After working for months (and years) to come up with your brand and build your distillery, bottle closure decisions often get left to the very end. But beware, the closure is one of the critical components of packaging and a part of the final product that your customers (not to mention reviewers, judges, and journalists) will interact with.
The closure is typically the only "moving part" of the final package. It serves both a functional role that seals the bottle during transport and storage, an active role in that it is handled each time one of your many customers opens and closes the bottle, and it is a marketing role in package appearance. For these reasons, it is important to plan well in advance when selecting your closure.
This article is intended as a brief summary of bottle closures (often called stoppers) for high-proof spirits. Wine and beer closures have different properties and require varied guidelines.
Type of Closure
The bottle entrance bore (referred to as bottle "finish") will need to match the closure that you select. There are generally two types of glass bottle finishes for distilled spirits: screw cap (sometimes referred to as GPI or Stelvin finish) or bartop (also called t-top or plate finish). Both screw caps and bartop stoppers act in a functional role by sealing your bottle. They also have a platform for "the cap" of the closure. The cap is the part of the closure that is visible outside the bottle.
Whereas the screw cap has some functionality considerations, such as thread type, a bartop closure has a whole range of options. The common bartop stem (also called a shank) is either made from real cork or synthetic (plastic) cork> In addition, glass stems are also appearing on the market more frequently, not to mention other closure types available (i.e. the Zork). Discuss these options with a closure company, and it can get complex. Considerations such as the design of the glass bottle, the type of spirit you are bottling (dark or light), and even the proof of the spirit will impact the closure you ultimately select.
The cap can be made from the same material as the stem, or it can be made from something else. Common materials for a bartop cap are plastic or wood. However, the design possibilities are limitless and include metal, glass, and ceramic.
Planning
Try to have all aspects of bottle design complete six months before you plan to bottle. That's right, a full half-year before bottling! Why so long? Industry-standard lead times for non-stock closures can be produced in 8-10 weeks (that's 2-1/2 months)! Some common closures and stock closures can be produced and shipped much quicker. However, if you're looking for something a little more custom, plan ahead. Sampling alone can require a back-and-forth process that can take weeks to complete. In addition, there are often factory shutdown periods around holidays and during the month of August. To avoid disappointment, don't leave packaging to the last minute.
Beyond planning well in advance, it is helpful to include a closure company in the early design phase. There are certain items that need to be taken into account when designing packaging. A creative designer can come up with a brilliant design that works well from a marketing standpoint but can have problems at a functional level.
Tolerance
Closures and glass bottles both have a tolerance range. Depending on the manufacturer of the glass bottle and manufacturing method. These tolerance considerations can be extreme. Glass bottle specifications generally indicate the tolerance range. Still, it is always a good idea to send the closure company at least one or two sample bottles. If the bottle design changes, send them a new bottle not only so they can have it on file for sizing but also for future questions or changes.
It is wise to send an updated glass bottle each year. Over time, the inside diameter of the glass bottle can change while staying within the acceptable tolerance spectrum.
Fill Level
One of the more common issues that can crop up is bottles being overfilled. Do not overfill your bottle! Glass bottles that have been professionally designed and manufactured take into account not only the liquid that the bottle contains but also the volume of headspace (also called ullage or air space) between the closure and the spirit. It is critical that this volume of air meets a minimum threshold. If there is not enough headspace during warm weather(or high seas if exporting or a bumpy road during domestic transport), the expansion of air will put pressure on the cork. This can cause the cork to lift and the tamper-evident seal (tamper tape, strip, capsule, or wax seal) to break. Review the glass bottle technical diagram. This may indicate a fill level including minimum and maximum thresholds.
Tightness
Bartop closures should be very tight upon initial insertion. The closure is fresh from production and will condense over time. The humidity in the bottle can soften the stem, while extraction and reinsertion of the closure can lubricate the inside of the bottleneck. Always test a bartop by inserting it into a bottle filled (to the appropriate level) with spirit (not water). Invert the bottle three times. Then remove and reinsert the stopper five times. How does it perform now?
Finally, review and re-evaluate after your initial bottling runs. Expect to make changes. It can take a few years to get things perfect, even after careful planning. Just like everything else in the distilled spirits world, the closure at every level of necessity is both a science and an art.