You might have inherited your crystal decanter from an aunt or grandparents or your parents, or you might have gotten it as gift, and you know its value if its a true piece, particularly such as the antique decanter. The more quality a decanter has, the more it needs proper care and cleaning to retain its sparkle that is so characteristic of items made of crystal.
I got my first crystal decanter from my grandma for a birthday gift several years ago. It was in the family for a long time and every generation took really good care of it. I remember when she gave me the best wine decanter, she told me how to keep it clean and take care of it. She told me that a good housekeeping always entails having your crystal glassware sparklinkg clean. Needless to say, I put it away and forgot all about it. Recently I got some nice crystal glasses - again for my birthday - from a dear friend, and this is when I remembered my Riedel crystal decanter given by my grandmother. I took it out from its hiding place and remembered how to clean it nicely.
In the meantime I've come across various other ways to clean a crystal object, so I'm going to give you here several tips to do it. You pick and use the one tip that suits you mostly. Btw one thing I never do is putting my crystal decanter in a dishwasher. Crystal can easily break, scratch and leave marks that can never be removed afterward.
Depending on style and design of the decanter you happen to have at home, you might have some with a wider neck, and others with a longer and thinner neck that doesn't give much room for putting your hand inside. This is when you need to get creative.
Cleaning the crystal decanter the grandma way
The way my grandma taught me to clean my decanter (it is an older Riedel decanter with a very tight neck) is to use ice. If it's summer and you can't get hold of ice, use some from your freezer. Depending on the ice box you have it in, the actual piece of ice might be just right for the decanter opening or it might not fit in. If it doesn't, break a couple of ice cubes into smaller pieces. Don't crush them so they become ice powder, leave them still in pieces form, as they need to be solid for this cleaning process to work.
Add the ice cubes in the decanter and pour a bit of water on top. Next spin, swirl and shake the decanter lightly so that the ice cubes touch and scrape the sides of your decanter.
Basically the process of spinning and scratching will remove the dried up sediment from the sides of your crystal piece. After a while, when you see the sediment removed, simply throw out this water (by now the ice is also melted) and use a bit of soapy water to rinse it out. If you still see some deposits at the bottom of your crystal piece, do the process again and rinse it out afterward again.
As a side note, I've recently learned that you can also add a bit of salt together with the ice cubes, this helps the scrubbing action even better for removing those stubborn Port stains.
As a last step, use regular water to remove any soap that was left over from the previous rinse. This works quite well and while at times it seems that you need to swirl and swirl that decanter in your hands until the sediments gets unstuck, it's worth the effort and it hardly takes a few minutes anyway.
Something much simpler that I've read about before and also tried out was to basically use warm water and some dishwashing detergent. Don't get the water too hot though as the crystal might break. Then again rinse out with pure warm water to remove any spots left behind by the soap or dishwashing. This mostly works when the stains are not too dry and strong.