Chemical properties of optical glass: Test procedures, and considerations for design and assembly
AbstractThis paper will summarize the considerations of chemical stability of optical glasses, with an emphasis on design considerations relevant to common optomechanical systems. It will consider the categorization of chemical stability into climatic (water) resistance, stain resistance, acid resistance, alkali resistance, and phosphate resistance, with descriptions of relevant test procedures. Then, a brief overview of the considerations with respect to fabrication and handling of lens systems will also be given.
Background Review of Schott Technical Information The Schott technical paper “TIE-30: Chemical properties of optical glass” gives an overview of the methods used by Schott to characterize its optical-glass products for the purpose of writing specifications. First of all, the paper briefly describes the chemical principles behind corrosion of optical glasses. SiO2, Al2O3, TiO2, and rare-earth oxides are poorly soluble in aqueous and acidic solutions, so they are not highly susceptible to leaching and local corrosion. On the other hand, alkali and alkali earth oxides are more soluble and typically lead to formation of corroded layers. A thickness of 0.1 µm is used as the threshold for a significant chemical change resuilting from acid, alkali, or phosphate exposure. Next, the report gives descriptions of each test procedure and the corresponding inspection of the tested element. For climatic resistance, the test elements are placed in an atmosphere saturated with water vapor. The temperature is cycled between 40°C and 50°C with a period of one hour. The elevated temperature accelerates the aging process, while the temperature cycling is needed to produce condensation. The test elements are left in the climatic chamber for a total of 30 hours. The surface of the element is then observed using a spherical hazemeter to determine the level of transmission haze and the increase in haze resulting from the aging process, as given in Table 1.
Table 1: Criteria for climatic resistance (CR) classification. Adapted from Table 2.1 in ref 
As examples of glasses in the worst CR class (class 4), the report gives KZFS12, N-LAK21, N-SK14, and N-SK16. This is an important concern in the design of achromatic lenses and other multi-element lenses, where such glasses are common; an example will be discussed later.
Next, the report discusses stain resistance, defining it to describe the effect that results when weakly acidic aqueous solutions come in direct contact with the glass surface. Stain resistance is not necessarily coupled to climatic or acid resistance; some PSK and LaK glasses appear to form no stains in the stain-resistance test because a layer of glass is completely etched away without stain formation. The test procedure involves the pressing of the test sample into a cuvette in which a spherical depression of up to 0.25 mm depth is loaded with a few drops of the test solution so that the solution forms a film over the surface of the glass). Test solution I consists of a standard (unbuffered) acetate with pH = 4.6, while test solution II is a sodium acetate buffer with pH = 5.6.
The test element is left in contact with the solution at 25°C temperature until the first brownblue interference stain occurs; the time is used to determine the stain resistance class as given in Table 2, below. If no interference stain is observed after 100 hours of exposure, the glass is classified as FR 0.
Table 2: Criteria for stain resistance (FR) classification. Adapted from Table 3-1 
Examples of glasses with poor stain resistance include SF57, SF66 (FR 5), N-KZFS2, and N-SK16 (FR 4).
The tests for acid resistance involve a larger volume of solution with an optionally lower (more acidic) pH than that of the stain resistance test, to allow for gross decomposition of the glass to be observable. The first test solution is a 0.5 mol/L nitric acid solution with pH = 0.3, and the second is a standard acetate solution with pH = 4.6 (the same as test solution I for stain resistance) The former is most commonly used, in order to achieve a sufficiently aggressive reaction with common glasses; the latter is used for less acid-resistant glasses so as to allow a reasonably long duration of testing. Physically, acid resistance is limited by the concentration and distribution of the sparingly soluble substances within the glass. In some cases, it is impossible to include a sufficient quantity of such solutes while still maintaining the desired optical properties. This makes the glass poorly acid-resistant. The test proceeds until a layer thickness of 0.1 µm is removed. (Since this does not necessarily involve the formation of an interference layer, direct dimensional measurements before and after the acid treatment are needed. The Schott report does not recommend a specific method for such measurements.) The acid resistance class (SR) is given as in Table 3.
Table 3. Criteria for acid resistance class (SR). Adapted from Table 4-1 
- 0 = no visible changes
- 1 = clear but uneven surface
- 2 = interference color(s) visible
- 3 = firmly adhered layer of cloudy leaching material
- 4 = thick, loosely adhering layer of material
Table 4: Criteria for alkali resistance class (AR) and phosphate resistance class (PR). Adapted from Table 5-1 
Examples given of glasses with alkali rating 4.3 are KZFS12, KZFSN4, KZFSN5, N-KZFS2, and N-LAK21. NKZFS2 and SF66 have phosphate resistance rating 4.2; 13 glasses have phosphate resistance rating 4.3.
