diff --git a/ClosedXML/ClosedXML/ClosedXML/ClosedXML.csproj b/ClosedXML/ClosedXML/ClosedXML/ClosedXML.csproj
index 3e55efc..7f22e1d 100644
--- a/ClosedXML/ClosedXML/ClosedXML/ClosedXML.csproj
+++ b/ClosedXML/ClosedXML/ClosedXML/ClosedXML.csproj
@@ -138,6 +138,7 @@
     <Compile Include="Excel\Misc\XLCallbackAction.cs" />
     <Compile Include="Excel\Misc\XLReentrantEnumerableSet.cs" />
     <Compile Include="Excel\XLConstants.cs" />
+    <Compile Include="Utils\GraphicsUtils.cs" />
     <Compile Include="XLHelper.cs" />
     <Compile Include="Excel\AutoFilters\XLFilteredColumn.cs" />
     <Compile Include="Excel\AutoFilters\XLCustomFilteredColumn.cs" />
diff --git a/ClosedXML/ClosedXML/ClosedXML/Excel/Cells/XLCell.cs b/ClosedXML/ClosedXML/ClosedXML/Excel/Cells/XLCell.cs
index 09c6e3f..e7a6c06 100644
--- a/ClosedXML/ClosedXML/ClosedXML/Excel/Cells/XLCell.cs
+++ b/ClosedXML/ClosedXML/ClosedXML/Excel/Cells/XLCell.cs
@@ -2062,19 +2062,19 @@
                                     if (row1String[0] == '$')
                                     {
                                         row1 = "$" +
-                                               (Int32.Parse(row1String.Substring(1)) + rowsShifted).ToStringLookup();
+                                               (Int32.Parse(row1String.Substring(1)) + rowsShifted).ToInvariantString();
                                     }
                                     else
-                                        row1 = (Int32.Parse(row1String) + rowsShifted).ToStringLookup();
+                                        row1 = (Int32.Parse(row1String) + rowsShifted).ToInvariantString();
 
                                     string row2;
                                     if (row2String[0] == '$')
                                     {
                                         row2 = "$" +
-                                               (Int32.Parse(row2String.Substring(1)) + rowsShifted).ToStringLookup();
+                                               (Int32.Parse(row2String.Substring(1)) + rowsShifted).ToInvariantString();
                                     }
                                     else
-                                        row2 = (Int32.Parse(row2String) + rowsShifted).ToStringLookup();
+                                        row2 = (Int32.Parse(row2String) + rowsShifted).ToInvariantString();
 
                                     sb.Append(useSheetName
                                                   ? String.Format("'{0}'!{1}:{2}", sheetName, row1, row2)
diff --git a/ClosedXML/ClosedXML/ClosedXML/Excel/Coordinates/XLAddress.cs b/ClosedXML/ClosedXML/ClosedXML/Excel/Coordinates/XLAddress.cs
index 8d0f040..d9b9a8e 100644
--- a/ClosedXML/ClosedXML/ClosedXML/Excel/Coordinates/XLAddress.cs
+++ b/ClosedXML/ClosedXML/ClosedXML/Excel/Coordinates/XLAddress.cs
@@ -207,7 +207,7 @@
             {
                 retVal += "$";
             }
-            retVal += _rowNumber.ToStringLookup();
+            retVal += _rowNumber.ToInvariantString();
             return retVal;
         }
 
@@ -215,23 +215,23 @@
         {
             if (referenceStyle == XLReferenceStyle.A1)
             {
-                return ColumnLetter + _rowNumber.ToStringLookup();
+                return ColumnLetter + _rowNumber.ToInvariantString();
             }
             if (referenceStyle == XLReferenceStyle.R1C1)
             {
-                return String.Format("R{0}C{1}", _rowNumber.ToStringLookup(), ColumnNumber);
+                return String.Format("R{0}C{1}", _rowNumber.ToInvariantString(), ColumnNumber);
             }
             if (HasWorksheet && Worksheet.Workbook.ReferenceStyle == XLReferenceStyle.R1C1)
             {
-                return String.Format("R{0}C{1}", _rowNumber.ToStringLookup(), ColumnNumber);
+                return String.Format("R{0}C{1}", _rowNumber.ToInvariantString(), ColumnNumber);
             }
-            return ColumnLetter + _rowNumber.ToStringLookup();
+            return ColumnLetter + _rowNumber.ToInvariantString();
         }
         #endregion
         #region Methods
         public string GetTrimmedAddress()
         {
-            return _trimmedAddress ?? (_trimmedAddress = ColumnLetter + _rowNumber.ToStringLookup());
+            return _trimmedAddress ?? (_trimmedAddress = ColumnLetter + _rowNumber.ToInvariantString());
         }
 
