Reprinted from the ACJ - April, 1995
Have you ever been to a coffeehouse? The deep rich smell of java. A couple of longhairs play folk songs while aging, anemic hippies dressed in black read from Kerouac or Dante novels. Each of them has a cup in front of them the size of your bathtub filled with espresso and topped with the brown-gray, dry foam of boiled milk and cinnamon. The cappuc-cino machine cranks out cup after cup behind the counter.
Now, I have to admit this scenario is a little dated. The last time I was in a coffee house, everyone in there was younger than me, had shorter hair, was reading John Grisham novels and drinking decaf. But if I didn't set up this Maynard G. Krebs style stereotype, I couldn't smoothly lead into my article. So here goes.
Slap a handle on your solder pot as it reaches the working temperature. The brown-gray, dry foam of solder dross floating on the surface. Hire an aging hippie to stir the mess and viola, a ridiculous analogy ready to serve.
It is not the coffee we are going to talk about, but the boiled milk floating on top, solder dross. The solder dross that floats to the top of a molten solder bath is made up of many compounds. It is not something we think of too often, except to scrape it out of the way so we can get at the pure solder. Often it is skimmed off of the surface and collected for disposal or reclamation. A regulatory note: If you send out your dross for reclamation, and you should since it is loaded with recoverable metal, make sure the firm taking it is licensed, certified and doing what they are supposed to with the material.
The hazards associated with dross material are minimal. Not because of low toxicity, but because of the form the dross is in and the way it is normally handled. The components of the dross utilize the inhalation and ingestion routes of entry, but are not absorbed through the skin. So only if the dross is scattered as a dust into the air that we breath, or is eaten, do the hazards come into play.
Thus, if we handle the dross gently and don't skim and collect in such a way as to send up a cloud of dust, we minimize the danger. To clean up any dross dust that may be covering the floor, vacuum it up with a HEPA filtered vacuum. Never sweep it up unless you wet it first, or you will be working in a hazardous cloud. Wearing a HEPA dust mask while skimming or cleaning is also recommended.
The bulk of the dross is made up of the metals in the alloy, usually tin and lead. They are trapped among oxides of the metals present. The major metal oxides — SnO and PbO — are common, as well as the oxides of the contaminants in the alloy such as iron (FeO), aluminum (Al2O3), calcium (CaO), manganese (MnO), magnesium (MgO), and zinc (ZnO).
A quick procedural note: The amount of oxide formed is affected by stirring. A solder pot needs to be stirred to avoid separation, but avoid pulling air down into the molten solder. The more oxygen that contacts the metal, the more oxide and therefore dross will be formed. Sulfides are also present in small amounts due to the sulfur content of the lead used for alloying. PbS, CaS, MnS, FeS and ZnS are sometimes present.
Other than the pure metal that is trapped in the dross, oxides of these major metals are most prominent. PbO is considered toxic due mostly to the lead content, the oxide portion of the molecule adding little to the hazard. Of the lesser dross components, iron oxide (FeO) is often the most abundant. It is responsible for the reddish tint dross sometimes has.
Inhalation of FeO dust can cause acute metal fume fever if enough is inhaled at one time. Repeated lower exposures are suspected of causing numerous chronic ailments. Bronchitis, lung and liver cancer are just a few. It's also damaging to reticuloendothelial tissue. Don `t bother looking that up in last month's definitions. This is the first time I've heard the word.
The same sort of maladies can be expected of the other oxides in the dross. In addition, calcium oxide (CaO) is a skin, eye and lung irritant, due to its corrosive nature. Aluminum oxide (Al2O3) dust has been associated with a lung ailment known as Shaver's Disease. Magnesium oxide (MgO) is an experimental carcinogen in mice. Zinc oxide (ZnO) is listed as a skin and eye irritant, but I take this with a grain of salt … or cream and sugar if you prefer. ZnO is the white stuff that all the cool lifeguards at the beach have on their noses. The things some folks will do for fashion.
The sulfides that are present in the dross do not seem to raise as many health concerns as the oxides, although calcium oxide (CaO) is listed as a poison via inhalation. The other metal sulfides are not considered "toxic" in dust or solid form. I still don't recommend taking a big sniff above the solder pot as you would a cup of fresh coffee.
So it would seem the trick to avoiding the hazard of solder dross is to keep the dust to a minimum. Also keep your hands clean so you don't swallow any dust during lunch or a cigarette break. In case any of you folks who only use a torch in your work are still reading, remember we are talking of oxides of solder. So even though you can't skim the dross off of your header or tube repair, oxides are present along with the metal fumes. Make sure the work area is well ventilated and/or you are using the proper fume and HEPA respirator.
As is obvious from my opening to this article, I wrote this during breakfast. Now that I have drained the cappuccino from my cup, it is time to fill it with bathwater, hop in and clean myself up for the workday. Anybody know any good folk songs?
The above article was written by David M. Brown, Chief Engineer of Johnson Manufacturing Company, Inc. and is published by JOHNSON with the expressed approval of the National Automotive Radiator Service Association and the Automotive Cooling Journal. Other reproduction or distribution of this information is forbidden without the written consent of JOHNSON and NARSA/ACJ. All rights reserved.
JOHNSON MANUFACTURING COMPANY
114 Lost Grove Road / PO Box 96 / Princeton, Iowa 52768-0096
Phone 563-289-5123 or Fax 563-289-3825