I can't explain why one solder pot would be lower in tin than another, 
     but I can explain why the tin content would drop: dross is mostly 
     solder mixed with a little tin oxide, not the combined oxides of tin 
     and lead.  The more dross that forms, the more the tin concentration 
     drops.  Since the formation of dross also removes about a hundred 
     times as much solder (as solder) as it does tin (as oxide), and since 
     solder is always added to replace that lost to dross, the rate of loss 
     of tin is slow.
     
     In my opinion it's not worth getting a solder pot analyzed month after 
     month.  The most practical way to tell if the contamination level is 
     too high is just to look at a quickly-frozen solder connection.  With 
     pure eutectic solder, such a connection will be "smooth and shiny", as 
     the specs used to demand. The first effect of contaminants is to make 
     the connection look gritty. (Some use the term "grainy", but this term 
     applied to surface appearance has three different meanings and so is 
     ambiguous). The second effect (higher concentration) is to strengthen 
     the connection somewhat (the particles reinforce the solder, making it 
     more resistant to creep).  The last effect (higher concentration yet) 
     is to restrict the flow of solder (because the contaminants are now 
     present as solid particles and so make it sluggish).  Concentrations 
     this high almost never occur in practice.  It is fair to say that of 
     all the possible causes of solder defects (defect meaning a condition 
     of a connection that renders it unreliable), solder composition is at 
     the bottom of the list.
     
     Technet has been carrying a discussion on how to remove the most 
     common contaminant, copper, virtually for free, from a wave solder 
     pot, just by letting the solder stand undisturbed at a temperature 
     just above its melting temperature, where the solubility of copper in 
     molten solder is low.
     
     The visual effect of solder in a solder pot being low in tin (compared 
     to the eutectic composition) is to increase, in the wave-soldered 
     connection, the amount of lead that freezes before the tin starts to 
     freeze.  If the solder freezes slowly, this lead will be observable as 
     many small protrusions on the surface.  These protrusions are the ends 
     of lead dendrites that formed during the early stages of freezing.  
     Because solder contracts about 4 percent during freezing, the last 
     liquid in the connection to freeze has receded, leaving the ends of 
     the dendrites exposed.  When the solder freezes quickly, this all 
     takes place so fast that the lead dendrites that form are too small to 
     be noticeable at the surface.
     
     (It is interesting to note that even eutectic solder does not freeze 
     instantly, although this is sometimes asserted.  It does not even 
     freeze at the eutectic temperature.  The temperature drops below 183 C 
     by dozens of degrees (a phenomenon called "undercooling": Spontaneous- 
     ly forming crystal nuclei necessarily start out very small, and very 
     small nuclei have a lower melting temperature than big crystals), and 
     then lead (Pb) starts to precipitate.  This causes the tin concentra- 
     tion in the remaining melt to rise.  The temperature rises, too, due 
     to release of the heat of fusion.  Eventually, tin also starts to 
     freeze along with the lead, and finally the temperature starts to drop 
     again.)
     
     The practical effect of solder being low in tin is nil, unless the tin 
     concentration has dropped to the point where the liquidus temperature 
     approaches the solder temperature (in which case, flow is restricted, 
     due to premature freezing of solder in the connection).

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