Silicone ink sacs: additional notes

We've been selling (and using) silicone pen sacs for several months now. Here are a few notes on what we've learned from the resulting in-the-field experience:

• Although we started out using and recommending the traditional standby, shellac, for attaching these sacs, we have found that a stronger and more reliable seal is provided by a silicone sealant such as Devcon Silicone Adhesive 12045 (Loctite 908570 is another brand successfully used by customers). It should be noted that these silicone adhesives give off acetic acid as they cure, and most contain some petroleum distillates. Sac nipples of hard rubber will not be affected, nor are hard plastics such as acrylic and polycarbonate likely to be vulnerable. Celluloid could be affected, but this is more a theoretical than a practical concern given the very small quantity of sealant required for sac attachment. In practice, I have had no problems to date using silicone adhesive to attach silicone sacs to celluloid sections (noting that the solvent in shellac, alcohol, is also a solvent for celluloid, which would seem much more problematical in comparison).
• The traditional sac sizing scale is always explained as representing the outside diameter of the sac in 64ths of an inch. Conventional latex sacs, all of which are now made by dip-molding, vary significantly in wall thickness and hence in outside diameter, but even the largest in a batch of conventional #18 sacs are going to be well under 18/64". Why is this? As far as I can tell, the sac size number in fact represents the inside diameter of the smallest barrel a given sac will fit -- a measurement necessarily  slightly greater than the outside diameter of the sac. Our first size of silicone sacs measures a true 18/64", slightly larger in diameter than conventional latex #18 sacs. Future sizes will be adjusted to bring them into line with extant sac size charts; we are considering re-labeling our #18 sacs as #18 1/2 for the same reason.
• Silicone rubber offers a number of advantages over latex, but there are areas where latex is superior. Gas permeability is one: air and water vapor will diffuse through silicone much faster than through other rubbers and plastics -- around 26 times faster than through PVC, according to one source. Knowing this, we put our first silicone sacs through extensive real-world tests. Our main concern was that ink might thicken due to evaporation, but in actual use we ended up refilling the test pens well before any noticeable thickening occurred. Recently, however, we have become aware of another way in which silicone's permeability can be an issue, albeit an easily managed one. It seems that if a pen with a silicone sac is left lying horizontally, after a couple of days the feed will become saturated and will drop ink into the cap. The solution is simple: if you aren't going to use a pen for a while, store it with the nib elevated. In fact, this was standard advice from all the old-time pen companies, and is a practice that is second nature to vintage pen enthusiasts. The pen doesn't have to be stood on end -- resting on a slant is quite enough.

More sac sizes will be on their way in the next few months, starting with the 14 to 17 range. Getting the molds adjusted properly takes some trial and error, but better to get the sacs right even if it takes a bit more time and expense up front.