Ink and Writing Instrument Analysis

September 02, 2025

Ink and Writing Instrument Analysis

Originally written September 2, 2025

Forensic document examination is the scientific evaluation of documents to determine authenticity, origin, and potential alterations. It is a multidisciplinary field that encompasses multiple elements, including:

Handwriting Analysis: Comparing letter shapes, slant, spacing, line quality, and pressure patterns to establish authorship.
Ink and Writing Instrument Examination: Identifying pen types, ink characteristics, detecting alterations, and potentially linking to a manufacturer.
Paper Analysis: Studying texture, weight, fiber composition, watermarks, and color to trace the source and detect forgeries.
Printing and Typewriting Examination: Analyzing typefaces, mechanical alignment, toner or ink, and device-specific characteristics.
Alterations, Erasures, and Indentations: Revealing modifications, overwritten text, or hidden writing through visual, optical, or chemical methods.
Linguistic and Content Analysis: Evaluating grammar, vocabulary, punctuation, recurring phrases, and psychological cues for behavioral insights.
Postal and Envelope Evidence: Examining postmarks, stamps, and mailing patterns to provide geographic and temporal context.

Together, these elements provide a comprehensive framework for investigating documents in criminal, civil, or intelligence contexts.

Everything we come across, as it pertains to law enforcements investigation of the Zodiac Killer’s letters/envelopes, has been the detection of fingerprints, attempted detection of DNA via envelope flaps and stamps, and mentions of the type of paper used (Woolworths 5th Avenue) via watermarks.

When I first started the case, it was my understanding that the first letter written by the Zodiac (trinity letters), and every letter after the debut letter (August 1969), was written on Eaton paper and the debut letter was written on Woolworths 5th avenue paper. Or vice versa. I can't really remember now (It’s been 15 years). Through the high-res images it has been determined that it was Woolworths and not Eaton. It is also included as a source within the FBI files. One question that remains is whether the debut letter (August 1969) was also Woolworths. It makes more sense to me that the debut letter would be Woolworths because of his continued use of that paper source and given to its perhaps signature behavior of the Zodiac. However, Eaton has been mentioned as a source here, and perhaps only, here:

So, going on with the assumption that the Zodiac would likely continue this "signature behavior" of using Woolworth's brand 5th Avenue paper, I must try and rectify why it would be mentioned that "Eaton" was a source. Nothing but Woolworth can be corroborated as a paper source. Either "Eaton" was drummed up out of thin air or "Eaton" was something else. It’s this lack of corroboration, believing that it potentially was a determination made from the analysis of Zodiac’s written communication but with unknown origin, that led to the thought of perhaps “ink” being the source for the “Eaton” mention.

Ink analysis served multiple investigative goals:

Identify Pen Type and Ink Class: Different pen types (fountain, ballpoint, felt-tip) produce distinctive line quality, pressure, and ink flow patterns.
Detect Multiple Inks or Alterations: Identifies post-composition changes, additions, or tampering.
Narrow Down Manufacturer: Chemical techniques could sometimes link inks to specific commercial formulations using reference libraries.
Support Authorship and Document Comparison: Confirms whether documents are consistent or produced by different authors.

Methods Available in the 1960s:

Microscopy/Visual Examination: This type of examination examined line quality, stroke sequence, pen lifts, and pressure patterns. It would be able to determine pen type (fountain, ballpoint, felt-tip, etc). Local labs (police department/colleges), private examiners, state and federal labs would have the potential of making this determination.

Example:

Ultraviolet / Infrared Light Examination: This type of examination detected fluorescence or absorption differences in the ink that’s invisible to the naked eye. It can suggest ink families or classes of ink. State (CDOJ), FBI, and Secret Service labs would have been able to do this. I would not discount Postal Inspection labs.
Thin-Layer Chromatography (TLC): Small samples of the ink would be taken from the source (noninvasive) and would then chemically separate dyes to create an ink “fingerprint”. Once completed, this could sometimes match the ink to a manufacturer using reference inks. State (CDOJ), FBI, and Secret Service labs would have been able to do this. I would not discount Postal Inspection labs.

However, unless proven through additional research, only the FBI and Secret Service had a reference ink library.

FBI International Ink Library wiki: https://en.wikipedia.org/wiki/International_Ink_Library

Secret Service Library article: https://nypost.com/2019/07/08/inside-the-secret-service-lab-with-more-than-15k-ink-samples/

Ink Solvability / Spot Tests: Solvents would be applied to test solubility and chemical reactions. It would distinguish ink families and is used with Thin-Layer Chromatography (TLC). All labs could do this. Cost was minimal.

