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Hi there, 

I have Amaco Potters Choice glazes and they say that they should be fired to cone 5/6. I am very new to pottery and was wondering what this means in terms of the firing schedule in my Nabetherm kiln. I have been trying to get the effects as shown on their website but they do not look very similar! What would the different outcomes of cone 5 and 6 be? Thanks so much and really appreciate any help, lara

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Hi Lara!

Your glazing results will very likely be influenced by the clay you are using.

Typically, glaze fire to bring the clay to full maturity - where sufficient temperature and time (together, "heat work") have vitrified the clay (Maturity (digitalfire.com) - and fully melt the glaze. Also typical, avoid over firing, which can cause a myriad of problems with both the clay and glaze, and under firing, which may leave the clay porous (immature) and glaze not fully melted.

Please identify the clay(s) you are using, your firing schedule, and post pictures of your glazed work as well as examples of the results you're looking for?

So much depends on the clay, thickness and size of the work, the surface of the work, how the work is bisque fired, the glaze(s) selected, how the glaze is applied, etc.

Cone chart indicates just 53F degrees (at 108F rise per hour) between cones five and six, however, a half cone can make a big difference - depends on many factors. I'm firing to cone five now after starting out shooting for cone six. Some of the clays and glazes I'm using tolerate a wider firing range - looks fine at five or six - but two of the clays I use definitely come out better firing a bit cooler and also benefit from a drop and hold.

Fahrenheit Cone Chart (clay-planet.com)

Perhaps you are looking at glaze effects that result from layering two or more glazes?

Edited by Hulk
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22 minutes ago, Sorcery said:

Usually thickness is the problem with PC glazes.

This can be a factor with any glaze, especially when  trying to replicate someone else's results with the same glaze.

If your colors are under-developed, you may be applying too thin - even with the 'correct' number of coats.  The recommendation of 3 coats brushed is based on the assumption you are applying each coat the same thickness they do.  I've found that I sometimes need to apply four coats to get the total thickness needed, because some glazes flow more, and wind up going on thinner than others.

Also - when brushing, make sure you apply each coat in a different direction from the previous coat.  For example:  I usually apply the first coat up & down, second coat sideways (around the pot), and third coat up & down.  This helps even out the thickness, and avoid noticeable streaks & brush-marks when it's fired.

Hulk mentioned glaze results being 'influenced' by the clay you're using...  Most of the examples shown on Amaco's website an literature are on a very light colored clay (generally a buff stoneware).  If you're using a darker clay, the glazes will look different - even if you apply and fire exactly the same way they do.

If you can post some pictures, in addition to telling us what clay you're using, tell us which glaze(s) you used on each piece.  There are folks on the forum that use a lot of PC glazes, and knowing which glazes might lead to experience-based tips for those specific glazes.

 

Re: firing schedule - I recently emailed Amaco, asking about their firing schedules, and they sent me the attached.  I haven't actually tried it yet and, as it says, "all kilns are different" - but it might give you a reference point to compare with.

PC_FiringSchedule.pdf

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Hi there. 

Thank you SO much for all your help - I really appreciate it! 

The clay that I am using is White Stoneware PF560 (It fires 1120∞C - 1280∞C ). 

The firing that I did on bisqueware was using a Nabetherm Kiln:

1) room temp --> 600degreecelcius at 90 degree per/hour

2) 600 --> 1220degreecelcius at 130 degree per/hour

3) soak for 30 minutes

For all glazes, I did 2 layers of one glaze followed by 2 more layers of another glaze and so 4 overall - they have all come out as quite whiteish. 

The colours that I did were:

Iron Lustre (under) + Light Sepia (over)

Iron Lustre (over) + Light Sepia (under)

Blue Midnight( under) + Light Sepia(over) - I am quite happy with this result

 

Do you think that the colours would be more pronounced at different temperatures? The outcome does not really look like it was intended to (I have attached the images of how it is meant to look)! I would love to know how I should adjust the firing schedule! Thanks so much for all your help :)

 

 

 

Glazes.jpeg

WhatsApp Image 2021-01-03 at 22.55.50.jpeg

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Screenshot 2021-01-03 at 23.28.41.png

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Your Light Sepia over Iron Lustre looks very much like the example here, which says it's two coats of each.

I haven't tried the #34 - but have done a lot of combinations with # 33 - and Iron Lustre definitely needs to be thicker to get the blues to show up.

Looking at the sample pic's you posted, and others on Amaco's site, it looks like the white in the Light Sepia tends to dominate whatever it is applied over.  On your two pieces with #33 & #34, it looks like that white float is there - but the glaze just wasn't applied thick enough to really develop to the extent you see in their examples. 

I would say don't mess with the firing schedule, just apply the glazes thicker...  either thicker coats, and stay with two of each,  or apply each coat the same as before, but do three coats of each.

For what it's worth - even though it's not what you were aiming for, I think  the Iron Lustre / Light Sepia combination still produced an interesting result, and a couple of nice looking bowls.

