preparing cerium oxide

[crashtestdummy]

Should hopefully be starting the polishing phase of my grind this week so have been reading up a bit about it and it seems a lot of people soak their cerium oxide the day before so wanting some oppinions on this.I did notice with 600 Alu oxide that when drier it caused lots of stiction so should I soak it and if so, how are people doing it or should I just apply it as I have with other grit-wet the blank, sprinkle on the grit and then dampen down with water?

[Glasspusher]

Rich, no need to soak it, just add a teaspoon to about 300 cc of filtered water. Mix it in a squeeze bottle which can be used to dispense it, give it a shake before use.

Good luck,

John

[crashtestdummy]

Cheers John, I spoke to Damian this afternoon and he jogged my memory about what you had said.

Cast my pitch lap today but think I may redo it.I put the pitch on the blank too watery to start with but it dried ok and is the required thickness but it's a bit crumbly in places at the edge which I think I didn't dry long enough before breaking the excess off.I had to repair a bit on the edge by melting some pitch to fill a bit that broke off.I also make the mistake of trying to cut the channels with a grinder which is a bit silly when the pitch just melts and fills the channels back up going to cut the channels deeper with a saw blande and may try and see how it work for an hour or you think I should just start again?

[crashtestdummy]

Tried to cut the channels out with a saw blade and the pitch just started disintegrating so I'm goi g to start again from scratch.I've started chipping the pitch off the wooden disk but it's going to be a long process.next time I'm going to put the pitch a bit thicker and leave it a full day before I start doing anything with it or should I cut the channels early while it's still warm?always said I dreaded the pitch lap stage and I'm defonately dreading redoing it even more

[Glasspusher]

Rich, best to let the pitch cool off fully before cutting the channels.

Cheers,

John 

[crashtestdummy]

Ok John.I'm going to chip it all off on Friday and reheat it and cast it again and take my time.will leave it over night to make sure

[JRM]

Rich,

 if you can stick the tool in a freezer and get it cold when you take it out you can easily remove pitch from tool,the difference in expansion between the too

makes the pitch come of real easy.

B.T.W... adding some beeswax will help with the chiping , an ounce for every 2 lbs will do wonders..

Rick M

[Peter Drew]

I used the Texereaux method, casting strips of pitch then cutting them up into squares with a warm knife when cooled. Sticking them on with beeswax to a warmed tool makes a very smart looking lap. You can also have some spares in the event of a mishap. 

[crashtestdummy]

Sounds like a good meathod Peter

[mapstar]

Rich, no need to soak it, just add a teaspoon to about 300 cc of filtered water. Mix it in a squeeze bottle which can be used to dispense it, give it a shake before use.

Good luck,

John

What do you use to filter the water John? Is It better to distill some?

[crashtestdummy]

I still have about 4L of distilled water so sorted there.I got 5L off eBay for about £13

[Astrobits]

When Cerium Oxide is ground it is done in a water slurry. When this is dried to give the powder we use there is a tendency for some particles to stick together and form agglomerates ( this happens with nearly all powders when dried from a slurry ). These agglomerates tend to give rise to small scratches we call sleeks as they break up. Sleeks are very subtle and can be polished out quite quickly. During normal polishing this happens and we don't see the sleeks after a prolonged polishing wet. However, when figuring, the polishing wets tend to become short enough for these sleeks to still be present when we clean off the mirror for testing and we immediately panick about contamination.

The best way to to avoid these sleeks is to make up your polishing mix and leave it for a couple of days before use as the agglomerates usually berak down with water. Even better is to purchase the slurry form of Cerium Oxide which has not been dried but this doesn't seem to be easily available anymore.

Nigel

[Glasspusher]

Damian, I just use a water filter unit bought from a supermarket, the one I use is by Brita and has replaceable filters.

