Hi -
Has anyone ever had the issue when they are doing Loss on ignition analysis in a muffler oven at 1000C? Previously I worked with a newer muffler oven and had no issues weighing and baking soils at 1000C. I recently started using an different muffler oven which is a little older, but is properly functioning. I baked my soils and crucibles at 550C and it was fine. I then turned it up to 1000C and baked it at 1000C for 6 hours and turned it down to 105C to take the crucibles out after cooling. However, after I took the crucibles out all of them seemed to be changing colors from neutral ceramic white to green from the bottom up. Also it looks like there are small crystals or precipitate forming on the crucibles, more densely from the bottom up. Attached are a couple of pictures. Does anyone have any experience with this or have any suggestions on how to get to the bottom of this or fix it?
Edit: I just want to note I tried to clean these crucibles with an HCl acid bath, and the greenness did not go away and the precipitate crystals seems to arise once it is fully dry again.
Looks dirty...
Did you check the temperature inside the oven? May be it's too hot.
Thanks for your response Rémi Losno .
Previously I have used the same crucibles and heated them to 1000C in a different oven and had no issues. I tried doing some maintenance (at 880C) burning/cleaning today, then I will wipe down the inside of the oven when it is cool for any residue. I have never had this issue with another muffler oven, and especially have never seen it change color at 1000C which is why I was concerned
Were the samples wet or were they contaminated with waste or hydrocarbons? This staining is unusual and may indicate that the furnace was contaminated by a spill of some sort in the past. I've never seen this amount of dicoloration of porcelain crucibles I've seen them develop a gray or slight reddish tint when used over many many ignition cycles but never to this extent. The reddish tint i have seen I've attributed to Fe oxide formation due to Fe impurities in the crucible but never to this extent. To me it appears to be something that is a contaminant in the furnace that volatilizes/condesnes when heating the furnace to high temperature..
If your are going to clean it up, you should heat it to at least the temperature with which you are igniting your samples or higher in order to get rid of whatever is causing the problem.
Larry Cihacek , Thanks for your help. The soils were not wet at all and should not of been contaminated so the oven must of been contaminated prior to use. I am familiar with the reddish oxide you mentioned and have experienced that on every other use, but yes never anything to this extent. Is there any good method you recommend for cleaning the oven other than heating it up? I have tried that and tried scrubbing it carefully at room temperature with a toothbrush, but it seems the problem is still present. It is hard to heat clean this oven because there is no upper vent and it is hard to keep the door open while in use, so any suggestions are helpful.
The problem never arises at 550C when doing organic matter, but has come up as an issue on all my other runs on this oven at 1000C. The oven I was using prior to this with the same samples did not have this issue ever.
I just want to note I also tried to clean these crucibles with an HCl acid bath (along with a standard DI, soapy water wash protocol), and the greenness did not go away and the precipitate crystals seems to arise once it is fully dry again.
Without knowing what the material is, I really don't have any good suggestions. Was it ever used in firing ceramics? Without knowing what it is it is difficult to know just what material the contaminants are soluble in. Also without knowing what it is, one doesn't know if the material is hazardous (eg. heavy metals). Mu suggestion would be to find another muffle furnace just for peace of mine.
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