Jump to content
Scott G

2 part: 1 glazing greenware (single fire) 2 getting an even thin clear coat

Recommended Posts

Will post pictures as soon as I get home, but I read somewhere that one does not really have to fire greenware before glazing. I understand there is a risk, and I am increasingly frustrated with the quality of the pieces coming out of my kiln.  Kiln fires to Cone 6, I bisque to Cone 06.  I bought a 25lb mix of the Amaco Ironstone and have been super disappointed.  Switched to clear glaze (amaco mixing clear) because I started doing work with mason stains and marbeling clay bodies together, wedging nicely, and then throwing the pieces.  They turned out great, but I started experimenting with dipping greenware in the glaze, and brushing it on.  The dipped pieces came out just as beautiful as the brushed pieces, but the glaze pooled and or looks slightly milky in the areas where a little extra glaze collected after dipping. (clear glaze was not cheap, I want to try making my own, and I know there are many recipes but never sure where to start...

 

I triple checked that I measured and calculated the specific gravity properly (1.41 g/ml) and I'm wondering what the pros and cons of direct firing a greenware piece with glaze on it.  The color seems to have come out the same, but dipping pieces are tough cause I cant use tongs, and small buildup seems to be inevitable with a hand held dipped piece due to awkwardness of holding while dipping.  Also dont want handles to fall off into expensive glaze bucket...

  Brushing worked ok, but perhaps I should try spraying a light layer instead?

 

Current picture is greenware, but hoping this shows that I'm not some n00b with terrible output.  2 marbling types to date.  Ones for practice with terra cotta and stoneware, and the second with porcelain and  porcelain mixed with mason stains with the same wedging and throwing process.  I have lots of pictures documenting the process but I don't know if this would ever be requested for a How-To.  

 

Mostly looking for advice on:

1 - achieving a thin clear glaze (brushing vs spraying vs making my own glaze - since the 10lb bucket of mixing clear set me back $76!)

2 - pros and cons of direct glazing said greenware marbled pieces since coat will be thin and all the detail work lies within the clay so no thick / glaze run expected.  

 

marbeled mug.png

Share this post


Link to post
Share on other sites

Nothing wrong with being a noob.  Once-firing is an option, probably a better option for someone who mixes their own glazes since there are some things that can be adjusted.  Works better with some glazes than others too.  I've done it once, with my clear glaze I added 2% bentonite to help the glaze adhere better to the greenware.  Worked ok, I got a lot of microbubbles because my clay body is high in iron and I probably went a little too fast through the organic burnout period.  

If the clear glaze you bought was formulated for brushing, that might explain why it pooled more.  A dipping glaze will dry faster so you can simply hold the piece upside down until it stops dripping, no pools

 

Share this post


Link to post
Share on other sites

Single firing complicates things a lot. Glaze application is much easier with bisque ware. Every glaze is going to apply differently, depending on what's in the recipe. The nice thing about mixing your own is that you can tweak things, because you know the formula. As for your specific questions:

1. A thin coat of clear may not be a sufficient layer of glaze to make the piece feel smooth and finished. There's a minimum amount of glaze needed to really seal things over so the surface isn't rough. If you're thinking it need to be thin in order to remain clear and not go milky, that's a problem with the glaze, not the thickness. I've got several clear recipes that will remain clear even if they're on double thick. Mixing your own glazes gives you a lot more control over the properties of the glaze, and will save you a ton of money.

