You must have certainly seen cork used in wine bottles. Why exactly do we use it? In this article, we are going to answer this question.
We will discuss how cork is produced and its various properties, especially its interaction with water. Then we will look at how these properties lend it to different uses such as in wine bottles, flooring, and other floatation devices.
Table of Contents
- What is Cork?
- Does Cork Absorb Water?
- Various Properties of Cork
- Does Cork Sink or Float in Water?
- Are Corks Airtight?
- Most Popular Waterproof Uses of Cork
- How to Make Cork Waterproof?
- Can Cork be Stained?
What is Cork?
Cork is an impermeable buoyant material. It is the phellem layer of bark tissue that is harvested from Qurecus Suber (cork oak), which is found in southwest Europe and northwest Africa. The montado landscape of Portugal produces about 50% of the cork harvested annually. Cork is made up of suberin, which is hydrophobic (molecules that repel water).
Cork has been used by humans for over 5000 years, and since antiquity, it has been used in floating devices and as a stopper for beverages, especially wines. Today, cork is put to a variety of other applications too: as a gasket material, in shuttlecocks, for acoustic and thermal insulation, etc.
Cork has a cellular structure in which cells have pentagonal or hexagonal shapes. The cellular wall of the cork consists of three layers: a thin, lignin-rich middle lamella, a thick secondary wall made of alternating suberin and wax lamella, and a thin tertiary wall of polysaccharides.
Does Cork Absorb Water?
No. Cork is almost impermeable in water. This is why it has been used as a stopper for thousands of years. It is also used in lifejackets, buoys, and other floatation devices because it does not absorb water and can remain buoyant for a long time.
One cubic inch of cork immersed in water for 48 hours will gain less than 3% in weight due to water absorption. This percentage is much lower than what a cubic inch of wood would gain when immersed in water for the same duration.
However, corks are not entirely waterproof. On some occasions, cork can taint the taste of the liquid in the bottle. This is why wines with a strange taste are often called “corky”. About 1-2% of corks end up harming the wine in this way through a substance called trichloroanisole(TCA).
This TCA has a very strong moldy smell and is created by a series of chemical reactions: chlorine from the environment reacts with the lignin molecules of cork to make trichlorophenol, which is then methylated by mold. This is why it’s always good to ask restaurants to let you taste the wine before pouring it to see if it is tainted.
Various Properties of Cork
Cork has the following properties:
Naturally, cork is quite resistant to water. It absorbs very little amount of water, which is why it has been used as a bottle stopper for centuries. Moreover, it’s low density and buoyancy also make it suitable for creating floating devices like life vests and fishing floats.
2. Low Heat Transfer
Cells in cork are small and tightly packed. The gas enclosed within these cells also has low thermal conductivity. As such, heat cannot easily pass through the material. This is also why cork is fire retardant.
Because of its cellular structure, it can be easily compressed during insertion into the bottle and then expands to form a tight seal. This property of cork is of great significance because the interior diameter of the neck of bottles is inconsistent—cork can contract and expand as required.
Cork has a bubble-form structure and is a natural fire retardant. This makes it suitable for acoustic and thermal insulation in floors, ceilings, and walls. Corkboard, a by-product of stopper production, is now also being increasingly used as an insulation product. It is seen as a non-allergic, safe, and easy-to-handle alternative to petrol-based chemical insulators.
Another wonderful thing about cork is that its extraction does not require cutting down trees. Only the bark is stripped to harvest the cork, and the tree continues to live and grow. Therefore, cork production is environmentally sustainable.
Does Cork Sink or Float in Water?
Cork floats in the water. The density of an object determines whether it floats or sinks in another object. Cork, like wood and ice, has a density that is less than that of water and therefore floats. Due to this, cork was used in floatation devices.
Another reason is buoyancy. Buoyancy is affected by several factors, one of which is air content. If we observe cork under a microscope, we will see that it has a large number of holes. These holes trap air, which makes the cork extremely buoyant.
Before 1900, life jackets were made from cork and balsa wood. Eventually, a material called kapok began to replace cork in life jackets. Today, plastic is used to make life vests.
