Can a drawing by itself count as prior art, or is a text description of the drawing needed as well?
For example, fig.9 of US3110626 was used against EP0030192 T204/83. Was this finding appropriate?
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For a European application, the answer is also yes, but only if the drawing (alone) contains enough detail to provide some technical teaching to the skilled person.
The decision you identified, T204/83, was one of the earliest EPO Board of Appeal decisions on this point, and effectively established the rules about drawings as prior art. The important principle of this case was stated at para 4 of the reasons:
When a feature is shown solely in a drawing without any other clarifying description a careful check should be made to establish whether the mere diagrammatic representation enables a person skilled in the art to derive a practical technical teaching therefrom. It would be impossible to lay down general rules as to how this should be done because the reply in each case will depend on the knowledge of the person skilled in the art and of the way in which the feature is shown in the drawing. Thus if a drawing is specifically devoted to disclosing a certain feature or if that feature is systematically included in a number of drawings a person skilled in the art will see it as an important item of technical information even if the illustration is unaccompanied by a verbal description or numerical information. If on the other hand, the feature is shown in the drawing merely as an ancillary element of a complicated device a person skilled in the art will sometimes be unable to elicit useful technical information from the drawing, at least without the information supplied in the newly filed patent application.
As shown in the official case law compendium (at I-C 3.6), this approach is still regarded as good law by the EPO. Because of this, it would be hard to argue that the decision was inappropriate.
A drawing can certainly act a prior art on its own. The MPEP section on Drawings as Prior Art states (citations omitted):
Drawings and pictures can anticipate claims if they clearly show the structure which is claimed. However, the picture must show all the claimed structural features and how they are put together. The origin of the drawing is immaterial.
When the reference is a utility patent, it does not matter that the feature shown is unintended or unexplained in the specification.
You've asked about a European application, but I've answered regarding the process for a U.S. application since that's the scope of this site (and where I'm most knowledgeable). The answer is likely the same for Europe, but I'm not certain.