Polishing pitch is used in the finishing of precision optics for laser applications, from NIF phosphate glass laser slabs used in the National Ignition Facility (NIF) at LLNL, to large aspheric focusing lenses in OMEGA, to the refractive optics in UV steppers for photolithography. The reasons are as follows:

• Pitch polishers give the smoothest surface finishes on most glasses.
• Pitch polishers may be scaled up to polish very large, meter class flats with corners (rectangle, square).
• Three people (one per shift) can operate several large planetary polishers, each with several polishing stations, 24 hours per day to produce relatively large quantities of large parts.

Optical polishing pitch is the critical consumable for high precision optical finishing in most shops. The recipes for the manufacture of a very popular product, Swiss Gugolz® pitch, was sold by the originators three years ago. Those companies in the USA that depended upon the consistency of the Gugolz® products were thrown into disarray when performance of new batches, made according to the "old recipe", were judged to be unacceptable. They are still recovering.

The LLE Optical Fabrication Shop is a typical example. Months of shop time were lost to "bad" pitch on the 36" continuous polishing machine (CP), since the only measure of acceptability was to pour the pitch onto the CP, "cut" a pattern into its surface, and polish parts. The pitch was judged unacceptable because, after 4 weeks of trial and error on fused silica parts, our optician could not produce flat parts. He could not find a parameter space (temperature, location of weighted conditioning flat, type and concentration of polishing abrasive, groove pattern in pitch) that would yield surfaces flat to l/20 @ 633 nm over a 190 mm clear aperture. He removed the "bad" pitch and tried another candidate.

Pitch is a very complex material. [See http://www.physics.uq.edu.au/pitchdrop/pitchdrop.shtml] Under load, it exhibits an instantaneous elastic strain, a delayed elastic strain, and creep or permanent deformation. Its ability to transfer load to the polishing abrasive that contacts the glass surface is one key to material removal and smoothing to sub-nm levels. The ability of pitch to creep or flow under load to match the surface being polished is the other key attribute for uniform removal over the part surface, and for figure control.

There is limited information in the literature regarding the composition and properties of optical polishing pitch. Compositions are mostly proprietary. It is generally understood that the material consists of various amounts of the following: residues distilled from tar, oil, or wood; rosin, a derivative of turpentine which comes from sap of pine trees or stumps, to increase melting point and tackiness; beeswax or linseed oil to lower melting point; asphalt; flake shellac; paraffin wax; wood flour; or walnut shell flour. Properties of importance are viscosity (stated to be in the range of 10⁷ to 10⁹ Pa·s), softening point, (55-70 °C), penetration hardness (60-80 by Shore D), coefficient of friction (tackiness), and groove pattern.

Rupal Varshneya Optics '05 Performing a Pitch Characterization Test

In 2001 we began a three year research project to establish a database of physical properties for optical polishing pitches. With commercially available instrumentation we began to characterize batches supplied to us from our own shop and other shops across the country. We are looking to understand why there are batch-to-batch variations, and just which properties are best able to predict consistent performance.

             Hardness                              Softening Point               

Sample Pitch Database Pages

In 2004 we conducted a series of characterization tests on a synthetic polishing pitch called Acculap (Sutton Technologies, Star NC). Because it is a totally synthetic product developed without the organic volatiles present in natural pitches, it is thought that this form of pitch might exhibit smaller batch-to-batch variations.

R. Varshneya, J. E. DeGroote, L. L. Gregg, and S. D. Jacobs, "Characterizing Optical Polishing Pitch," Optifab 2003 (SPIE, Bellingham, WA,2003), Vol. TD02, pp. 87-89.

J. E. DeGroote, S. D. Jacobs, L. L. Gregg, A. E. Marino, J. C. Hayes, and R. Varshneya "A Data Base for the Physical Properties of Optical Polishing Pitch," in Optical Fabrication and Testing Digest (Optical Society of America, Washington, DC, 2002), pp. 55-59.

J. E. DeGroote, S. D. Jacobs, and J. M. Schoen, "Experiments on Magnetorheological Finishing of Optical Polymers," in Optical Fabrication and Testing Digest (Optical Society of America, Washington, DC, 2002), pp. 6-9.

J. E. DeGroote, S. D. Jacobs, L. L. Gregg, A. E. Marino, and J. C. Hayes, "Quantitative Characterization of Optical Polishing Pitch," in Optical Manufacturing and Testing IV, edited by H. P. Stahl (SPIE, Bellingham, WA, 2001), Vol. 4451, pp. 209-221.