 
@@ -362,13 +362,13 @@
         {
             String address;
             if (referenceStyle == XLReferenceStyle.A1)
-                address = String.Format("${0}${1}", ColumnLetter, _rowNumber.ToStringLookup());
+                address = String.Format("${0}${1}", ColumnLetter, _rowNumber.ToInvariantString());
             else if (referenceStyle == XLReferenceStyle.R1C1)
-                address = String.Format("R{0}C{1}", _rowNumber.ToStringLookup(), ColumnNumber);
+                address = String.Format("R{0}C{1}", _rowNumber.ToInvariantString(), ColumnNumber);
             else if (HasWorksheet && Worksheet.Workbook.ReferenceStyle == XLReferenceStyle.R1C1)
-                address = String.Format("R{0}C{1}", _rowNumber.ToStringLookup(), ColumnNumber);
+                address = String.Format("R{0}C{1}", _rowNumber.ToInvariantString(), ColumnNumber);
             else
-                address = String.Format("${0}${1}", ColumnLetter, _rowNumber.ToStringLookup());
+                address = String.Format("${0}${1}", ColumnLetter, _rowNumber.ToInvariantString());
 
             if (includeSheet)
                 return String.Format("'{0}'!{1}",
diff --git a/ClosedXML/ClosedXML/ClosedXML/Excel/Ranges/XLRangeAddress.cs b/ClosedXML/ClosedXML/ClosedXML/Excel/Ranges/XLRangeAddress.cs
index 1a02e91..d5b0e94 100644
--- a/ClosedXML/ClosedXML/ClosedXML/Excel/Ranges/XLRangeAddress.cs
+++ b/ClosedXML/ClosedXML/ClosedXML/Excel/Ranges/XLRangeAddress.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Diagnostics;
+using System.Globalization;
 using System.Linq;
 
 namespace ClosedXML.Excel
@@ -64,7 +65,7 @@
                 else
                 {
                     FirstAddress = XLAddress.Create(worksheet, firstPart + "1");
-                    LastAddress = XLAddress.Create(worksheet, secondPart + XLHelper.MaxRowNumber.ToStringLookup());    
+                    LastAddress = XLAddress.Create(worksheet, secondPart + XLHelper.MaxRowNumber.ToString(CultureInfo.InvariantCulture));    
                 }
             }
 
diff --git a/ClosedXML/ClosedXML/ClosedXML/Excel/Style/Colors/XLColor_Static.cs b/ClosedXML/ClosedXML/ClosedXML/Excel/Style/Colors/XLColor_Static.cs
index dfaaaa7..d66e191 100644
--- a/ClosedXML/ClosedXML/ClosedXML/Excel/Style/Colors/XLColor_Static.cs
+++ b/ClosedXML/ClosedXML/ClosedXML/Excel/Style/Colors/XLColor_Static.cs
@@ -7,14 +7,19 @@
 {
     public partial class XLColor
     {
-        private static Dictionary<Color, XLColor> byColor = new Dictionary<Color, XLColor>();
+        private static readonly Dictionary<Color, XLColor> ByColor = new Dictionary<Color, XLColor>();
+        private static readonly Object ByColorLock = new Object();
+
         public static XLColor FromColor(Color color)
         {
             XLColor ret;
-            if (!byColor.TryGetValue(color, out ret))
+            lock (ByColorLock)
             {
-                ret = new XLColor(color);
-                byColor.Add(color, ret);
+                if (!ByColor.TryGetValue(color, out ret))
+                {
+                    ret = new XLColor(color);
+                    ByColor.Add(color, ret);
+                }
             }
             return ret;
         }
@@ -44,50 +49,65 @@
             return FromColor(ColorTranslator.FromHtml(htmlColor));
         }
 
