Roman Engineering- Porous Plug Colliquiaria

[tiwanaku]

In a particular ancient Roman siphon at Aspendos (in Turkey)where 30cm holes are chipped through a long chain (> 1.5 km) of cemented together 1.0 m^3 stone blocks, some of the (now scattered) blocks show a curious transverse hole from the main water-conducting hole to the outside air. If the hole were open, this hole would hardly be consistent with normal siphon operation. It may be that a porous plug of some material was forced into the tranverse hole the putpose of which would be to release trapped air (at about 70 psia) while only permitting very limited water leakage. The question is then if anyone knows of some naturally occuring or easily manufactured material (wood, palm wood, fired brick, packed cloth, porous rock,...,?)that has such properties and would be available to Roman engineers. Some of the holes are conical with the large end towards the interior of the pipe while other holes are straight through.
Typical tranverse hole sizes range from 3 to 5 cm but it is not known where the transverse holed blocks are located in the block sequence. Any suggestions as to a material that could be used for this application greatly appreciated as it may possibly provide a meaning for the Latin word "collaquiaria" which is only used once in an ancient Latin text in reference to siphon operation but not explained other than in vague terms related to air pressure relief.
Frontinus (tiwanaku)

 

[saxon]

tiwanaku, Have you thought of a raw wool plug? It's porous, formable and readily available for the time.

BTW, Have you checked with Vitruvius?

saxon

 

[prex]

The characteristic you are looking for is nowadays available by means of special films (Goretex is an example brand name) where microholes are obtained in a plastic film with a suitable and fairly constant size, such that gas molecules may pass through, whereas liquid ones are stopped just because of their dimension.
However this kind of material will properly function over time only due to its limited thickness: any higher thickness material would be rapidly clogged by impurities and also by gas or liquid molecules trapped into holes restrictions or change of direction.
What I want to arrive at is that for sure romans could have invented a filler that was able to let air escape whilst retaining water (cork is an addition to your list that comes to mind), but that all of these possible solutions would inevitably last a limited time.
Isn't it possible that simply a plug was used on these holes?

prex

http://www.xcalcs.com

Online tools for structural design

 

[tickle]

have you thought that the tapered holes may have had floats in them like current day steam traps?