A Dowel is just a cylindrical rod of wood or metal. You might have one as a closet rod or in your garden and they can be found in nearly every craftstore and hardware store.. Wood dowels are milled with the grain as straight as possible along the length of the dowel. This makes them strong and stable. They are cut with the endgrain at the top and bottom of the dowel, just as a tree would stand. So they are more absorbent at those ends. This will often cause them to look darker than the surrounding wood, even of the same species, as they absorb more oil or finish than the facegrain If the grain pattern matches well, or you can see face grain, it is more than likely a plug and used to cover a screw or bolt.
Dowels are a great mechanical joint and can aesthetically complement a piece of furniture. A well fitted dowel will add extra strength when driven through an existing joint, as opposed to glue alone. Dowels are often glued into the piece, but are sometimes left unglued if the furniture needs to be taken apart later.
Be cautious though, what appears to be a dowel may be a plug to cover up an inset screw. Greene and Green used their ebony plugs masterfully as exemplary architects of the Arts and Crafts movement. So while this isn't always a bad thing, screws can loosen over time, making a piece wobbly, especially if they are used as the main method of connection. If the screws do become loose and are covered by a plug, there will be no way to tighten them without drilling out the plug.
A well fitted dowel will have a paper thin border around it. There will be no wood torn out and the dowel will be flush with the outside of the cabinet. See above image.
A poorly set dowel happens when the drilled hole is larger or becomes damaged by the bit. There can be chips of wood missing or a gap. Here the gap is filled with a putty that doesn't quite match the surrounding wood. It won't really affect the structure of the piece, but can be a bit of an eye sore.
Keep an eye out and you will notice dowels are used in lots of modern and old furniture. If your buying a piece, ask if they are dowels or plugs. And keep an eye out for an over abundance of plugs in a piece, this may indicate that screws are the dominant joinery and it can make a piece feel a little visually busy. But ultimately, its all up to you.
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