Markings on glass

On our glass tempering furnace, we are seeing marks on the thick float we have never seen before. from 12mm to 19mm glass there is almost a pitting that occurs. the weird thing is that it looks like debris ot dust but there is nothing there to the touch. Its almost like the rollers themselves have imprinted the marks onto the glass. we have cleaned the rollers several times but this does not seem to help. the exit temp. of glass is 618-620 , so its not over heating. please assist where possible. I have never seen these marks in the 8 years that we have had this furnace, we use very little SO2 gas, in fact almost nothin at all. COULD IT BE THE ROLLERS???

Guest User
Thu, 07/08/2008 - 15:55

For thick glass 12 – 19 mm your exit temperature should be around 603 to 600. Providing your exit temperature reading device is correctly calibrated you could try reducing all zones (top and bottom) by 15 degrees and running the glass at your normal heating time and quench settings. If everything is okay (reduced marking) then check the break pattern still meets code, and if not, increase the quench air to compensate for the reduced exit temperature.

Guest User
Mon, 11/08/2008 - 07:37

Thank you , for your valued input. My furnace settings with regards thickers float are more bottom heat than top heat to aviod edges being chipped, is this incorrect! for example on 12mm glass my bottom heat is set at 714 degrees and top heat 695 degrees. could this be the cause of the excess markings. i will try what you have recommended and post my results. regards

Guest User
Mon, 11/08/2008 - 10:35

The reason you get chipped corners on thick glass is that the large mass of glass entering the furnace causes a large drop in temperature which in turn causes the elements to go to full power to recover the temperature. The upper elements radiate directly into the upper surface of the glass but the lower surface of the glass is shielded from the lower elements by the roll bed and element covers. Therefore there is a differential heating effect on the glass with more heat entering the upper surface causing it to expand more and go into a water shedding shape driving the corners of the glass into the rolls and chipping them.

Your heat profile helps to balance the glass temperatures top to bottom but the roll bed is very hot at 714 (and the hotter the roll bed the bigger the probability of marking the glass). A far better option is to ask the furnace manufacturer to give you a simple software modification to limit the maximum power in the upper zone elements for the first third to half the heating time. You will find that the furnace will still recover temperature within the heating cycle, and the marking and chipped corner problems will go away.

To prove the point manually, set your furnace to about 695/685, load a bed of glass and as the glass enters the furnace switch off around 80% of the upper elements. At half the heating time switch all the upper elements back on. (For safety in this trial I would recommend that you increase the heating time by around 5%).

Let me know how you get on.

Best regards

Guest User
Mon, 11/08/2008 - 12:18

wow , what a difference , i lowered bottom heating to 685 and top heating to 690 , heating time of 520 seconds on 12mm , its amazing !!!
my exit temp is around 600 -604 degrees, no shelled corners and no sheeled edges, the quality is very very good. I cannot thank you enough.
I do have the power split option and will run tests on delayed heating for top furnace, i was however a little concerned about the white haze coming into play. Once again , thank you. Question: can i use this as a template for my thicker products as well as the thinner ones.

Guest User
Mon, 11/08/2008 - 16:05

You should not need delayed upper heating on glass 8mm or thinner, but the thicker you go the more advantageous it becomes. If central white marking becomes an issue try increasing the percentage of power available on the top elements during the early stages of heating (my earlier suggestion was 20%)

Guest User
Tue, 12/08/2008 - 07:15

Jonathan Barr, i cannot thank you enough for all your assistance, tell me ! Are you available to consult with us. We have 6 furnaces within our group , so we are relatively small. Would you be interested in assisting us with prehaps a lecture or on site training. please contact me at

Guest User
Wed, 17/09/2008 - 19:19

Hi Peter D. In twenty years of working on tempering plants I have never seen white spots on rolls associated with SO2. What you normally get is a build up of Sodium Sulphate on the rolls (SO2 gas mixing with the Sodium in the surface of the glass being processed) and this appears as a brown stain, or when severe, as brown spots on the rolls. They will simply wash off with hot water (60 – 70 degree) do NOT use solvents or abrasives on the rolls. If the spots are left to grow in size they will damage the roll surface because they form hot (Sodium Sulphate is liquid at furnace temperatures thus providing a lubricant between the roll and the glass). When the furnace is cooled down the ceramic roll has almost zero coefficient of expansion, however, Sodium Sulphate spots do have a coefficient of expansion, and will therefore contract, and will crumble the surface of the roll. This in turn leads to more marking, so the operator uses more SO2 gas and you get into a tail chase situation. Basically do not use SO2 unless you absolutely have to and then turn it off as soon as any marking problem has reduced or gone away.

Guest User
Tue, 16/09/2008 - 22:56

Can you tell me, did you remove the rollers to clean them or run a cleaning board (scourer/carpet) over them while still in the furnace. The markings on your glass as described are consistant with the use of S02 Gas which over time will build up very small deposits on the rollers (about the size of a seseme seed). These little lumps are very difficult to see because they are the same colour as the rollers. Durring your next maintenance period if you remove a couple of the rollers you will be able to feel them when running your hand over the roller. Alternatively if you do not want to wait till your next shutdown try this test first. Run a 12 or 15 mm glass approx 2mt x 1mt with the gas turned on all through the cycle, set the S02 at a higher than normal setting giving you a strong bloom over the glass. If the build up is present on the rollers you will nottice when looking at the glass after furnacing that there is tiny little spots missing from the overall bloom covering the glass. This is because it has remained on the roller adding to the build up. With respect to Johnathon, he has offered you some very sound advise for improving the quality of your glass, but if these deposits are present on the rollers even these settings will fail you as the build up continues. Kind Regards Peter D

Guest User
Wed, 17/09/2008 - 22:35

Hi Jonathan. Firstly I must apologse for miss spelling your name (rude of me). I never said the spots were white! simply the same colour as his rollers, after 8 years that he has been running this machine if his rollers are still white (I want to know his secret:) I merely wanted to know if he has checked them to see if the spots are present. If not I suggested a quick method of doing so. I agree with you whole heartedly, no one should use S02 Gas unless you absolutely have to. Like yourself I have spent 35 years commissioning furnaces and teaching people how to use them to achieve optimum quality both here and abroad. I enjoy reading your comments
Kind Regards
Peter D

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