood around a two or three burner hot plate, and provide an opening that is near the size of the plate, you'll have all the opening you need for good ventilation. Be certain that you have adequate head room, so you can stir a tall beaker with a glass rod when necessary. 

If, by chance, I didn't understand your question, and what you're asking is how large should the connection between the blower and hood be, the intake of the blower will dictate the size. Blowers usually come with a coupler of sorts, to which you can attach tubing or vent pipe. Duplicate that size and you'll be fine. 

By the way, I fabricated my duct work from Transite water pipe. It was, at that time, readily available from plumbing supply houses. The place I got mine actually gave me the pipe because it was damaged on one end. No matter, I had to cut it up, anyway. The rectangular transition to round pipe was made of pieces of Transite plate, epoxy assembled. It was still holding up quite well after ten years of operation. 

I used an acid resisting epoxy paint to cover the paddle wheel and the entire blower housing (all of which was well sand blasted to white metal before painting, to insure adhesion. That, too, seemed to work out nicely. I had to replace the motor one time in over ten years, because it got noisy. Single phase motors tend to be much louder in operation than three phase. They "hunt" constantly. 

Harold


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## lazersteve

Harold, 

I'll take your advice on the paddle fan. I read in Hokes book that the fan should be belt driven with the motor outside of the hood, is this how you constructed yours?

Steve


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## Harold_V

Steve,
I'll post the same picture of my hood I've posted before, for ready reference. It shows the blower assembly, and the pipe that hooks to the hood, to the rear of the blower. I had provided for a future hood, which accounts for the light brown (held in place with duct tape) cover you see at the end of the hookup pipe. It was never built, but only because I never took the time. I really needed two hoods because I had to stop refining when I incinerated. 

It isn't necessary to have a belt, and I would suggest that you do not. Keep it as clean and simple as possible, although if you have something specific in mind and a belt is required, I see no problem with using one. The nice thing about the blower in the picture (from Grainger) is that there is nothing on the outside that creates a hazard, very unlike a belt. 

I made one serious misque when I wired the lab. I had a 1 horse blower in the turret, which was my melting room for the large furnace and my small crucible furnace. I wired it to the same ciruit that shared the hood, and it would knock a breaker on rare occasion. That's the reason for the extension cord you see in the picture. The quad receptacle, for all practical purposes, was worthless because of the 1 horse blower, and I was way too busy to screw around with rewiring. Talk about bush-league!  

Harold


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## Palladium

8)


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## aflacglobal

8)


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## 71smithg

Howdy,
I've been reading about the hoods. Is it possible to buy a premade hood somewhere or do you have to build them. We probably could build it ourselves if it comes to it. We can work stainless steel. Is that a good hood to use?

Thanks


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## Harold_V

71smithg said:


> Howdy,
> I've been reading about the hoods. Is it possible to buy a premade hood somewhere or do you have to build them. We probably could build it ourselves if it comes to it. We can work stainless steel. Is that a good hood to use?
> 
> Thanks


Hoods are available commercially, although they're not cheap. You may have good fortune and find one used, however. 

Stainless is not a good choice, although it would be better than mild steel. The acids we use in refining will dissolve most metals. There's nothing quite like asbestos, but as we all know, it is no longer an option. 

Harold


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## LeftyTheBandit

May I request or offer my service to re post this fume hood information on a thread other than "How to clean a crucible..."

The data here is valuable and I fear anyone interested in fume hood creation may miss this gem.

Regards
Richard


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## Harold_V

LeftyTheBandit said:


> May I request or offer my service to re post this fume hood information on a thread other than "How to clean a crucible..."
> 
> The data here is valuable and I fear anyone interested in fume hood creation may miss this gem.
> 
> Regards
> Richard


Because the posts cover both topics, what might be a good idea is for you to post a link to this thread, including comments that it offers more information on building fume hoods. 

Good idea, by the way.

Harold


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## glibbudd

Found that if you simply heat your silica melting dish that is full of dirty borax with a low heat torch(like with a tank that is running out of fuel...) , will almost turn it black and start to burn, then let it cool and tap it with your graphite stir rod and it will come off leaving you with basically a new crucible....


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## heliman4141

If i may add,
Ive had super success using dilute nitric & distilled water in a 20% dilution heated on a hotplate to very quickly clean crucibles & not remove crucible material. They end up looking new, i just allow them to dry a few days to a week then re-season with borax.


My .02 Dave


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