Debate Responses
The dye derived from the חלזון deteriorates close to its death.
שבת )עה א( “דכמה דאית ביה נשמה טפי ניחא ליה כי היכי דליציל ]י”ג: דליצלח [123] [ ציבעיה”. Shabbos (75a): “As long as [the חלזון ] is still alive it is better for him, so that the dye should be clear / successful.”
This phenomenon is recorded clearly by Roman sources in regard to the Purpura dye:
Pliny: “It is a great point to take the fish alive, for when it dies, it spits out this juice.” [124]
Aristotle (Historia Animalium, Book V, 15): “Fishermen are anxious always to break the animal in pieces while it is yet alive, for, if it die before the process is completed, it vomits out the bloom; and for this reason the fishermen keep the animals in creels, until they have collected a sufficient number and can attend to them at their leisure.”
The “spitting out the juice” stated by Pliny seemingly refers to a loss of quality in the dye, or the absorption of the dye into the flesh of the snail. This loss of quality is described in other books as well [125].
We decided to perform an experiment. We took two snails and broke the shell. For the first snail we immediately extracted the dye and rubbed it into a cloth. We placed the second snail aside for about a half hour before extracting the dye and rubbing it into another spot on the same cloth.
Two observations became clear:
1) Soon after the snail is killed, the dye fluid begins to leak and is absorbed into the flesh. It also begins to change color from black to purple.
2) The first dye stayed fast after washing the cloth, whereas the second dye washed out almost completely.
The phenomena we observed has a simple explanation: Once the snail is broken the dye begins to leak and is exposed to air. This causes the dye to oxidize (hence the change in color). Since the dye must be in a state of reduction in order to bond with cloth, once it has been allowed to oxidize it will wash out. Although it is possible subsequently reduce (de-oxidize) the dye, this process was time consuming and not always 100% successful using the ancient methods.
The phenomenon observed by Chazal exists with this snail and was clearly documented in other sources as well.
Footnotes
123 כן היא גירסת הר”ח והרמב”ן )שם(, וכן גרס הערוך, ובתשובות רב נטרונאי גאון )ברודי )אופק( תשובות פרשניות סימן תפא( ור”א בן הרמב”ם )שו”ת ברכת אברהם סימן יט(.
124 Pliny, Ch. 36. See also: Aelian, De Natura Animalium (16:1) where the same phenomenon is described.
125 Aelian, De Natura Animalium, Book XVI Chapter 1. See also: Rawlinson, 1889.