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Chemical Tempering of Glass Lenses

 

Chemical Tempering (Chemtempering) Strengthens glass lenses by putting the surface of the glass into compression. The process by which this occurs is the exchange of ions. During the chemtempering process lenses are submersed in a bath of molten salt at a prescribed temperature. The head causes the smaller ions to leave the surface of the glass and the larger ions present in the molten salt to enter it. Once the lenses are removed from the bath and cooled they shrink. The larger ions that are now present in the surface of the lens are crowded together. This creates a compressed surface, which results in a stronger lens that is more resistant to breaking.

 

Chemtempering creates a uniform layer of stress. This is because the ion exchange occurs uniformly on all surfaces. Unlike the air tempering process, chemtempering has no relationship to lens thickness. This difference is evident when lenses tempered in these two way are viewed in crossed polarized light. The air- tempered lenses typically display various stress patterns; one of the most common resembles a Maltese cross. In the uniformly tempered lenses, such patterns do not exist. Drop-ball tests support the superior impact strength of chemtempering.

 

X-CEL Optical does not endorse heat or air tempering and does not offer any opinion in regards to this form of strengthening.

 

Glass Type Bath Composition Time Temperature
White Crown & Fixed Tints 100% KNO3 16 + 2 hours 450º + 5º C
Photochromics

60% KNO3
40% NaNO3

 

16 + 2 hours 400º + 5º C
Photochromics 100% KNO3 Minimum 2 hours
Maximum 4 hours
450º + 5º C

 

Never mix photochromics and fixed tint glass in the same chemical bath.

 

Preheat, Post-cool, General Precautions

An Important 15 Minutes: Preheat and Post-cool Lenses

 

Photochromic Lenses

 

PREHEAT: Photochromic lenses should be allowed to preheat over the chemical bath for 15 minutes before immersing them in it.

 

Post-Cool: After photochromic lenses are chemtempered, suspend them over the chemical bath for post-cooling. The post-cooling time should never exceed 15 minutes. Remove photochromic lenses from their suspended position over the bath as quickly as possible in order to preserve photochromic properties.

White Crown and Fixed Tint Lenses

 

PREHEAT: Crown Lenses should be allowed to preheat over the chemical bath for at least 15 minutes before immersing them in it. If breakage occurs, preheat 30 to 60 minutes.

 

Post-Cool: After crown lenses are chemtempered, suspend them over the chemical bath for at least 15 minutes. This will allow them to cool. If breakage occurs, post cool for 30 to 60 minutes.

 

 

  • Before Processing, make sure lenses are clean and dry.
  • After lenses have been processed in the salt bath and cooled, rinse salt away in warm to hot water. Do not use cold water as it may cause thermo shock, and result in breakage. Then dry lenses thoroughly. Allowing them to drip dry may cause spotting.
  • If lenses are to be coated, the coating application must be done after they are chemically strengthened.
  • Coated lenses must be drop-ball tested after the coating is applied.

 

Glass Color Chemical Bath Bath Time Heat Tempered
Clear Crown Standard Crown bath 16 hours Yes
*All fixed Tints Standard Crown bath 16 hours Yes
1.60 Clear 16™ Standard Crown bath 16 hours No
1.70 Hi-Lite Standard Crown bath 16 hours No
1.80 Hi-Lite Standard Crown bath 32 hours No
UVSG Standard Crown bath 16 hours Yes
ACE

Standard Crown bath
But at a temperature of 530° C

16 hours No
Photo Gray Extra Photochromic bath 2 or 16 hour bath Yes
Photo Brown Extra Photochromic bath 2 or 16 hour bath Yes
Photo Gray Thin & Dark™ Photochromic bath 2 or 16 hour bath No
Photo Brown Thin & Dark™ Photochromic bath 2 or 16 hour bath No
Photo Sun II Photochromic bath 2 or 16 hour bath Yes
Autumn Gold™ Photochromic bath 2 or 16 hour bath Yes
1.60 Photo Solar™

Photochromic bath

16 hour bath No
1.60 Photo Gray Extra Photochromic bath 16 hour bath No

 

    *Fixed Tint Glass includes these glass colors:

    - Green 1,2,3 - Gray 1,2,3 - Rose 1,2,3 - G-15 - Rose Didymium - Tan 1,2,3 - Rosetta - Canary Yellow

 

Glass lenses which cannot be tempered:

      - 1.80 Leaded X-Ray Glass - KG-3 Infrared filter - All Laminated lenses including polarized.  


Trouble Shooting Guide:

 

  Problem   Probable Cause   Suggested Correction
  Hazy Surface   - New bath-not cured for long enough before use

- Polish lenses to remove haziness

- Allow bath to cure for at least 24 hours
(Place no lenses or dummy lenses in bath during this time )

 

  Hairline surface cracks seen after chemtempering  

- Salt contamination and/or ph too high

- Bath temperature too hot

- White crown or fixed tints in photochromic bath

 

- Replace salt

- Check bath temperature with accurate thermometer and correct temperature

- Discard lenses and redo in proper bath

 

  Excessive breakage during drop-ball test  

- Edging flaws

- Insufficient grinding and/or polishing on concave side of lens

- Bath temperature too hot or too cold

- Check edging wheel and retrue or replace

- Check generator wheel and retrue or replace

- Increase fine grind time

- Check bath temperature with accurate thermometer and correct temperature

 

  Pitted lenses   - Bath temperature too hot  

- Check bath temperature with accurate thermometer and correct temperature

 

  Spotted Lenses  

- Lenses not thoroughly cleaned before tempering

- Lenses were allowed to drip dry, resulting in spotting before they were chemtempered, tempering enhances spots

 

- Make sure lenses are thoroughly cleaned before tempering

- Do not allow lenses to drip dry

 

Salt Bath Tempering Guide:

 

  When to check the temperature   How to check the temperature   What temperature range is required
 

Routinely and...

24 hours after filling

Once molten salt temperature is reached, prior to loading lenses

After every addition of fresh salts once temperature has stabilized

24 hours after complete refill

 

Use a calibrated thermometer

Insert directly into salt bath

Check several places across the bath

16 hour photochromic bath:
395º - 405º C (743º - 761º F)

2 hours fast photochromic bath:
445º - 455º C (833º - 851º F)

16 hour white crown/fix tint bath:
445º - 455º C (833º - 851º F)

If temperature is outside that range, make adjustments as directed as detailed below.

 

 

How to adjust temperature:

 

If the temperature is too high or to low:
1) Reset the temperature control by the appropriate amount to correct the bath temperature.
2) wait 2 - 4 hours.
3) Re-measure temperature with thermometers.
4) Repeat steps 1 - 3 until temperature is correct.

Please note:


1) Consult manufacturer's instructions manual for information on how to adjust temperature of the chemtempering unit.
2) Over time, the thermocouples on chemtempering units may become less sensitive and allow chemical bath temperature to become too hot. Eventually, they will need to be replaced.

 

 

 

 

 

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