Agree with monstercookies about everything, though I might also include FimoSoft
(FimoClassic is fine) in the list of too-brittle clays along with Sculpey III (and SuperSculpey, and original Sculpey).
You'll also want to be sure the clay you use for the buttons is well conditioned to make sure all the ingredients are well mixed, and also be sure the clay has been fully baked/polymerized, for the best final strength.
There's also loads of info on making polymer clay into buttons
(for functional or decorative use) on this page of my polymer clay "encyclopedia":http://glassattic.com/polymer/buttons.htm
Also as mentioned, you can use various materials and/or techniques to give polymer clay a shiny surface (polymer clay alone doesn't need "sealing" though).
Liquid "varnishes" of various kinds can be used, though for a glossy appearance you'd want to use one that came in a "Gloss" version. Some types of those cannot
be used on polymer clay though since it is a plastic (any that are petroleum-solvent based and must
be cleaned up or thinned with paint thinner/etc--also no dry cleaning for polymer clay), and some of them just aren't as tough as others (and may become scratched or become cloudy from absorbing humidity).
The toughest clear liquid finishes are the clear polyurethanes (water-based) like the Varathane brand, and acrylic floor polishes like Future or Mop 'N Glo. Some of the brands of "sealer" or "varnish" you'll see in craft stores are actually the same thing just rebottled (e.g., Studio by Sculpey's Gloss Glaze finish --note that this is different from just "Sculpey Gloss Glaze" which is clear in the bottle and probably more like their older, less-wondeful version of a gloss finish).
Less-tough liquid finishes are things like clear epoxy resins, "acrylics mediums" intended to mix with acrylic paints, permanent white glues or "dimensional" white glues, clear embossing powders, etc.
None of those really make the clay item much stronger (unless perhaps naturally thick, or applied with multiple coats for a little more strength), but using liquid polymer clay can make a clay item stronger (though if it's not a very thin coat or heated high enough, can make the surface a bit cloudy). For most button shapes you wouldn't need to worry about strength though (as long as well conditioned and baked, and only strong clays are used), but you'd probably want to avoid any "projections," or any thin areas next to thick areas so that lot of stress could be put on them.
There's much more info about using liquid finishes
, as well as liquid polymer clay and epoxy resins, on these pages:http://glassattic.com/polymer/finishes.htmhttp://glassattic.com/polymer/LiquidSculpey.htmhttp://glassattic.com/polymer/other_materials.htm
Because polymer clay is plastic it can also be "sanded-then-buffed
" (after curing) to get even smoother surfaces that can range in appearance from beautifully-satiny all the way up to super-high-gloss (all the way to gloss will require an electric
buffer instead of buffing by hand). Here's more info about doing that:http://www.glassattic.com/polymer/sanding_tumbling.htmhttp://www.glassattic.com/polymer/buffing.htm
(and when using a Dremel/rotary tool:) http://www.glassattic.com/polymer/tools_Dremels_worksurfaces.htm
A lot of clayers prefer the look of sanding and buffing to using liquid finishes but many clayers also use polyurethane/Varathane in particular when they have something they want a gloss on (or need to seal something onto the surface or prevent things added to the surface from oxidizing).
As for washing and drying
, those things won't really affect bare clay negatively and probably wouldn't affect the regular liquid finishes (though Future is thin so may eventually disappear) unless there were really-rough-surface things in the machine that would be rubbing against them for awhile (acting like coarse-grain sandpaper). In fact polymer clay can come out kind of buffed in the process of being heated and rubbed on by clothing (and some people even intentionally put clay beads in the dryer awhile to buff them on purpose).
You'd also want to wait at least a week for any finish to "cure" as well as to dry (and perhaps also do the "re-baking" technique which hardens finishes like that even more --see Baking
page at my site for more on that).
And you'd also want to avoid actually soaking
the buttons/clothing for several hours or more, since long-term contact with water can affect acrylic paints and finishes over time (making them cloudy, or even loosening from their surfaces).
Have fun and show us what you make!