-        private static Dictionary<Int32, XLColor> byIndex = new Dictionary<Int32, XLColor>();
+        private static readonly Dictionary<Int32, XLColor> ByIndex = new Dictionary<Int32, XLColor>();
+        private static readonly Object ByIndexLock = new Object();
+
         public static XLColor FromIndex(Int32 index)
         {
             XLColor ret;
-            if (!byIndex.TryGetValue(index, out ret))
+            lock (ByIndexLock)
             {
-                ret = new XLColor(index);
-                byIndex.Add(index, ret);
+                if (!ByIndex.TryGetValue(index, out ret))
+                {
+                    ret = new XLColor(index);
+                    ByIndex.Add(index, ret);
+                }
             }
             return ret;
         }
 
-        private static Dictionary<XLThemeColor, XLColor> byTheme = new Dictionary<XLThemeColor, XLColor>();
+        private static readonly Dictionary<XLThemeColor, XLColor> ByTheme = new Dictionary<XLThemeColor, XLColor>();
+        private static readonly Object ByThemeLock = new Object();
+
         public static XLColor FromTheme(XLThemeColor themeColor)
         {
             XLColor ret;
-            if (!byTheme.TryGetValue(themeColor, out ret))
+            lock (ByThemeLock)
             {
-                ret = new XLColor(themeColor);
-                byTheme.Add(themeColor, ret);
+                if (!ByTheme.TryGetValue(themeColor, out ret))
+                {
+                    ret = new XLColor(themeColor);
+                    ByTheme.Add(themeColor, ret);
+                }
             }
             return ret;
         }
 
-        private static Dictionary<XLThemeColor, Dictionary<Double, XLColor>> byThemeTint = new Dictionary<XLThemeColor, Dictionary<double, XLColor>>();
+        private static readonly Dictionary<XLThemeColor, Dictionary<Double, XLColor>> ByThemeTint = new Dictionary<XLThemeColor, Dictionary<double, XLColor>>();
+        private static readonly Object ByThemeTintLock = new Object();
+
         public static XLColor FromTheme(XLThemeColor themeColor, Double themeTint)
         {
             XLColor ret;
-            Dictionary<Double, XLColor> themeTints;
-            if (byThemeTint.TryGetValue(themeColor, out themeTints))
+            lock (ByThemeTintLock)
             {
-                if (!themeTints.TryGetValue(themeTint, out ret))
+                Dictionary<Double, XLColor> themeTints;
+                if (ByThemeTint.TryGetValue(themeColor, out themeTints))
                 {
+                    if (!themeTints.TryGetValue(themeTint, out ret))
+                    {
+                        ret = new XLColor(themeColor, themeTint);
+                        themeTints.Add(themeTint, ret);
+                    }
+                }
+                else
+                {
+                    themeTints = new Dictionary<double, XLColor>();
                     ret = new XLColor(themeColor, themeTint);
                     themeTints.Add(themeTint, ret);
-                }
-            }
-            else
-            {
-                themeTints = new Dictionary<double, XLColor>();
-                ret = new XLColor(themeColor, themeTint);
-                themeTints.Add(themeTint, ret);
 
