This can be created by heat or moisture, or from both heat and moisture. Most commonly from a hot coffee cup or from water left on the surface for a long time.
There are two types of finishes that are used on wood. They are either polyurethane based (water) or oil based. I'm not sure what the type of finish is on my surface, but the softened hardness of the finish, smoother finish, and cloudy nature of the stain leads to the probability that it is water based.
This unfortunate little blurb on the internet recommended the following:
Q: One of my relatives placed a hot cup of tea on our wood dining room table and it left a huge white ring? How do I remove the ring from the table?
A: The white ring is moisture that has been absorbed by the finish. When you do get white rings appearing, this is a sign that the finish is wearing out and losing its ability to repel moisture. The heat from this cup of tea softened the finish and allowed the moisture to get into the top layer of finish. In this case, since it is a new white ring, sometimes you can be successful using a hairdryer on the low/warm setting to evaporate that moisture back out. However, if you have a piece of furniture with a white ring that has been in there for some time, we need a little bit of abrasiveness to remove it. I usually get a pad of 4-0 steel wool - the finest steel wool they make. The steel wool by itself is going to leave scratches, so I first pour onto the white ring a finishing product, usually Minwax® Antique Oil or Minwax® Wipe-On Poly, and then use the 4-0 steel wool on top of the oil to lightly abrade the surface. Take a rag, wipe it off, check it, and gradually you'll see that white ring disappearing. Then take Minwax® Wipe-On Poly or Minwax® Antique Oil and go over the entire surface of the table, because that white ring indicates the finish is worn out. It needs another coat of finish to prevent that from happening again.
This recommendation is pretty bad because the stain has probably penetrated the full thickness of the finish which would require the removal of all the finish and probably a pretty ugly refinish if you can't find the exact stain or finish used. Also, I don't have the patience to sit around with a hair dryer to see if that would work.
Try this method which requires next to no skill and with tools you probably have around your house already! This instructable will work with fresh or even really old stains!
Step 1: Tools you will need:
One iron, and some clean fabric, preferably 100% cotton, clean and dry. I'm using a pillow case and some scrap pieces of fabric.
Step 2: Prep the Iron
Lock off any steam function and set the temperature at the absolute lowest setting.
The temperature should be low enough so that you would be able to put your hand on the surface comfortably.
This will be your starting setting.
Step 3: Prep the fabric
Step 4: Place fabric and iron over the stain.
The fabric disperses the heat evenly and gently over a large surface reducing hotspots.
At the lowest setting, you can leave the iron on the surface for hours. *AT YOUR OWN RISK*
Your two controls are the thickness of the fabric and the temperature control of the iron.
As the heat moves from the iron through the fabric, it will be absorbed by the wood over a large area. As long as the temperature remains low and the fabric thick enough, you will not damage the surrounding surface of the finish. If you use this instructable properly, you will be using NO MORE THAN 10% of the heat range of the iron.
On occasion, you can test the heat transfer by sliding off the fabric and placing your hand on the surface. The temperature you are trying to approximate is a warm cup of coffee. (Not a cup of freshly boiled water)
If you cannot touch the surface, YOU HAVE STARTED THE SETTING ON THE IRON TOO HIGH!
Always start at a lower setting and slowly move up. If you check repeatedly, you will not damage the finish.
If you want to transfer more heat to the stain, remove layers of fabric between the iron and the surface, or increase the setting on the iron (incrementally). Remember 'warm cup of coffee' temperature.
The slower and gentler the process, the safer you will be to the surrounding finish of the stain. If you keep to the warm cup of coffee temperature, you can take as long as you need to 'lift' the stain.
Depending on the stain, I've taken as long as an hour to remove a stain from surface. If you have the temperature at a safe even level, you can leave the iron on the surface almost indefinitely (at your own risk).
If you are doing it properly and the finish is the right type for this instructable, then you will see the stain will change from cloudy the opaque to smaller to clear. If you see changes, then you are at the right temperature. The heat will cause the stain to slowly release the moisture back into the fabric. If the fabric started off a completely dry, the fabric will absorb whatever little moisture that is in the stain.
As the stain releases most of it's moisture, the last remnants will take the longest as it's the deepest part of the stain. Do not increase the temperature. You are at the right temperature. It will just take a long time. DO NOT FORCE IT BY INCREASING THE TEMPERATURE.
Step 5: And you're done!
Well it's not ready yet. I did say that it would take more than an hour. Besides, it would be the picture of a normal coffee table. Pretty boring.
While I was typing this out, I went to check the temperature and it turned out to be too low. It was the temperature of lukewarm coffee!
So going back to my own instructable, I raised the temperature on the iron leaving the thickness of the fabric as is.
Start low and slowly move up. The slower you transfer heat from the iron through the fabric to the surface, the less likely you will raise the temperature so fast that you will scorch the surface. The older the stain, the lower the heat.