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6 hours ago, larathompson288 said:

I would love to know how I should adjust the firing schedule

I would fire to cone and I am not a fan of a soak at the top unless one has a specific proven reason for it. My experience it often causes more issues. If you do not fire by cone it is fairly simple to get a good approximation for it by following the Orton chart. The requirement is in approximately the last 200-250f or 100c of the firing (your last segment)  pick a final temp and speed from the cone chart. So if you look below and we pick cone five from the chart using the center column (60c/hr) we subtract 100c which is our starting final segment temperature.

So final segment = start @ 1086, rate = 60c per hour, finish at 1186c  This is generally a reasonably accurate way to finish at cone 5. The last 100c is where most of maturation is done so getting this correct is pretty critical. Witness cones are always suggested to confirm slightly over / under firing.

 

 

6BB28364-C56F-40C4-9B31-CBE9B8962994.jpeg

Edited by Bill Kielb
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7 hours ago, Rockhopper said:

s you posted, and others on Amaco's site, it looks like the white in the Light Sepia tends to dominate whatever it is applied over.  On your two pieces with #33 & #34, it looks like that w

@Rockhopper thanks so much for the advice on glazing - I will try and apply some thicker coats and maybe take some more detailed notes of exactly what I have done so that I know for sure what is happening (it is easy to lose track I find:unsure:)

@Bill Kielbthanks so much for the info on the firing schedule. I am not too familiar with glazing cones but that is really helpful. The Amaco glazes say fire to  'Amaco Potters Choice Glazes | 1200°C - 1240°C'  - and so will it not be underfired if I go to 1186degress? (Sorry I am very unknowledgable)!

Would it therefore make sense, based on the chart to do the following in the kiln for the Amaco glazes:

Segement 1: room temp --> 600degC at 100 degree/C per hour

Segment 2:  600 -->1086 degree at 150 degree per/hour 

Segement 3:  1086 --> 1186 at 60degreeC per /hour

Segment 4: soak 20 mins

 

I am slightly confused about what the difference in the higher/lower firing ramps in the initial segments produce. Below is the advice that I got from a pottery supplier:

80-100˚c to 600˚C then 100-150˚c to 1220 -1250 ˚c with a soak of 20 -30 minutes --> for example, what is the difference between going at 80degrees and 100 degrees in the first segement?

 

THANK YOU SO MUCH FOR ALL YOUR HELP - really appreciate it :)

 

 

 

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@larathompson288  Potters choice is a cone 5/6 glaze, see below.      Which at 60c per hour is 1186c  to 1222c which is a good speed to penetrate most wares and even things out.  The faster you go the more likely the kiln will be less even. The center column on the Orton table is 60 degrees per hour because it is a common ideal speed for many kilns and most wares.

so let’s say you insist on 1200c as your final temp, then 1200 - 100 = 1100c. Your final segment starts at 1100 and fires to 1200 @ 60c per hour

so 

Segment1 - Room temp -> 600c @ 100c per hour

Segment  2 - 600c -> 1100c @ 150c per hour

Segment 3 - 1100c -> 1200c @ 60c per hour

No soaking unless you would like to fire to cone 7

And most common is to add witness cones to the firing to see where it ends up and make minor adjustments to your top firing temp from there. Without witness cones, you have no idea of what cone this was really fired to. However, using the correct speed for the final segment will very likely get you close

Amacos own schedule above shows they fire just over cone 5 and not quite cone 5-1/2. If you want to match their results, best you fire similar to them, applied like them on similar claybodies.  Firing to cone 6 or 7 because of a hold likely will reduce the chances of matching their firing.

When folks fire to cone 5 with a 20 minute hold their witness cones end up at cone six. By adding extra time they increase their heatwork one extra cone without hitting cone 6 typical peak temperatures.

No holds at the top unless you have a real reason such as above. It generally doesn’t even things out and usually causes folks trouble. Fire the right speed per the chart and this will generally give repeatable results.

Witness cones are made of glaze and calibrated to bend when the right amount of heat and time has passed. This is known by many as heatwork.  
Glazes are made of silica, alumina and flux. The silica and Alumina will not melt below about 1600c on their own so the flux helps them to melt into a nice glass. How fast does this happen? Follow the Orton chart. It’s your best consistent indicator you have supplied the right amount of energy and most of this melting occurs during the final segment! Go 60c per hour from [desired cone temp - 100c] to [desired cone temp] and you have done your best.

 

272DEEE2-A9B9-4320-B27C-3A3CC9FD4B42.jpeg

Edited by Bill Kielb
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Lara,

I did not see a reference to your bisque fire temp/cone? Many times the adhesion of the glaze in application depends on your bisque temp, and your pre-glazing preparation. Many have had to adjust their bisque temp to balance the absorption of the glaze in application. At the same time washing a pot in different ways affects the way the glaze is applied. Could you describe your glazing prep for glazing? Washing the pot with a damp sponge, dipping in water, rinsing under running faucet, no washing?

 

best,

Pres

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