-                byThemeTint.Add(themeColor, themeTints);
+                    ByThemeTint.Add(themeColor, themeTints);
+                }
             }
             return ret;
         }
diff --git a/ClosedXML/ClosedXML/ClosedXML/Excel/Tables/XLTable.cs b/ClosedXML/ClosedXML/ClosedXML/Excel/Tables/XLTable.cs
index 112221e..a090dfc 100644
--- a/ClosedXML/ClosedXML/ClosedXML/Excel/Tables/XLTable.cs
+++ b/ClosedXML/ClosedXML/ClosedXML/Excel/Tables/XLTable.cs
@@ -396,7 +396,7 @@
             foreach (IXLCell c in range.Row(1).Cells())
             {
                 if (XLHelper.IsNullOrWhiteSpace(((XLCell)c).InnerText))
-                    c.Value = GetUniqueName("Column" + co.ToStringLookup());
+                    c.Value = GetUniqueName("Column" + co.ToInvariantString());
                 _uniqueNames.Add(c.GetString());
                 co++;
             }
@@ -410,11 +410,11 @@
             if (_uniqueNames.Contains(name))
             {
                 Int32 i = 1;
-                name = originalName + i.ToStringLookup();
+                name = originalName + i.ToInvariantString();
                 while (_uniqueNames.Contains(name))
                 {
                     i++;
-                    name = originalName + i.ToStringLookup();
+                    name = originalName + i.ToInvariantString();
                 }
             }
 
@@ -446,7 +446,7 @@
                     foreach (IXLCell c in headersRow.Cells())
                     {
                         if (XLHelper.IsNullOrWhiteSpace(((XLCell)c).InnerText))
-                            c.Value = GetUniqueName("Column" + co.ToStringLookup());
+                            c.Value = GetUniqueName("Column" + co.ToInvariantString());
                         _uniqueNames.Add(c.GetString());
                         co++;
                     }
diff --git a/ClosedXML/ClosedXML/ClosedXML/Excel/XLWorkbook_Save.cs b/ClosedXML/ClosedXML/ClosedXML/Excel/XLWorkbook_Save.cs
index 21072b9..70ef083 100644
--- a/ClosedXML/ClosedXML/ClosedXML/Excel/XLWorkbook_Save.cs
+++ b/ClosedXML/ClosedXML/ClosedXML/Excel/XLWorkbook_Save.cs
@@ -1580,11 +1580,11 @@
             if (context.TableNames.Contains(name))
             {
                 var i = 1;
-                name = tableName + i.ToStringLookup();
+                name = tableName + i.ToInvariantString();
                 while (context.TableNames.Contains(name))
                 {
                     i++;
-                    name = tableName + i.ToStringLookup();
+                    name = tableName + i.ToInvariantString();
                 }
             }
 
@@ -3282,7 +3282,7 @@
                 maxColumn = xlWorksheet.Internals.CellsCollection.MaxColumnUsed;
                 var maxRow = xlWorksheet.Internals.CellsCollection.MaxRowUsed;
                 sheetDimensionReference = "A1:" + XLHelper.GetColumnLetterFromNumber(maxColumn) +
-                                          maxRow.ToStringLookup();
+                                          maxRow.ToInvariantString();
             }
 
             if (xlWorksheet.Internals.ColumnsCollection.Count > 0)
@@ -3637,7 +3637,7 @@
                 }
 
                 if (maxColumn > 0)
-                    row.Spans = new ListValue<StringValue> {InnerText = "1:" + maxColumn.ToStringLookup()};
+                    row.Spans = new ListValue<StringValue> {InnerText = "1:" + maxColumn.ToInvariantString()};
 
                 row.Height = null;
                 row.CustomHeight = null;
@@ -3679,7 +3679,7 @@
                 {
                     foreach (var delCo in kpDel.Value.ToList())
                     {
-                        var key = XLHelper.GetColumnLetterFromNumber(delCo) + kpDel.Key.ToStringLookup();
+                        var key = XLHelper.GetColumnLetterFromNumber(delCo) + kpDel.Key.ToInvariantString();
                         if (!cellsByReference.ContainsKey(key)) continue;
                         row.RemoveChild(cellsByReference[key]);
                         kpDel.Value.Remove(delCo);
diff --git a/ClosedXML/ClosedXML/ClosedXML/Extensions.cs b/ClosedXML/ClosedXML/ClosedXML/Extensions.cs
index cac055e..dbc2e57 100644
--- a/ClosedXML/ClosedXML/ClosedXML/Extensions.cs
+++ b/ClosedXML/ClosedXML/ClosedXML/Extensions.cs
@@ -9,6 +9,7 @@
 using System.Xml.Linq;
 using System.IO;
 using System.Xml;
+using ClosedXML.Utils;
 
