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Minnesota Clay Co. USA Technical Info - Mixing & Using Stoneware Glazes

Minnesota Clay Stoneware Glazes are leadless and vailable in powder and liquid form. Mix powdered glaze with tap water at a ratio of 12 fluid ounces per pound of glaze. Using a blender will help to activate the suspending agents in the mixture and the resulting slurry should be poured through a 60 mesh sieve. The resulting mixture will likely need an addition of water to obtain a slightly thicker than whole milk consistency. If you have a hydrometer, a 1.45 -1.5 reading is a good target (1.45 reading for BT-1 Clear). Premixed Liquid Glazes should be shaken well and poured into a glaze bucket through a 60 mesh screen. Add water if necessary to achieve a slightly thicker than whole milk consistency.

Rather than discuss each glaze individually, we will discuss each general grouping/series. We ask that you make specific allowances for each glaze, considering personal preferences and your own given situation. The glazes are applied to either the #1 Stoneware (and variations with Grog or Manganese), #2 Stoneware, #3 Stoneware, Rainy Lake or MB bisque fired to cone 08-04; then glaze fired primarily to cone 6 in oxidation. The glaze firing should reach cone 6 in approximately 8-9 hours for best results.

Application Techniques: Dipping is the best method, but if you must use a brush, brush 2-4 coats, one coat directly after the other. Or, Dip 1-2 coats, applying overlapping coats before the previous coat has lost its wet sheen.

Minnesota Clay Stoneware Glaze Series:

BT-Series Bright Transparent Glazes - 14 glossy colors that allow some or much of the clay to show through. Apply to cone 08-04 bisque ware and glaze fire from cone 6-9. Mauve Red and Purple provide the best color from cone 5-7.

The transparent series is formulated primarily on a single base glaze. We recommend just over 1lb. of water to 1lb. of dry glaze. This glaze tends to flocculate (thicken on standing) and requires slightly more water than other glazes of this temperature range.

We recommend a fairly thin application to retain the transparent quality and prevent crazing. Better brilliance will be achieved if your pieces are on a white or light clay body(#3, MB, Northern Light, Polar, Rainy Lake). A single-dip/thin application, or even spraying the glaze, will give the best results.

HG-Series High Gloss Opaque Glazes and SG-series Slip Glazes - 16 glossy (HG-14 is a semi-gloss) colors that will largely hide the clay body that it is covering. Slip Glazes can be applied over other stoneware glazes to create atmospheric effects. Apply to cone 08-04 bisque ware and glaze fire from cone 5-9. HG-4 (2nd Hand Rose) looks best fired from cone 4-5.

Although there are many bases involved in the High Gloss series, they all require about the same amount of water-to-dry ratio for workability. We recommend slightly less than 1lb. of water to 1lb. dry glaze. The best results may be obtained by applying the glaze in two medium-thick dipped coats.

One exception to the double-dip application might be the HG-14 Oil Spot Black glaze, which seems to work better when applied slightly thinner. In this case a single-dip should be adequate. When the application is just right you should have a smooth, satin gloss surface with numerous "oil spots." When the application is too thin, the color will tend towards the black-brown and lack the "oil spots".

HM-Series Matt Glazes - 5 opaque colors with matt surfaces. Apply to cone 08 -04 bisque ware and glaze fire from cone 5-9. Matt glazes will become smoother and more glossy when fired toward the top end of the recommended firing range.

A number of different formulas make up the matte series, however; all seem to require just over 1 to 1-1/4 lb(s.) of water to 1lb. of dry glaze. The best color and surface are achieved by two dipped coats (3-4 brushed). Avoid too rapid a firing as this series need adequate time to "flux out." We fire our tests is about 8-1/2 hours.

NG-Series New Glaze Series - 17 new glazes added over the past few years. Most of these glazes are relatively opaque hiding the clay body's color. Apply to cone 08-04 bisque ware. All of these glazes have been sucessfully fired at cone 6. Individual firing temperatures can be found on the product pages.

Although there are many bases involved in the New Glaze series, they all require about the same amount of water-to-dry ratio for workability. We recommend slightly less than 1lb. of water to 1lb. dry glaze. The best results may be obtained by applying the glaze in two medium-thick dipped coats.

One exception to the water to dry glaze ratio might be the NG-4 Sapphire Blue glaze, which seems to work better when applied slightly thicker. In this case less water will be needed to get the desired thickness. When the application is just right you should have a high gloss surface with a considerable amount of variations in the blue glaze. When the application is too thin, the color will tend towards the green-black side and lack the blue variations.

Helpful Glaze Hints:

  • One pint of water = approximately one lb. of water.
  • 454grams = 1 lb.
  • Stir glazes thoroughly prior to applying.
  • Two thinner coats of glaze is better than one thick one. If too thick, the glaze may crack away from the ware before or during the firing. If the application is too thick, completely wash off the glaze and re-glaze after the piece has dried.
  • Apply the second glaze coat just before the first coat has lost its sheen.
  • Store left-over glaze in airtight containers to maintain its water/dry glaze ratio.