2. Glazing bisque is easier.

Share this post


Link to post
Share on other sites

Scott G, I have been spraying and single firing for 2 years now. I mix my own glazes, but have never altered any glaze just because it was being applied to greenware. If spraying, pay attention to the way the glaze looks while you are spraying. It took me 5 glaze firings to get the correct glaze thickness. Each glaze is different and will need to be learned how much to apply. I look for a velvet look, OldLady, who also single fires, calls it "chunky velvet". If you go past the velvet look the glaze surface will look filled in and smoothed over, and the glaze will likely run. If you get less than the velvet look the glaze will be rough. I have most glazes in the range of 1.45 - 1.55 for specific gravity, though a few are less and a few are more. Some folks glaze the greenware when it is leather hard or when it still holds some moisture. I wait till they are bone dry. Pour the glaze into closed forms like you do now for dipping, wait till they dry overnight then spray the exterior. Then wait till they dry overnight before loading into the kiln. Or you can wait several days between interior glazing and exterior glazing. I work after my day job hours so I can't get to glazing them all at one time. Pouring is easy and I will pour all in 1 night. Then they wait their turn till I can get to glazing the exterior, usually no more than a week. 

Search Steven Hill, he gives all kinds of advice on spraying and single firing.

Edited by dhPotter

Share this post


Link to post
Share on other sites

It can be tricky applying a clear glaze over darker clay. Any clouding will be much more apparant. The firing cycle you use for your clear glaze can affect it dramatically: a soak hold at the end of your firing to allow bubbles to clear is really important, and being aware of what temperatures the materials in your clay and glazes begin to decompose and loose their carbonates is also helpful. If you fire too quickly through the wrong temperature, any fluxes that begin to melt before your clay body is done releasing gasses will trap more air bubbles. This is why they sometimes recommend a hotter bisque for red or other dark clays, because they tend to have more sulphur and carbon compounds to release than more refined clays. I'm a believer that all things are possible, and that getting a nice application of clear over marbled clays in a once fired situation is very likely doable with enough testing. I personally choose to bisque my dark clays because as already noted, glazing bisque is easier, and a larger chunk of off-gassing is already done.

ps. Nice handle!

Share this post


Link to post
Share on other sites

I think Neils pots above covers most what I was going to say.

Making your own glaze is always cheaper that commercial glazes and you learn 100% control of a glaze that way.

I never consider the cost of a glaze I like and am using. I buy all my materials in bulk that way the cost starts low to begin with.

For basic materials-like you would use in your clear glaze consider buying them in 50# bags .

Share this post


Link to post
Share on other sites

Thank you all for the well thought out responses.  I have the hand pump sprayer which I will attempt a few pieces for single firing.  I did notice a few defects in the dipped bisqueware too, any comments happily welcome.

This whole size limiting thing makes it tough to share issues.  Had to screenshot the picture and then threw yellow stars where the pinholes and red stars with bounds around it to show the milky run from a not perfectly dipped piece.  Don't like that it is cloudy, would rather a thinner glaze to simply protect the body.

 

glazing_issues.png

Share this post


Link to post
Share on other sites

Has a drip run down from the rim where the vertical stripe is? Sometime where a clear is thicker it is milkier.

When you dip and lift the item out keep holding it upside-down so the drip collects on the rim. One or two (careful!) shakes will remove the drip, or you can touch it with your finger or a sponge which will attract the excess. Wait until the sheen is going off before you turn it the right way up and your drip won't run down.

If you have difficulty holding the base when you dip then you can turn a small ridge 5mm up from the bottom. This allows your fingers to get a grip and catches any runny glaze to stop it hitting the shelf.

I raw glaze too - sometimes you do have to experiment with the best approach especially with commercial glazes where you can't adjust the recipe. Experiment with small changes of water content (specific gravity) of the glaze slurry. This will change how quickly the thickness builds up during the dip.

Joe

Share this post


Link to post
Share on other sites

To check thickness of sprayed glaze, I've applied cut pieces of tape, which can be removed when the glaze is dry, leaving a bare spot with nice sharp clean edges - looks like an on purpose accent! ...which I typically fill with a contrasting colour. Any road, as the glaze on the tape goes away in the end, I'll scrape off a bit to check thickness once the sprayed layer has set up.

