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

 

 

• 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.