Ink and paper would be analyzed together to provide a complete forensic profile:

Ink Analysis: Determines how the document was written, identifies pen type, detects alterations, and may trace ink to a manufacturer.
Paper Analysis: Determines source, consistency, and contextual information about origin, time, or region.

Combined, they allow examiners to:

Authenticate documents and detect forgeries.
Establish authorship or identify multiple contributors.
Link documents to specific sources or regions.
Generate investigative leads for law enforcement.

If they did one, they surely did the other in some capacity.

Standards For Writing Ink Identification: https://www.swgdoc.org/documents/SWGDOC%20Standard%20for%20Writing%20Ink%20Identification.pdf

The above link, I believe I read, was dated 2013. So not very old. However, if you scroll down to the end of the article, reference is given to an article written on the subject in 1972. If you follow that title you can make your way to the actual publication in 1972 for download.

Law enforcement, at the time of the Zodiacs communications, had the ability to determine pen type and color. Testing of the ink through the help of either the CDOJ, the FBI, and possibly even the Postal service document examiners would reveal the chemical structures of the ink. This, in conjunction with the ink library maintained by both the FBI and Secret Service, would reveal brand, manufacturer, type of ink, and date range of ink. From there, law enforcement could go to the manufacturer with the ink type and chemical profile and request batch and dye lot information. This would determine distribution regions, stores, and time frames where the pen was sold.

While we haven’t seen the documentation, law enforcement (at some point), would have known a lot more than it was a “blue felt-tip pen” and would have known this at minimal cost (probably not even through them but through the USPS investigation). So, I have no doubts they determined a manufacturer and possibly even a location or general location for the pen source. The problem is that it wouldn’t have been “Eaton” in any early investigations.

So, can anything be said about “Eaton”?

In 1893, the Hurlbut Paper company purchased the Terry Clock Factory of Pittsfield, Massachusetts and set up a stationary division at this location. This stationary division was led by Arthur W. Eaton. Soon after, in 1899, Hurlbut Paper was absorbed by the American Writing Paper Company. The American Writing Paper Company decided to get rid of the stationery division, and Arthur W. Eaton along with other investors, decided to buy it. The new stationary company became Eaton – Hurlburt. In 1908, Crane & Co. principals made a significant investment in Eaton’s company, along with William Pike of the Hard (Bradley A. Hard) and Pike Paper company. This prompted the changing of the name to Eaton, Crane and Pike – Arthur W. Eaton, Winthrop Murray Crane, and William Allen Pike. By 1934, Crane and Pike had both left the company, and the name was reduced to simply Eaton.

Eaton is famous for producing fine writing papers that were often sold in boxed sets. These sets would include paper (of course), matching envelopes, and also “bonus” items like decorative seals and stamps. These were not postage stamps - they were decorative or sealing stamps that customers could affix to envelopes and possibly along the pointed seal.

Ebay, 1932 New York world's fair.

This above image was just one example found on eBay. I've seen them with bridges, ships (specifically Dewey), trains, and many others. But they also came as gold or silver foil wafer seals and embossed monograms, flowers, and crests. These were used from at least the early 30s to the 1950s when they were considered elegant touches for personal correspondence. This was more common with premium stationery lines targeted at gift buyers. It's also highly collectible.

All was good until demand for stationery began to fall during the mid to late 1950s, driven by a general decline in letter-writing and traditional stationery use. By 1967, business was in steady decline and Textron acquired the Eaton company. The interesting thing is that the year before, in 1966, Textron also acquired the Sheaffer Pen Company. Sheaffer was founded in 1912 by Walter Sheaffer in Fort Madison, Iowa, and incorporated in 1913. Sometime between 1967 and June 1976, Textron decides to merge these two companies into a single division named Sheaffer-Eaton. They discuss the merger in the Sheaffer company magazine called the “Sheaffer Times”.

This magazine, along with pens, other pamphlets, brochures, and repair guides dating back to 1913 can be found here:

The Glideriter was their first felt-tip pen and came in colors of Blue, Black, Red, and Green. I had previously found online one in blue that was of the shade we see in the Zodiac’s writing, but I have, sadly, since lost it. This particular pen had replaceable cartridges and I believe I read they were screw in. I believe I also seen examples where the tip could be unscrewed and replaced with a fountain pen tip (but don’t quote me

)

Needless to say, there is a potential “Eaton” tie to the communications via pens and ink through the Sheaffer / Eaton merger of roughly June 1976.

Would law enforcement have known all of this? Based on procedures above and Shimoda mentioning the Zodiac’s familiar blue felt-tip pen in 1978, I’d say yes.

Will we be able to prove conclusively? I don’t know. Was Graysmith confused? No, I don’t think so. I think he probably knew or was provided somewhat misleading information like Morrill did here (Outlined in red):