You did not get a defective mug. That hole you see in the bottom of your ceramic travel tumbler is supposed to be there.
It is a unique feature of certain types of porcelain to go cups and without it, they would not work.
If you have a single walled porcelain tumbler, or one made from metal, glass or plastic, you should not be seeing a hole. If you are, there is a good chance you DID receive a defective product.
But if you have a double walled ceramic travel mug, it will almost certainly have a hole in the bottom. In fact, any porcelain cup with a double walled construction will have this hole. On some mugs, the hole will be plugged up, while on others, it will be open.
Here is why it they have a hole.
Why Do Coffee Cups Have Holes?
There are actually two main reasons.
First, when the pottery is fired in the kiln, water that was trapped in the clay is boiled out. In a double layered mug, the steam that forms from water evaporating in between the two layers needs to escape. Hence the hole.
Without it, the mug would burst open from the pressure created by the steam looking to escape.
You may be wondering why they don’t fill the hole after the cup has been fired?
Well, that brings us to the second reason for the hole.
The space in between the two layers of ceramic is filled with air. When air heats up, it expands. Every time you pour a hot drink like coffee or tea into your mug, it heats the air in between the layers, causing it to expand. The hole offers a path for the expanded air to escape. Without it, the cup could burst.
If you’ve got one of these travel mugs, you’ve likely noticed a big problem with the hole. When you wash the cup, water can get inside the hole. It can be difficult to dry out the inside once it has gotten wet, which can lead to the formation of mold.
For this reason, the hole is often plugged up. If it isn’t, you’ll want to take extra care when washing your tumbler. Do not put it in the dishwasher, since that guarantees water will get inside.
Instead, hand wash the mug, taking care not to get water in the hole.
If you do get some water in between the two walls, never fear. This post tells you how to get it out.