Sidebar List of relevant ISO standards
- Climatic resistance: ISO/CD 13384 (proposal), “Raw optical glass – Testing of the climate resistance CR (resistance to humidity) at temperatures changing between 40°C and 50°C and classification”
- Stain resistance: No applicable standard
- Acid resistance: ISO 8424, “Raw optical glass – Resistance to attack by aqueous acidic solutions at 25°C – Test method and classification”
- Alkali resistance: ISO 10629, “Raw optical glass – Resistance to attack by aqueous alkaline solution at 50°C – Test method and classification”
- Phosphate resistance: ISO 9689, “Raw optical glass – Resitance to attack by aqueous alkaline phosphate-containing detergent solutions at 50°C – Testing and classification”
Discussion Design and fabrication considerations of glass choice in a multi-lens system
Fig 1: Schematic of achromatic doublet lens with mount. The right end of the barrel is intended to be sealed by the presence of additional (chemically insensitive) lenses, a diaphragm, and/or connection to a larger optical system. (Illustration: Own work)
One flint glass with relatively high dispersion is KZFS12, with vd = 36.29; compare the lower dispersion vd= 64.17 for N-BK7 . The borate flint (KZFS) class has generally poor chemical strength in all categories. Glasses containing both borates and at least 20 molar percent lanthanum perform better, but such glass compositions are not routinely manufactured by Schott. In the final assembled system, only the convex BK7 element comes directly in contact with environmental influences such as atmospheric moisture. However, during assembly, great care must be taken to avoid exposing the KZFS12 element to high levels of moisture (CR 4), strong acids (SR 53.3), or any alkalis or phosphates (AR 4.3, PR 4.3), especially at elevated temperatures. As KZFS12 is rated at FR 1, weaker acids are not of significant concern. Of course, skin contact must still be avoided in order to prevent fingerprints and other organic contamination.
The required assembly conditions can be achieved by using gloves for all human handling of the negative element. The requirements on these gloves are not particularly demanding, due to the glass’s good stain resistance; ordinary latex gloves are sufficient provided that the environment is suitably controlled to eliminate sources of steam and strong acids/alkalis. Additionally, automated assembly machinery may be needed for the cementing of the elements and subsequent installation into the barrel. The handling requirements become less stringent once the system is fully assembled. At that point, the priority is to prevent leaks of steam or aqueous chemical solutions from entering the space of the barrel, where they would contaminate the back surface of the negative lens. This requirement is easy to achieve for common visual optics, provided that the barrel is a single solid piece or that the system is operated in a suitably controlled environment.
Finally, computer simulations and accelerated-aging experiments on prototypes may be vital tools to evaluate the reliability of the system. If screw threads are present in the system, either as lens mounts or to join barrel segments, such tests may be valuable to determine the thread dimensions and machining grade necessary to control gas leakage. Other special situations that warrant attention may include:
- Demanding operating environments, such as high temperatures in combination with chemical fumes
- Barrel designed to be disassembled and reassembled while the system is in use
- Redesign of the lens as a dialyte lens, in which the separation between the lens elements exposes both sides of each to environmental attack
- Constraints in the material choice; for example, operation at UV wavelengths where BK7 is unsuitable due to poor transmission (< 300 nm), or when new innovations make some other glass or polymer composition more economical
On the other hand, if the lens were redesigned as a cemented triplet, the central element would not be directly exposed to environmental stresses under proper mounting, so the chemical requirements on that element would be relatively lax, and the burden would shift to the cementing process.
An overview of the Schott technical report “TIE-30: Chemical properties of optical glass” is given. The test procedures described in the report are summarized, and the chemical parameters of representative glasses are evaluated with regard to said procedures. Also, the considerations of chemical stability for the design and assembly of a lens system are discussed.
- “Bk7 Optical Glass Flats from VPG.” http://www.vpglass.com/optical_glass/bk7_glass.html. Volume Precision Glass, Inc. Accessed 1 Dec 2013.
- “Borofloat Optical Glassfrom [sic] VPG.” http://www.vpglass.com/optical_glass/borofloat_glass.html. Volume Precision Glass, Inc. Accessed 1 Dec 2013.
- “Optical Glass – Description of Properties.” Schott, June 2003. Supplied by University of Arizona, OPTI 521 class notes, 2013. pp. 6-33.
- “SCHOTT BOROFLOAT® 33 Borosilicate Glass.” http://www.us.schott.com/borofloat/english/index.html. Schott North America, Inc., 2013. Accessed 1 Dec 2013.
- “TIE-30: Chemical properties of optical glass.” Technical Information – Optics for Devices. Schott, July 2004. Supplied by University of Arizona, OPTI 521 class notes, 2013. pp. 1-9.