With this method, the first thing is to mostly grind out a hole in the mirror with a piece of iron (or other metal if desired) and then you make the plaster tool and finish the grinding with the plaster tool that you make.

       We do this by putting a pinch of the coarsest grit that you have and a few drops of water (we'll call this a "wet") on the piece of glass selected for the mirror and take the iron end cap or other metal piece and start pushing it around on the glass. You will hear a loud grinding sound which is normal. At the beginning, you can go over the whole surface just to smooth it out a bit. Spend most of the time in the middle of the glass as this is where you will need to remove most of the material. Don't work in just one small area though or you will not make the surface very spherical. You will quickly see that the grit becomes quieter in it's sound and the grit starts looking a medium grayish color. This is the point where it's time to quickly wash off the old grit (you don't need to get everything off, just most of the grit and glass fragments) and put on a fresh "wet" of grit and do some more grinding. The gray color is from the glass (pretty much a white color at that size) mixing with the grit. After the initial reduction in sound level, you may want to wipe around the sides of the mirror and pick up all of that grit that you pushed off and put it back on the face of the mirror again. Do this only with this coarse grit and you will be able to go farther with the grit that you have. Don't do it with the finer grits as you may pick up a piece of dirt and thus make a scratch on your almost finished mirror.

       One of the things you may want to make if you're a detail nut is a cardboard, wood or plastic template of the radius of the mirror you're going to make. You can do this by using a string attached at one end to a nail and a pencil (or pen) on the other end the distance apart of the radius that you're going to make on the material you've selected and then cut the curve out by whatever means best for the material. You want the convex curve in the material to mate to the mirror for a regular newtonian mirror. Regular use of the template will insure that the hole is pretty spherical and give you a rough idea of how deep you are. If you really want to keep track of your depth, you can also make a spherometer. There are several places on the web where pictures and even some crude plans are of spherometers are. They aren't hard to make if you have a lathe but they do want a good dial indicator or micrometer head in order to work well.

       Measure the Sagitta of the hole you are grinding as you go and when you get to about .020" of the final value it's time to make the Plaster Tool and finish the grinding. The butt end of a drill or a feeler gage can be used to test for the correct depth along with a straight metal ruler. Remember that you want to make a spherical surface and you may want to measure the depth you are doing in more than one place to insure that you are indeed getting something resembling a sphere. The accuracy doesn't need to be all that great but the shape shouldn't be a hole in the middle of the glass and the outer 1/2 be a flat surface that hasn't been hardly touched.

       After you have the mirror carved to about 80% of the depth, we'll start making the plaster tool by placing the tiles on the mirror and trimming any tiles that are over the edge of the mirror. We do want the tiles to go as far as they can to the edge of the mirror without hanging over the edge. If you do this with the plaster dam already in place then you will know what the edges of the mirror are. You don't have to exactly fill out to the edge all the way around either. If desired, you can just use whole tiles to fill out the area of the mirror. You can either crunch the tiles closer together to make them fit or cut them with a tile cutter, it's your choice. The tiles should be laying down flat on the mirror or the plaster will quickly flow underneath the tiles when you do your casting.

      After the tiles are sitting on the mirror, it's time to make the dam to hold the plaster you're going to pour. The cheap and dirty way is to just use some aluminum foil or such thing and hold it on with duct tape. For really cheap, just duct tape is ok but you may run into the stuff grabbing itself and making it hard to get a nice looking tool. You do want to make the tool about 1:6 ratio or so for thickness to diameter but don't get to picky as that's just an approximate size. After you're happy that the dam will hold the plaster you can put the tiles in the bottom of the cavity you have now made. If you desire, you can use some cooking oil or other mold release stuff to keep the plaster from sticking to the mirror and holding down the tiles.

       Mix the plaster so that it is a moderately stiff mix (the approximate method) or, if there are specific instructions for the ratio of plaster to water, use a scale to measure the water to add to the plaster. Mix the plaster well and make sure that there aren't any dry chunkies in the fluidized plaster or thick parts in it or the plaster won't cure properly. You have about 3-4 minutes generally to do the mixing and pouring. If you aren't sure about how long you have, make a small mix and see how long it takes before setting up. You will also have about 5 minutes from starting to clean up your tools before the plaster adheres to them so plan your work and work quickly. Your wife won't like it very much if you use her power mixer and don't clean the thing up. Just putting the stuff in water won't clean it up as plaster will setup very nicely even under water. You will also want to mix up enough plaster to make a tool that is about a 1:6 ratio for thickness. You can always mix up a second batch quickly if you didn't do enough the first time as long as the second batch is poured onto the first before the first sets up. This is akin to the continous pour method that is used for large projects with plaster and concrete.

       When you have the plaster mixed thoroughly, pour some into the dam and then shake or bang the plaster down a bit and then pour the rest in. Try not to make large bubbles with the pour. After pouring, pound or otherwise vibrate the mirror lightly on the table to get the bubbles that you did get into the mix out of the plaster. Don't get to rambunctious or you may get the tiles to lift off of the mirror and then it will take a lot longer to mate the tiles to the mirror. Just a couple of gentle whacks on the table top is quite enough to make the bubbles float up and away from the surface of the tool.

       After a bit you will notice that the plaster is getting warm. This is what plaster normally does and it will usually get warmer than about 120F or so. This heat is from the chemical action of the hydration of the plaster and is quite normal. The heat is from the plaster setting up and shortly after the tool gets as warm as it gets, and you can scratch the top without having water well out of the plaster after you make the scratch, you can start removing the dam. At this time, the plaster is soft but will hold it's shape. After getting the dam off, it is then the best time to get the back of the tool flat enough to not rock when you use it on the bottom as the plaster is still quite soft. Take a flat edge and just scrape the backside. Be careful with the tool as it has no real strength yet. It is also a good time to separate the tool from the mirror after you have flattened the backside. The time taken from the pouring of the plaster and the carving time is suprisingly short and is usually on the order of about 5 minutes or so after the plaster becomes stiff.

This picture is of a plaster tool after having finished grinding a mirror. The tiles are fully buried in the plaster so they won't have any problems with them coming loose. This particlar tool got made with a plywood backing so that it had solid material on the backside so that a socket for a grinding machine could be put on it. When you first cast the mirror you will note that the tiles don't fully contact the glass and some plaster got on top of the tiles but don't be afraid of this and destroy the tool as this is normal and the inital grinding will put them in good contact. Note in the lower left side, the tiles are not as well aligned as the rest of the mirror. This is also OK as you don't have a flexible surface like a lap does to cause grinding problems.

      Let the tool sit for a day or two to fully harden. You can use it right away but the plaster isn't up to it's full strength yet and you may knock an partial edge tile loose if start too soon.

       Now that you have the tool made, you can now finish grinding the mirror with the tool using the roughing stroke with the mirror on top of the tool. You will need to get all of the tiles in contact with the mirror at this point and you may have to do a little bit of extra grinding in order to insure that all of the tiles are in contact. If the radius starts getting too short, you can lengthen the radius by putting the tool on the bottom and the mirror on the top.

       As soon as all of the tiles are showing at least 90% of their surface is ground, you can call the tool and mirror properly mated and you can start down through the grits. You're then done with this section and can go and do your fine grinding.