Glaze "Overlaps"

[Rick Wise]

I regularly use 2 base glazes shown below -- one a gloss (Fat Cream) and the other a matte (Val Cushing VC 71 Matte Base).  If I have a large piece and have to dip with an overlap using the Fat Cream -- no problem.  The overlapped double dipped line disappears in the firing.  But if I try the same thing with the VC Matte, I get a distinct line on the finished piece where the 2 dips overlapped.  One melts away in the fire.  The other wont.  Can anyone help me understand why this happens and how the VC Matte recipe might be adjusted to make the overlap disappear?

• Neph Sy      45
• Gerstly Borate  25
• Silica  20
• Ball Clay 10
• Zircopax  8
• Rutile   1

VC Matte

• Custer Feldspar  40
• Silica  16
• Whiting  16
• EPK 10
• Frit 3124  9
• Talc  9

[Bill Kielb]

The  top glaze has .48 Boron and likely melts at cone 04 (lowfire) so it probably levels out a bunch assuming fired to cone 6. The bottom glaze has virtually no boron and has gloss chemistry so likely it is matte because it is not fully melted to get the matte look. A wild guess is the VC turns glossy fired to its correct melting temperature. Not being fully melted would be a reason that the overlap would likely show up.

[neilestrick]

I would never expect an overlap to not show. I think what you have there is a rare occurrence.

[Hulk]

Might be able to get a sharper boundary with some help from tape and wax?  - tape, dip, wait for the glaze to dry some,  wax, wait for the wax to set up enough, remove tape, wait a while... dip the other glaze. 

...if you're using two different glazes.

On rereading, OP, you're asking about a boundary forming where same glaze overlaps, yes?

 

[Bill Kielb]

Could always hold plate by foot with thumb on one rim location, dip complete and touch up where your thumb was. Need big enough dipping container though and helps to have  something to strike the drip edge. Else two step with same method pouring back of plate first then flip using same technique to pour front. One touch up point - thumb.

[Rick Wise]

Hulk -- that is correct: same glaze.  The problem is just that the final product shows the fact that it was dipped in 2 motions.

16 hours ago, Hulk said:

Might be able to get a sharper boundary with some help from tape and wax?  - tape, dip, wait for the glaze to dry some,  wax, wait for the wax to set up enough, remove tape, wait a while... dip the other glaze. 

...if you're using two different glazes.

On rereading, OP, you're asking about a boundary forming where same glaze overlaps, yes?

 

 

[Rick Wise]

21 hours ago, Bill Kielb said:

The  top glaze has .48 Boron and likely melts at cone 04 (lowfire) so it probably levels out a bunch assuming fired to cone 6. The bottom glaze has virtually no boron and has gloss chemistry so likely it is matte because it is not fully melted to get the matte look. A wild guess is the VC turns glossy fired to its correct melting temperature. Not being fully melted would be a reason that the overlap would likely show up.

Thanks Bill.  Interested in what you say here.  If I am following you correctly, I could try to fire matte glaze higher which might result in its full melt (and thus a smooth no-line look) but might also make it gloss instead of matte?

[Magnolia Mud Research]

I solve the obvious 'overlap problem' by using adjusting the design of the item by converting the 'overlap' into a decorative feature.  some times this requires just changing the location of the overlap by the way I dip the item into the glaze slurry.  other times I use a brush loaded with the glaze to make additional 'overlaps' elsewhere.  

LT
 

[Mark C.]

20 hours ago, Rick Wise said:

Hulk -- that is correct: same glaze.  The problem is just that the final product shows the fact that it was dipped in 2 motions.

 

I think trying to hide how it was made or glazed is not a good idea. Overlaps very -depending on glazes-just part of the deal. If you want  this to be like car paint striping use the wax over the 1st glaze right to the edge and then dip and wipe off any left on wax-pain in the butt but very clean lines.I was taught that finger marks and throwing marks are all part of my process and should be left. I embrace this idea in glazing and all things pottery from reduction variables  in kiln to finger marks. 

[Bill Kielb]

4 hours ago, Rick Wise said:

Thanks Bill.  Interested in what you say here.  If I am following you correctly, I could try to fire matte glaze higher which might result in its full melt (and thus a smooth no-line look) but might also make it gloss instead of matte?

Yes,

It was an observation of why you might experience more trouble with one than the other in response to your question. Cone 04 boron not necessarily a great thing at cone 6. The limit generally is just south of 0.6 where the excess  starts  to become troublesome. Truth be told the earth is cone 10, it’s our geology and that is where the earth melts. Folks figured out how to lower the melting point to cone 6 and cone 04 most easily by addling boron.   (There are other ways)

To your point, on point observation  - when we design clear glazes to melt over heavy underglaze  (some are refractory) we generally include a little extra boron say instead of 0.15  (cone 6) more like 0.19 to melt closer to cone 4-5 ish.  When we test these I notice they melt so completely at cone 6  it’s nearly impossible to  tell on a test tile where the second coat begins on a double dip tile.

so your  first glaze  should melt at 04 (lowfire) and you hardly notice the intersection when half dipping the glaze. Makes sense!

As to  making the overlap disappear, most often it’s very difficult with most glazes  so I would  rather suggest you change your method if you want an even canvass. Spraying makes this easy and very uniform or there are pouring  techniques that allow you to get similar results  by carefully pouring and touching up where your finger or forceps marks remain.

As to matte finishes as a result of under firing a bit,  lots of opinions about this. This is common with many glazes created by multiple trials and firing to visual taste.  For me I  prefer to  create  mattes as true mattes that are mattes fired to their intended cone. They are mattes by chemical composition and not necessarily by under firing or slow cooling. A true matte over fired will be a runny matte with crystals at the bottom of the runs. Just a personal preference though and there are many many opinions on this. Some chemistry, some not, some experience, some trial and error. When I spoke  with John Britt  about this at one of his workshops, he freely admitted he was not a new chemistry guy and had a lifetime of accumulated knowledge. Who would disagree with that?

Great guy, fun workshop!

As to firing the second glaze higher, it probably won’t make your overlap disappear and will probably turn it more glossy.