 [assembly: CLSCompliantAttribute(true)]
 namespace ClosedXML.Excel
@@ -162,18 +163,12 @@
 
     public static class IntegerExtensions
     {
-        private static readonly NumberFormatInfo nfi = CultureInfo.InvariantCulture.NumberFormat;
-        private static readonly Dictionary<Int32, String> intToString = new Dictionary<int, string>();
-        public static String ToStringLookup(this Int32 value)
+       
+        public static String ToInvariantString(this Int32 value)
         {
-            if (!intToString.ContainsKey(value))
-            {
-                intToString.Add(value, value.ToString(nfi));
-            }
-            return intToString[value];
+            return value.ToString(CultureInfo.InvariantCulture);
         }
 
-
     }
 
     public static class FontBaseExtensions
@@ -190,7 +185,6 @@
             return font;
         }
 
-        private static Graphics graphics = Graphics.FromImage(new Bitmap(1, 1));
         public static Double GetWidth(this IXLFontBase fontBase, String text, Dictionary<IXLFontBase, Font> fontCache)
         {
             if (XLHelper.IsNullOrWhiteSpace(text))
@@ -198,7 +192,7 @@
 
             var font = GetCachedFont(fontBase, fontCache);
 
-            var textSize = graphics.MeasureString(text, font, Int32.MaxValue, StringFormat.GenericTypographic);
+            var textSize = GraphicsUtils.MeasureString(text, font);
 
             double width = (((textSize.Width / (double)7) * 256) - (128 / 7)) / 256;
             width = (double)decimal.Round((decimal)width + 0.2M, 2);
@@ -219,7 +213,7 @@
         public static Double GetHeight(this IXLFontBase fontBase, Dictionary<IXLFontBase, Font> fontCache)
         {
             var font = GetCachedFont(fontBase, fontCache);
-            var textSize = graphics.MeasureString("X", font, Int32.MaxValue, StringFormat.GenericTypographic);
+            var textSize = GraphicsUtils.MeasureString("X", font);
             return (double)textSize.Height * 0.85;
         }
 
diff --git a/ClosedXML/ClosedXML/ClosedXML/Utils/GraphicsUtils.cs b/ClosedXML/ClosedXML/ClosedXML/Utils/GraphicsUtils.cs
new file mode 100644
index 0000000..954c22c
--- /dev/null
+++ b/ClosedXML/ClosedXML/ClosedXML/Utils/GraphicsUtils.cs
@@ -0,0 +1,22 @@
+using System;
+using System.Drawing;
+
+namespace ClosedXML.Utils
+{
+    internal static class GraphicsUtils
+    {
+        public static SizeF MeasureString(string s, Font font)
+        {
+            SizeF result;
+            using (var image = new Bitmap(1, 1))
+            {
+                using (var g = Graphics.FromImage(image))
+                {
+                    result = g.MeasureString(s, font, Int32.MaxValue, StringFormat.GenericTypographic);
+                }
+            }
+
+            return result;
+        }
+    }
+}
diff --git a/ClosedXML/ClosedXML/ClosedXML/XLHelper.cs b/ClosedXML/ClosedXML/ClosedXML/XLHelper.cs
index 6bb853c..8f5f073 100644
--- a/ClosedXML/ClosedXML/ClosedXML/XLHelper.cs
+++ b/ClosedXML/ClosedXML/ClosedXML/XLHelper.cs
@@ -54,85 +54,52 @@
                       RegexOptions.Compiled | RegexOptions.CultureInvariant | RegexOptions.ExplicitCapture
                 );
 
-        private static Dictionary<string, int> columnNumbers = new Dictionary<string, int>();
         /// <summary>
-        /// 	Gets the column number of a given column letter.
+        /// Gets the column number of a given column letter.
         /// </summary>
         /// <param name="columnLetter"> The column letter to translate into a column number. </param>
         public static int GetColumnNumberFromLetter(string columnLetter)
         {
+            if (string.IsNullOrEmpty(columnLetter)) throw new ArgumentNullException("columnLetter");
+
             int retVal;
             columnLetter = columnLetter.ToUpper();
-            if (columnNumbers.TryGetValue(columnLetter, out retVal))
-                return retVal;
 