More Information on Using Minnesota Clay Glazes

Background:
Glazing can be a tricky business; therefore it is best to test any glaze that is new to you by preparing and firing a test piece.  The best kind of test piece will resemble the pots that you intend to glaze.  Also, you ought to attempt to use the same application technique that you will use on your finished pieces.  In preparing the test piece, make mental notes about the consistency of the glaze mixture.  For instance, does the glaze appear to be the consistency of milk, a light cream, or a heavy cream?  Also, examine the thickness of the glaze after it has dried on your test piece.  Using a fettling knife or other tool, remove a small piece of glaze to expose a cross section.  Does this cross section appear to be the thickness of a dime?  If so, the glaze may have been applied too heavily. 

The signs of an application that is too heavy are many: transparent glazes looking cloudy, rolls of glaze around the foot, crazing (or cracking), and crawling (where the glaze pulls away from the clay).  There is one main sign that a glaze is too thin, and that is a pebbly, orange peel texture.  Generally, glazing too heavily is a common problem; glazing too lightly is not.

Mixing Glaze:
Liquid glaze should be poured through a 60 mesh sieve into a bucket to remove any lumps.  Mix the glaze vigorously using a whisk or another tool.  If the glaze is new, you will probably need to add some water to thin the consistency.  Use your judgment and some test pieces to assess the thickness.  Remember that you can always add more water if it is too thick, but it is much more difficult to remove water.

Powdered Glazes:
Powdered glazes can be a little more work to prepare, but can be worth the effort because of the cost savings.  First, read and follow the safety precautions on the product label.  With powdered glazes it is best to wear a NIOSH-certified mask to avoid inhaling dusts and to try to limit skin contact by wearing rubber gloves.  The safety label will inform you of any precautions you may need to take.  The label will also give you a recommended water amount to start with.  This recommendation is by no means the definitive amount of water that you will need to add.  In fact, it is usually on the thick side; you will need to add more or less water as you see fit. 

Mixing methods can be as low tech as you like.  However, the greater amount of agitation that you can bring to your glaze during mixing, the less lumpy your glaze will be and it will go through a sieve more easily.  If you can dedicate a blender to this task, so much the better.  Just remember not to use it for frozen drinks any longer!  A technique that will allow you to virtually eliminate dust is to add water to your glaze in a watertight plastic bag and mix it by massaging the bag.

Using A Hydrometer:
A Hydrometer is a measuring tool that can help you control the consistency of your glaze.  It is a glass tube with a weighted end and a scale printed vertically along its length.  When it is inserted into the glaze mixture, it will come to rest at a certain point along the scale.  The resulting reading (between 1000 and 2000, but usually closer to 1450) can help fine tune your glaze set up and make glazes more uniform from batch to batch.  (Note: springtime can bring changes to water quality and is sometimes reflected in the viscosity of your glazes.  Watch for unusual hydrometer readings at this time of year and adjust your glaze set up based on how it builds up on test items as opposed to the hydrometer.) 

If you have a gram scale you can identify the Specific Gravity (S.G.) of the glaze.  The resulting figure can be useful and a highly accurate measure of glaze consistency.  Specific Gravity is normally measured as a ratio of grams (weight) per milliliter (volume).  Once you have recorded the correct S.G. of a glaze, it will be simple to set up future batches to the same consistency.
Water has a specific gravity of 1 because 100 grams of water has a volume of 100 milliliters.  Since glaze contains not only water, but solids as well, it will have a S.G. greater than 1.  It is very easy to find out the specific gravity of a glaze.  First, find a container that is tall and narrow like a juice can.  Punch a small hole in the center of the lid.  Place the container and lid on the gram scale.  Use the tare bar on the scale to zero the scale to the combined weight of the container and lid.  Fill the container to the brim with water so that when the lid is applied, a small amount of water trickles through the hole in the top.  Sponge away the excess water on the lid and weigh the container.  The resulting weight in grams will also be the volume in milliliters.  For example: if the water weighs 250 grams, the volume is 250 milliliters.  Now, pour the water out and dry the container and lid.  Fill the container with your glaze and weigh it as you did with water.  Let’s say the weight is 350 grams.  To find out the S.G. of your glaze, divide the weight (350 grams) by the volume (250 milliliters).  In this case, the resulting S.G. is 1.4.  Keep a record of your S.G. figures for future batches of the same glaze.

Application Techniques:
The main application techniques are: dipping, pouring, brushing, and spraying on glaze.  Of the four, the first two are the best for our glazes.  Pouring or dipping will provide you with the most even application and the most reproducible results.  Brushing can be highly unpredictable because it is very difficult to achieve an even, all-around application thickness.  Spraying can be a very good technique, but it must be performed in a properly ventilated spray booth in combination with appropriate respiratory protection.

Dipping & Pouring:
Thin the glaze to a milk-thin consistency for best clarity in a transparent glaze, and use a slightly heavier consistency for opaque glazes.  If dipping two or more glazes so that they overlap, apply each new coat before the previous coat has lost its wet sheen.  This will allow the two coats to dry together and help prevent peeling.  Glaze tongs work well if you want to dip an entire piece into one or more glazes.  Be sure that the glaze container has sufficient volume to submerge your piece without hitting the sides.  Try to locate the points of the glaze tongs on a structurally strong area of the piece and, if possible, away from areas that are decorative.  The four “snake bites” that a glaze tong leaves can be dabbed with a brush using the same glaze.

Brushing:
This technique can work for small quantities of glaze.  Three or four coats of light cream consistency glaze should give the proper fired thickness.  Try to overlap coats as you work around your piece and try not to allow any one area to become too dry.  Anytime you apply a wet coat of glaze to an already dried area of glaze, there is the potential for the topcoat to peel as it dries.

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