White haze and use of SO2 gas


Iam novice in glass tempering, only 3 years experience.

Recently we begin to have problems white white haze - a white abrasion on the bottom surface of the glass.

I know that are two reasons for this defect:

1. The glass is not flat at the begining of the heating phase.
2. The rollers are just clean and the contact betweem them and the surface of the glass is rough. Thicker glass increase this defect.

1. We ajust the settings and the glass stay flat during the heating phase.

2. We used SO2 gas according to the procedure described in the furnance manual: only during production, not on coating glass. The procedure say not to use gas excessively (to stop the injection when white haze disappear or when a bluish film is created on the surface of the glass).

But even we used SO2 gas (3 hours) the defect didnt disappear and the bluish film didnt form on the surface of the glass.

So, now I ask myself if is any gas in the recipient. The device shows that we have preassure and flow, but nothing happen. And I can not feel the smell. I know that SO2 gas smells like burned matches.

How can we check if the gas is ok? In winter we had -20 grd Celsius. Can low temperature desintegrate SO2 gas? How can we check if is any leak? How quicly the bluish film appears? 3 hours is not enough? I could not find pictures on the internet to make an ideea how it looks like.

Please help me in this matter with more information about the use of SO2 gas. The problem increase every day. First we had problems only with big sheets of glass, 8-12 mm, but now white haze appears also on 4-6 mm, small pices (400x1000 mm).


Guest User
Mon, 25/05/2009 - 09:46

common culprit is uneven heating.
If you have still problem with white marks
check if all rollers are rotating in same speed ( no issue if it is a chain drive).few drops of oil in belt system can cuse scratch(bottom scratch)

try to load glass diagonally and see if you are getting the same marks in same direction. If the washing machine have any debiries on brushes can cause some marks .
if same direction it is not from furnace.

Guest User
Thu, 23/04/2009 - 20:14

You say that recently you have been experiencing problems - what has changed in the furnace since before when you did not have any problems?
However, try putting two table-spoonsfull of sulphur powder (flours of sulphur) in a paper envelope on top of a piece of glass as it goes into the furnace. The sulphur powder will burn and produce SO2 gas. On that load, or the next load, you will see a greyish/blue cloudy deposit on the glass surface (it will wash off with water). I would recommend that you do not do this more than about three times a day for about three days to help condition your rolls.
Jonathan Barr

Guest User
Sat, 23/05/2009 - 23:34

For So2 leakage check,please contact with supplier about checking method of this.
For with haze i recommend the same with first comment and please check air jet flow and convection system of furnace the root cause of white haze is over temperature of fuse silica roller in furnace with glass is dirty before loading into furnace.
clean roller with sand paper #1000 up.
take hot water with roller.
reduce top/bottom temp.
reduce cycle rest time.
check roller belt system.

Guest User
Thu, 07/05/2009 - 05:37

The White line is a problem in my process. There are two furnace since 2004 (large and small).The white line appear for 2 year and it appear any size of glass. After I used SO2 so it can solve the white line in large furnace only but it can not small furnace. Also I try to do any method in manual such as

1.Low down the temperature setting or shorten the time gap (cycle rest time) between glass. loading into the firing.
2.Low down the temperature setting 5-10 C.
3.Use SO2 gas.
4.Cleaning roller and cleanliness inside the factory.
Any method can not solve a white line.

1.Grinding the rollers by sand paper NO. 1000.
2.Washing the rollers by methanol 100%.

How long rollers was expire ?
What is the white line in your opinion?

Please help me for method to solve a problem
detail and picture ----> http://www.mediafire.com/?sharekey=7171629570b0a2e6bda4076e811714c8e04e75f6e8ebb871
Thank you for your advise

Guest User
Tue, 05/05/2009 - 10:31

Hello Daniela,

Did you follow the advices from Jonathan Barr?
What are the facts?
When we started up our furnace in september 2007 I smelled the SO2. Now I can not feel the smell.
We just have similar problem with a white abrasion on the bottom surface of the glass. But especially we have it with thickness 6 mm grey.

thank you

Guest User
Sun, 31/05/2009 - 09:33


We manage to solve the problem, but I want to tell you all every step we made.

First time when we had white abrasion on glass it was 3-4 mounth ago. It was on 8 and 10 mm glass, 2400x3500 mm, but only close to the edges. We could not use SO2 gas because we had to temper coated glass also in that period. So we only ajust the settings of the furnance (top convection furnance) to keep the glass flat in the heating phase and the defect disappear.

Recently we stop the production and clean the rollers with water.

We decided to use SO2 gas when we started the production again (in the manual of the furnance is written to use SO2 after cleaning the rollers).

This time we had 6 mm glass, 900x1200, with the settings that we always used. After 3 loadings the white abrasion appear again but this time in the middle area. Loading after loading the defect increase and we decided to use little pices for tests (400x1000). We changed a little the settings but nothing happend. We stoped the injection of SO2 but nothing happend.

I can not use sulphur powder because my boss not agree with this. We have to use the cylinder with gas that we have.

Next day we continue with the injection of SO2 and test (6 mm, 900x1000). The white abrasion still remain. After a few hours we stoped the injection and continue with test. This time, loadind after loading the defect decrease so we decided to start production, but if we load only one pice of glass the final quality was good. If we load 2 pices: one was good, the other one had the white abrasion (it didnt matter the position on the bed of rollers).

The next day we could temper full loadings (3500 mm long, 6 mm) like we never had problems.

So we decided to contact the producer of the furnance and told him about this.

His answer was simple: The values in the manual for SO2 flow and pressure were for daily production. But if the white haze appear we must increase the flow and to inject SO2 for 3 or 4 loadings.

So that is why the white haze disepear only after a few days. We used to less SO2!

Thank you all for your opinion!


Guest User
Sun, 31/05/2009 - 14:04

Dear Sumeth,

I have seen your pictures and I can say that the white line is exactly the problem I also had: white haze.

I solved this problem using SO2 but our furnace is large. I can not say why it doesent work with little furnace.

About the rollers you should ask the producer of the furnance when they need to be replace. He sould know best.


I have read and I agree to glassonweb.com's Terms of Use, including Privacy Policy and Cookie Use.