+            //Extra check because we allow users to pass row col positions in as strings
             if (columnLetter[0] <= '9')
             {
                 retVal = Int32.Parse(columnLetter, NumberFormatForParse);
-                columnNumbers.Add(columnLetter, retVal);
                 return retVal;
             }
-            
-            var length = columnLetter.Length;
-            if (length == 1)
+
+            int sum = 0;
+
+            for (int i = 0; i < columnLetter.Length; i++)
             {
-                retVal = Convert.ToByte(columnLetter[0]) - 64;
-                columnNumbers.Add(columnLetter, retVal);
-                return retVal;
+                sum *= 26;
+                sum += (columnLetter[i] - 'A' + 1);
             }
-            if (length == 2)
-            {
-                retVal =
-                    ((Convert.ToByte(columnLetter[0]) - 64)*26) +
-                    (Convert.ToByte(columnLetter[1]) - 64);
-                columnNumbers.Add(columnLetter, retVal);
-                return retVal;
-            }
-            if (length == 3)
-            {
-                retVal = ((Convert.ToByte(columnLetter[0]) - 64)*TwoT26) +
-                       ((Convert.ToByte(columnLetter[1]) - 64)*26) +
-                       (Convert.ToByte(columnLetter[2]) - 64);
-                columnNumbers.Add(columnLetter, retVal);
-                return retVal;
-            }
-            throw new ApplicationException("Column Length must be between 1 and 3.");
+
+            return sum;
         }
 
-        private static Dictionary<int, string> columnLetters = new Dictionary<int, string>();
         /// <summary>
         /// 	Gets the column letter of a given column number.
         /// </summary>
-        /// <param name="column"> The column number to translate into a column letter. </param>
+        /// <param name="columnNumber"> The column number to translate into a column letter. </param>
         public static string GetColumnLetterFromNumber(int columnNumber)
         {
-            String retVal;
-            if (columnLetters.TryGetValue(columnNumber, out retVal))
-                return retVal;
+            var dividend = columnNumber;
+            var columnName = String.Empty;
 
-            #region Check
-
-            if (columnNumber <= 0)
-                throw new ArgumentOutOfRangeException("column", "Must be more than 0");
-
-            #endregion
-
-            var value = new StringBuilder(6);
-            int column = columnNumber;
-            while (column > 0)
+            while (dividend > 0)
             {
-                var residue = column%26;
-                column /= 26;
-                if (residue == 0)
-                {
-                    residue = 26;
-                    column--;
-                }
-                value.Insert(0, (char) (64 + residue));
+                var modulo = (dividend - 1) % 26;
+                columnName = Convert.ToChar(65 + modulo) + columnName;
+                dividend = (dividend - modulo) / 26;
             }
-            retVal = value.ToString();
-            columnLetters.Add(columnNumber, retVal);
-            return retVal;
+
+            return columnName;
         }
 
         public static bool IsValidColumn(string column)
diff --git a/ClosedXML/ClosedXML/ClosedXML_Net3.5/ClosedXML_Net3.5.csproj b/ClosedXML/ClosedXML/ClosedXML_Net3.5/ClosedXML_Net3.5.csproj
index b73f954..b41afcb 100644
--- a/ClosedXML/ClosedXML/ClosedXML_Net3.5/ClosedXML_Net3.5.csproj
+++ b/ClosedXML/ClosedXML/ClosedXML_Net3.5/ClosedXML_Net3.5.csproj
@@ -876,6 +876,9 @@
     <Compile Include="..\ClosedXML\PathHelper.cs">
       <Link>PathHelper.cs</Link>
     </Compile>
+    <Compile Include="..\ClosedXML\Utils\GraphicsUtils.cs">
+      <Link>Utils\GraphicsUtils.cs</Link>
+    </Compile>
     <Compile Include="..\ClosedXML\XLHelper.cs">
       <Link>XLHelper.cs</Link>
     </Compile>