Rich, the pitch you had was Gogolz 55 and would have been of the correct hardness. Overheating can drive off the volatiles making the pitch harder so you may need to soften it with beewax, as mentioned, or pure turpentine or boiled linseed oil. Sounds like you may have overheated first time round so you might need to add a little something to soften it, heat it slowly next time!!!!

John

[mapstar]

Thanks john think I have one somewhere so will have a look?

[crashtestdummy]

I heated it over about 20 minutes at light heat and avoided it boiling at all so I'm stumped really.

Where can I get beeswax and how much do I add??

[mapstar]

Boiled linseed oil is easy to get hold of Rich from the usual source.

[crashtestdummy]

I don't understand though.the nail test was perfect and the centre of the tool was fine it was just the edges that were a problem.I think I may have just needed to let it cool for longer.

[JRM]

Rick,

 Heating to fast loses the volitiles, and can change the hardness of the pitch. I have 2 kgs og gugloz 64 when i melt it it should take 1 or two hours,

i will add 2 ounces of beeswax to this and mix it in during the melt,you want to stur it every now and then when heating an do not get it to a point

that it boils, you can add some linseed oil to it as well (5 - 10 ml) to soften it if it is too hard, the beeswax will help with the chipping.20 minute melt is way to fast

and hot, slower is always better.

Rick M.

[crashtestdummy]

Have had another go today,took about 30 mins to melt which I thought was slow.I'm leaving it overnight on the mirror to set and cool properly then will take it steady getting rid of the excess, bevelling it and cutting the channels.if it doesn't work out I will get some beeswax and some linseed oil and start again.

[Glasspusher]

Good luck Rich.

John

[crashtestdummy]

Cheers fella, you will be the first to know if I need help

[crashtestdummy]

Well second time and still not lucky.left the pitch on the blank overnight and then removed the foil and a bit of pitch had run under the foil so to get the foil off I had to peel it from under the pitch which left a little depression.the main problem is cutting the channels now,it's just maddening and it just chips too easily on the surface plus it would take me hours.I think I'm going to cop out on this way and cast a plaster disc to the shape of the mirror then as said by Peter, cast some pitch pieces about 1 1/2" square and about 5mm thick and just epoxy them onto the plaster disc after I've resin sealed it.at the moment I'm not in the right frame of mind to have a third attempt at the original meathod or I'm going to end up throwing the thing if it doesn't work again.plus I'm going to end up just destroying £50 of pitch.

Any further advice much appreciated

[fwm891]

Tried to cut the channels out with a saw blade and the pitch just started disintegrating so I'm goi g to start again from scratch.I've started chipping the pitch off the wooden disk but it's going to be a long process.next time I'm going to put the pitch a bit thicker and leave it a full day before I start doing anything with it or should I cut the channels early while it's still warm?always said I dreaded the pitch lap stage and I'm defonately dreading redoing it even more

Best way I've found to cut channels is the first mark out on the lap where the channels are going - run a line of washing-up liquid along the channel line and cut with a sharp stanley knife - don't force it and cut both sides at a slight angle so you end up with a flat bottomed 'V' when to full depth.

I also cast my laps onto a sheet of ali foil spread over the mirror. Stops the lap sticking to the mirror and provides a surface to mark the channels on, and protection to the lap surface when cutting. The resulting squares of ali foil peel off easily

[crashtestdummy]

I did use Ali sheet but I couldn't find any to cover the blank in one piece.I'm considering casting some pitch squares and sticking then onto the lap with some wax or just warming them up a bit so they just stick onto the other pitch

[JRM]

Rich,

   if you do the squares you just heat the bottom of them till they look  wet, not to much, then attach them to the tool pressing them firmly for

good adheson, after they are all on do a good cold press to mate them all up.

the beeswax is added to the pitch and melted with it, mixing it in good before the pour whether you pour a lap or cast a block to cut into squares,

the wax helps with chiping but it still chips, pitch is a stick messy hateful stuff to work with sometimes, every experience I have had with it has

been just that... an experience, hope it helps

Rick M