Edited by Hulk
oh yeah

Share this post


Link to post
Share on other sites

Everyone has covered most of your questions; glazing greenware leather/bone dry comes with its own set of difficulties, which most potters stray away from and just bisque your pots. There are times when it would be really nice to omit that 24 hours of bisque time, have limited fuel sources for both bisque/glaze fires, or want to be "greener" about the ceramics process. While Ive done it (not a whole, whole lot), I find single firing to be a pain in my rear for all the issues you've mentioned, or others have too.

Here's a couple things not mentioned yet; the reason why your bone dry/leather hard pots take longer to dry (leading to runs/drips) is two fold. Your pots still have significant moisture in them, leaving no room for that moisture from your glaze to go. Not only does this lead to runs/drips, but it also weakens the mechanical connection of your glaze to the pot; this can be an even bigger issue if you are burnishing your pots to a high shine (which are already difficult to get glazes to stick to...tend to shiver/crawl). Secondly, most commercial glazes contain additives which improve the flow/brushability of glazes; essentially it makes them "wetter" and  wetter longer. For glazing greenware, when not spraying, you want your glaze to dry as quickly as you can, with as little moisture as possible, lest you deal with those runs/drips, or worse, your pot becomes so saturated with moisture it begins to break back down into slop.

Many, many a processes are possible in ceramics; Tony Hansen did an experiment to see if he could make, and glaze fire pots in a single day to cone 10, and back to room temp in a 24 hour period. Is it possible? Most definitely, does it make sense, heck no! In my opinion, single firing creates so many variables and details that need to be controlled that unless you have omitted a lot of other details from the process, its too much to manage. Factories who crank out hundreds of thousands of (toilets, sinks, lamp bases.....) regularly single fire, and save a lot of money on fuel/labor costs by doing so, but their process is highly specialized, are VERY good at what they do, and have engineers who can formulate very specific clay bodies, glazes, and firings to have a maximum yield.

For us meager studio potters, unless you CANT bisque fire, then single fire is your only option, but if you can, make your life easier and do so.

Share this post


Link to post
Share on other sites

sorry that so many of you think single firing is difficult, wrong or whatever.   

glazing bisque is very difficult for me.  if there is the slightest error, a drip, a run of the wrong color or a spot of wax in the wrong place, there is no alternative but tossing the whole thing.   a spot of something that is obviously not intentional cannot just be removed easily.   can you just scrape off an excess of wax on bisque?  no, refiring is the answer.   can you wipe off the cobalt drip that happened accidently on bisque? no, washing the entire thing and drying it MIGHT wash off the cobalt but a shadow of it will still be there.  

with greenware,  scraping with a sharp knife and a touch of a damp sponge wipes away any trace of the error.    easy.     

since there are so very many different ways to work, please do not discourage people who might want to do something differently from your way.

Share this post


Link to post
Share on other sites
3 hours ago, oldlady said:

if there is the slightest error, a drip, a run of the wrong color or a spot of wax in the wrong place, there is no alternative but tossing the whole thing. 

Not at all. You can burn out wax by firing it up to about 800 degrees. You can scrape off and wipe off glazes, too. The vast majority will clean off just fine. In my studio we never throw out work because of glaze application problems.

I don't think people are trying to say that single firing is wrong, but for most people it is more difficult. For instance, it would be impossible for me to single fire a large percentage of what I make without breaking it. I would not want to try to put a liner glaze in a 15" tall, wide, thin porcelain jar without bisque firing it first. Nor would I be able to glaze the outside without investing in a spray booth, which I can't fit in my studio. I also wouldn't want to try dipping my mugs in the green state, because they are very thin and would likely over-saturate and fall apart. I also need to bisque fire my underglazes before glazing, or the binders and hardeners will prevent a good glaze application. There are tradeoffs and allowances that must be made with single firing, so it is not a practical system with all types of work or studio setups.

Share this post


Link to post
Share on other sites

thank you neil for adding your opinion which is really the same as mine.  what you make dictates  how you fire.  

your previous post made it seem that any single firing was wrong and should not be attempted.  i just want to let new people know there are alternatives to what they may think of as definitely only one way to do something.   sometimes it seems that they leave their brains at the door and only do exactly what they are told.   potters are allowed to think and reason as well as follow instructions. 

when i only fire a few times a year, warming up a single piece to 800 degrees is a big deal so it is easier to toss that once a year mistake.

