Bubbles in Glaze

[colekeller]

Hey all, 

Lately I've been ramping up my slip-casting production and have been looking for a better way to cut off the sprue after the small vase is completely dry. I wait for the ware to be bone dry because this way it's rigid and I won't deform anything. I have found a solution in a miter/chop saw. It cuts the sprue perfectly every time and is super quick. The only problem is that when I dip glaze the objects small bubbles immediately appear on the surface where the saw blade cut the clay. It's as if the blade is leaving something on the surface that is not compatible with the glaze. This confuses me, though, because after cutting the sprue I sponge the ware in that exact location to remove the sharp edges and then the wares get bisqued. How could it be that something from the saw blade is still on the surface after all that? Could it be that the blade is burnishing the clay from friction? Still wouldn't the sponging fix that problem? I have tried multiple glazes and the problem persists, and the bubbles only happen where the blade touched. I have attached a photo of the wares after glaze firing. 

If anyone has any experience with problems similar to this or thinks they may have an answer as to what's causing it I would greatly appreciate your thoughts! I am hoping to continue being able to use the miter saw because it is dramatically faster and more precise than what I can do with a hand saw. 

I intend to thoroughly clean the saw blade and run a test but I'm afraid that if motor oil or lubricant are the cause of the problem those materials will continue to find their way to the outside of the blade due to the centrifugal action of the saw. 

Thanks again to this community for all the help,

Cole

[Babs]

Very fine dust?

Could try fettling the glaze on these spots prior to firing

Slight "burnishing" of pot prior to bisquing.

[colekeller]

@Babs Thanks for the reply! 

I don't think it's fine clay dust that is causing the problem, because even when I saw these by hand the problem does not occur (even though there is dust created by cutting the sprue)

I certainly will try fettling the glaze if the problem persists but I'm hoping I can fix the root cause of the problem, because it's only happening when using the miter saw and on the spot where the saw touches the clay. I'm worried that even with fettling the glaze is incompatible on these spots due to foreign bodies.

Cole

 

[neilestrick]

Is your saw blade coated? There are a number of different coatings used on blades, many of which will not necessarily burn out in the bisque. If you're not sure, go buy a cheap all-steel blade. No carbide, no Teflon, titanium, paint, etc on the teeth.

Personally, I don't like the idea of cutting clay with a chop saw. For one, you're making a lot of dust since you're doing it when the pot is dry. You need to have a really good venting system, or be working outdoors. Even if you're outdoors, you're going to be getting dust on your clothes and carrying it inside. Second, you're going to burn up your saw pretty quickly. Clay dust will get into the bearings and motor and destroy it.

[colekeller]

@neilestrick You bring up some really good points. I didn't even think about the coating on the blade.  I'm using a blade intended for wood with metal I believe, which makes me think it likely is coated. I'm now thinking that that is likely the problem.

Also, regarding dust, yes it's certainly an issue. I do work outdoors when doing this but as you said it's still not an ideal solution. I'm going to try this on some leather-hard pieces and see if that helps with dust while still avoiding warping. 

Thanks so much this clears a lot up!

Cole

[Magnolia Mud Research]

Try pre-wetting the rims before glazing.  I would consider applying a thin clay slip after the saw cut is completed to fill the pores in the edged. The pin holes in the photos seem to me to be related to air pockets bubbling through the glaze slurry.  The saw cut surface is different than the surfaces produced by the slip-casting molds.   Filling in the “pores” with either water or slip should make them go away.

[colekeller]

@neilestrick I just checked the blade I was using and it is in fact coated with carbide. Do you think this is most likely the cause?

[colekeller]

@neilestrick Disregard my last message - the blade is made of carbide steel. It is still, though, coated with a friction reducing rim coating. I will purchase an all steel blade and see if this solves the problem. 

[Chilly]

I think @Magnolia Mud Research has got it right.

The platelets align when slip-casting, and by sawing you've cut them in half across ways.  The speed of the saw is likely making this noticeable, 

No idea on how to cure it, sorry. 

Have you tried:

Really wetting the cut surface? 

Dipping it into more slip?