|The 2005 Gibbs stamp, with James Maxwell's thermodynamic surface, derived from Gibbs's work, shown in the background.|
The Gibbs stamp was issued in a set of four, along with geneticist Barbara McClintock, mathematician John von Neumann, and physicist Richard Feynman, in honor of great American scientists.  The stamp unveiling ceremony took place on May 4th, 2005 at Yale University, attended by John Willard Gibbs, a distant cousin, who spoke at the ceremony.  A framed picture of the Gibbs stamp adorns the wall at the entrance to the Dean’s office at the faculty of engineering at Yale University. 
The overall stamp design, showing what seems to be a three-dimensional graphical representation of various temperatures and pressures of the states of some substance, meant to represent the work “associated with major contributions made by the scientist”, was completed by American artist Victor Stabin, based on design input from American chemical engineer Kenneth Jolls, who specified the graph and the use of the Gibbs equation on the collar (not shown above).  Stabin had previously designed the 2004 commemorative postage stamp of American composer Henry Mancini, of which 87 million copies were made. 
The portrait of Gibbs is a photograph taken from collections of the American Institute of Physics' Emilio Segre Visual Archives; it was made toward the end of Gibbs' life, at about the time of his final publication in 1902. 
The origin of the stamp design traces to the webpages of American Kenneth Jolls, a professor of chemical engineering at Iowa State University, a site containing computerized Gibbs model graphical diagrams, based on research by Daniel Coy, Joll’s former doctoral student.  Jolls has devoted much of his professorial career to applying computer visualization technology to representing thermodynamic concepts that prior generations of researchers were forced to construct painstakingly from either wood or clay. In searching out a design, the USPS had come across Jolls’ website on Gibbs and retained his expertise to aid in the stamp design.  In working out the design, with the USPS, as Jolls recounts: 
“We started getting material together. We got through the first cut, and they wanted suggestions for a design to go along with Gibbs’ picture. I argued that the design had to be related to Gibbs' famous three-dimensional energy-entropy-volume surface, and we finally settled on the map of the USV surface that I had dug out of the tombs at the Berkeley Library from Maxwell's 1875 Treatise on Heat. USPS accepted the idea and sent a draft design to me about a year and a half ago for comments.”
The "famous" three-dimensional energy-entropy-volume surface, referred to here by Jolls is likely the “statue of water” plaster model (of which three were made, two remaining in Cambridge) that Maxwell had sent to Gibbs in New Haven in circa 1873.  JoIIs also convinced the USPS to rename Gibbs as a thermodynamicist, rather than its original title of chemist, upon his recommendation.
| Fig 26d. "Thermodynamic Surface" in James Maxwell's 1875 book Theory of Heat, visualized more fully in Maxwell's thermodynamic surface.|
The graph of the back of the stamp shows figure 26d, titled "Thermodynamic Surface", from Scottish physicist James Maxwell's 1875 book Theory of Heat.
The graph is meant to represent Gibbs 1873 publications on the graphical methods in thermodynamics ("Graphical Methods in the Thermodynamics of Fluids" and "A Method of Geometrical Representation of the Thermodynamic Properties of Substances by Means of Surfaces"), as fully described in a three-dimensional plaster surface of water Maxwell made in 1875, one of which he sent to Gibbs as a gift.
albeit the actual graph shown is not to be found in his major works.
The Gibbs stamp bears the two-dimensional map (pictured adjacent) of the energy-entropy-volume function, specified by JoIIs, from the fourth edition (1875) of "Theory of Heat" by Scottish physicist James Maxwell. 
All four 2005 American scientists’ stamps have a small equation on hidden on the collars of each scientist. Most of the stamps have such equation although, to note, some poster prints and web images (such as shown above) have no equations. In the case of the Gibbs stamp, it was Jolls who convinced the Postal Service to incorporate the differential equation of the Gibbs surface function, which in modern notation is:
dU = TdS – PdV
into the design as well. Gibbs, however, used Greek notation for his equation. Hence, the equation shown on his collar is:
dε = tdη - pdv
The tiny equation is mostly unnoticeable to the average observer. Trained thermodynamicists, however, are drawn to it and often excited to find it hidden. In 2006, for instance, MIT propulsion thermodynamics professor Zolti Spakovszky, discovered the equation after his wife had given him the book of stamps as a gift. In an April 08, 2006 letter to the editor of Mechanical Engineering, titled "Stamp of Authenticity", pictured adjacent, Spakovszky shows how one can find the the 1873 Gibbs equation on the collar of Gibbs’ shirt by magnification. 
Letter to editor: "Stamp of Authenticity", April 08, 2006 Mechanical Engineering, by Zollti Spakovszky.
“I couldn’t take my eyes off the Gibbs stamp. I noticed that there were some dark spots on his collar. A closer look under the microscope revealed a very pleasant discovery: the Gibbs equation is written on his collar. It is in the original form Gibbs reported on the second page of his paper, “Graphical Methods in the Thermodynamics of Fluids,” published in the Transactions of the Connecticut Academy, 1873.”
In 2009 commentary, Spakovszky affirms that finding the equation hidden on the collar was a “delightful experience”. 
On March 6, 2008, Postal Service issued the second in its American Scientists series of commemorative stamps, also designed by Stabin. The 41–cent stamps come in four designs in a pressure–sensitive adhesive pane of 20 stamps honoring theoretical physicist John Bardeen, biochemist Gerty Cori, astronomer Edwin Hubble, and structural chemist Linus Pauling. 
● Gibbs tombstone
1. Gibbs stamp (large .jpg image) – Yale.edu
2. The 2005 Commemorative Stamp Program – US Postal Service.
3. Gibbs Stamp Ceremony (images and notes), May 04, 2005 – Eng.Yale.Edu.
4. Picture of Faculty of Engineering (Doorway) – Eng.Yale.Edu.
5. (a) Letter to Editor (Stamp of Authenticity), Zolti Spakovszky, c. 2006
(b) Letters (April, 2006) – Mechanical Engineering.
(c) Josiah Willard Gibbs – Scientific-web.com
6. Anon. (2005). “Postal Service Pays Homage to Josiah Willard Gibbs”, Chemical Engineering Progress, 01 July.
7. Gibbs Models – Iowa State University (Kenneth R. Jolls).
8. Maxwell, James. (1875). Theory of Heat (Fig. 26d. “Thermodynamic Surface”, pg. 207). Dover.
9. Email communicate from Spakovszky to Libb Thims (18 Mar 2009)
10. Anon. (c. 2004). “Iowa State Chemical Engineer Drives Issue of New Stamp Honoring Father of Thermodynamics.” College Feature, Iowa State University – College of Engineering.
11. (a) Victory Stabin (overview) – StabinMorykin.com
(b) Victor Stabin - Homepage.
12. Hacker, Annette. (2004). “ISU Professor Helps Develop Postage Stamp Honoring Noted Scientist.”, News Service. Iowa State University.
13. Rukeyser, Muriel. (1942). Willard Gibbs - American Genius ("statue of water" model plaster surface, pg. 202). Garden City, New York: Doubleday, Doran & Co., Inc.
14. American Scientists (2008) – USPS.
● Gibbs Stamp (News & Events) – Yale Engineering.
● U.S. Postage Stamp Series Unveiling at Yale to Celebrate Exceptional Scientists – Yale News Release.