Share this post


Link to post
Share on other sites

I am blown away at the amount of helpful responses to this post.  Thank you all! I decided that single firing would be fun, but without the ability to spray the glaze on, dipping greenware makes me nervous, so I will plan to fire bisque first.  As for the glaze defects, I found a strainer and strained the crap outta the glaze and found the little chunks in the glaze, which is annoying cause AMACO dipping glaze instructions didn't even mention having to strain them.  Also, $70+ for the small 1.5 gallon bucket is outrageously expensive after using most of it already.

Slightly off topic, is there a favorite glaze book or recipe for a clear glaze that people are happy with?  I do most of my shopping at Rovin Ceramics in Ann Arbor MI and they have a wide variety of dry goods I can use.

https://rovinceramics.com/collections/dry-materials

Been experimenting with Mason stains and porcelain, clear glaze is almost gone and the next batch will be ready in ~1 month so I need to get a head start on glaze experimentation.  

2019_sjgmugs.jpg

Share this post


Link to post
Share on other sites

Whoops, good point Neilestrick.  Very nice gallery pictures btw. 

Firing to cone 6, and my marbled clay is rovin clay Terracotta with fine grog cone 5 and high fire light stoneware cone 6-10 and the marbled porcelain is mid range cone 6 as well.  Is there a versatile clear recipe that would suit all 3 clay bodies?  

Had such bad luck with actual glazes that I started doing the marbling to enable clear glaze usage and have liked results so I plan to do this for a while. 

My kiln is an old kilnsitter so I've been careful when it trips to turn back on right after for 20-30 mins with all 3 knobs at Medium to give glaze a chance to finish bubbles and controlled cool down. 

Share this post


Link to post
Share on other sites

First, get rid of the cone 6-10 clay, because at 6 it's underfired. Find a body that matures at 6. You've got 3 radically different clay bodies there, so it may be difficult to find a clear that will work on all 3. What brand/model of porcelain are you using? They're not all the same.

Share this post


Link to post
Share on other sites

scott,  there is a brand name somewhere on your kiln.  it is not "kilnsitter".   that is just an automatic shutoff device.  (their term)

look at the side of the box with the controls on it.   neil knows everything about kilns and can help you more with that information.

Share this post


Link to post
Share on other sites

My Skutt model 231 kiln can't make it much higher than cone 6, so I plan to keep that as my max.  Can't afford a new kiln and I finally got the kilnsitter setup to trip at the right point.   It's older than me according to https://skutt.com/skutt-resources/manuals/kilnsitter/  - says it is made before 1980 so it's basically a functioning antique.  In good shape too.. unlike me... 

I'm switching to a medium range stoneware (Firing range is cone 06 to cone 8) so it will be more mature.  I quoted the stoneware based off an empty box I had laying around the house but have since found the above empty medium range stoneware box in the house as well.   Not sure how that happened, but the mid range luckily matches the  same shrinkage rate at cone 6 so marbling should still be fine I assume.

  All the clay is mixed by Rovin Ceramics.  The porcelain is also Rovin - Firing range is cone 6, Shrinkage at cone 6 is 11.7%, Absorption at cone 6 it is 0.9%.  Description: RO-95 is a very white domestic kaolin porcelain for firing at cone 6. Well-suited for wheel work and hand-building, RO-95 is also used as a pattern clay for mold-makers.  https://rovinceramics.com/collections/white-moist-clay/products/ro-95-cone-6-porcelain

@neilestrick - 

Quote

What brand/model of porcelain are you using? They're not all the same.

- Did the above answer the question on the porcelain? 

Perhaps I need to find a clear glaze for the stoneware/terracotta and a separate for the porcelain?  I never would have thought of needing different clear glazes, another reason I'm glad this community exists.  

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.