Are Corks Airtight?
Yes. Corks are quite airtight but not completely. Most foods are best when fresh, but wines on the other hand, usually need some time to taste their best. Winemakers work hard to control the aging process and this involves choosing the right kind of stopper.
There are two aspects of wine aging. The first involves the reaction of fruit acids with alcohol, which reduces the sourness of the wine. This is especially important for tart wines.
The second aspect of aging is oxidation. When oxygen interacts with the wine, it produces many chances and ultimately yields an oxidized wine. Under anaerobic, that is, no-oxygen conditions, many wines develop undesirable aromas.
A small amount of oxygen helps to get rid of those thiol compounds that give a rotten smell. Oxidation also leads to stable pigments in red wine by reacting with the red anthocyanin molecules of the grapes.
Therefore, all bottle stoppers must allow some level of oxygen, and a typical cork lets in one milligram of oxygen per year. This might seem like a tiny amount, but over a few years, it makes the wine softer and mellower.
Most Popular Waterproof Uses of Cork
There are several uses of cork:
1. Bottle Stopper
As we have been discussing, cork serves as an excellent bottle stopper, especially for wine. Over the years, technology has really helped to improve every step of the winemaking process, from oak barrels to micro-oxygenation.
Yet, it is proving difficult to replace natural cork as a stopper. While manufactured stoppers have some advantages, cork’s connection to the natural environment and its centuries-old tradition make it hard to substitute.
Natural corks are used for about 60% of the wine bottles produced every year.
2. Floating Devices
As discussed earlier, the low density of cork allows it to float in water, which is why it can be used in a variety of floating devices such as fishing floats, and buoys.
It can also serve as an alternative to neoprene (synthetic rubber made by polymerization of chloroprene) to make fishing rods.
3. Cork Flooring
Cork is also used for house flooring because of its impermeability. Today, there are a variety of other flooring alternatives, but cork flooring is still a more environment-friendly choice; unlike hardwood, it doesn’t involve cutting down trees.
However, it’s important to consider that when used for flooring, cork needs special treatment to be able to withstand moisture and pressure. We will discuss this below.
Besides these, there are several other others of cork. For example, cork serves as an excellent gasket material (used for filling spaces between two mating surfaces), is used to make bulletin boards and floor tiles, and is essential for making badminton shuttlecocks.
Cork can also be mixed into concrete, and the composites made in this way have good energy absorption and lower thermal conductivity. It can also be used to make bricks for walls, in woodwind instruments, and as an alternative to leather in fashion products.
All these uses of cork benefit from the environment-friendly nature of cork production, making them much more sustainable than their alternatives.
How to Make Cork Waterproof?
Although cork is naturally quite impermeable, it is mostly used after being treated with sealants. In old times, waxes and animal protein binders were used to seal cork. Today, however, we can make cork waterproof by coating them with a layer of water-insoluble polyurethane binder.
Polyurethane binders are great because they do not leach into the ground and are much more resilient than other binders. They adhere to the granules of cork to form a highly impermeable composite.
Most surfaces require 2-3 coats of binder. But surfaces that come in regular contact with water should have 5-6 coats for added protection. Some people have many misconceptions about the waterproof quality of cork, but as experts point out, cork works perfectly well against moisture.
Can Cork be Stained?
Yes, cork can easily be stained or painted. Although it appears different from other wood species, the same sealer and stainer products used for traditional timbers (oak, pine, etc.) can be used on it.
This makes cork a great choice for flooring. Some people worry about the durability of cork flooring because they assume that it will be like bulletin board material. But in reality, the cork used in flooring is about 4-5 times the density and has a different binder material to provide moisture resistance.
You can check out this article on eHow to find out the right steps to stain a cork surface.
In this article, we discussed how cork is a naturally impermeable material. Because of its composition, it absorbs very little water, which is why is used as bottle stoppers and in floating devices.
Moreover, the cork industry is environment-friendly, giving all products cork products an upper hand over many other synthetic alternatives. As a wine stopper, there is really no substitute for cork at the moment.